BashNotesForProfessionals.txt

Bash
Bash
Notes for Professionals

Notes for Professionals

100+ pages

of professional hints and tricks

GoalKicker.com

Free Programming Books

Disclaimer
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Contents
About ................................................................................................................................................................................... 1
Chapter 1: Getting started with Bash ................................................................................................................. 2
Section 1.1: Hello World ................................................................................................................................................. 2
Section 1.2: Hello World Using Variables ..................................................................................................................... 4
Section 1.3: Hello World with User Input ...................................................................................................................... 4
Section 1.4: Importance of Quoting in Strings ............................................................................................................ 5
Section 1.5: Viewing information for Bash built-ins .................................................................................................... 6
Section 1.6: Hello World in "Debug" mode .................................................................................................................. 6
Section 1.7: Handling Named Arguments .................................................................................................................... 7

Chapter 2: Script shebang ........................................................................................................................................ 8
Section 2.1: Env shebang .............................................................................................................................................. 8
Section 2.2: Direct shebang .......................................................................................................................................... 8
Section 2.3: Other shebangs ........................................................................................................................................ 8

Chapter 3: Navigating directories ...................................................................................................................... 10
Section 3.1: Absolute vs relative directories .............................................................................................................. 10
Section 3.2: Change to the last directory .................................................................................................................. 10
Section 3.3: Change to the home directory .............................................................................................................. 10
Section 3.4: Change to the Directory of the Script .................................................................................................. 10

Chapter 4: Listing Files ............................................................................................................................................ 12
Section 4.1: List Files in a Long Listing Format ......................................................................................................... 12
Section 4.2: List the Ten Most Recently Modified Files ............................................................................................ 13
Section 4.3: List All Files Including Dotfiles ................................................................................................................ 13
Section 4.4: List Files Without Using `ls` ..................................................................................................................... 13
Section 4.5: List Files ................................................................................................................................................... 14
Section 4.6: List Files in a Tree-Like Format ............................................................................................................. 14
Section 4.7: List Files Sorted by Size .......................................................................................................................... 14

Chapter 5: Using cat .................................................................................................................................................. 16
Section 5.1: Concatenate files ..................................................................................................................................... 16
Section 5.2: Printing the Contents of a File ............................................................................................................... 16
Section 5.3: Write to a file ........................................................................................................................................... 17
Section 5.4: Show non printable characters ............................................................................................................. 17
Section 5.5: Read from standard input ..................................................................................................................... 18
Section 5.6: Display line numbers with output ......................................................................................................... 18
Section 5.7: Concatenate gzipped files ..................................................................................................................... 18

Chapter 6: Grep ........................................................................................................................................................... 20
Section 6.1: How to search a file for a pattern ......................................................................................................... 20

Chapter 7: Aliasing ..................................................................................................................................................... 21
Section 7.1: Bypass an alias ........................................................................................................................................ 21
Section 7.2: Create an Alias ........................................................................................................................................ 21
Section 7.3: Remove an alias ..................................................................................................................................... 21
Section 7.4: The BASH_ALIASES is an internal bash assoc array .......................................................................... 22
Section 7.5: Expand alias ............................................................................................................................................ 22
Section 7.6: List all Aliases .......................................................................................................................................... 22

Chapter 8: Jobs and Processes ........................................................................................................................... 23
Section 8.1: Job handling ............................................................................................................................................ 23
Section 8.2: Check which process running on specific port .................................................................................... 25

Section 8.3: Disowning background job .................................................................................................................... 25
Section 8.4: List Current Jobs ..................................................................................................................................... 25
Section 8.5: Finding information about a running process ..................................................................................... 25
Section 8.6: List all processes ..................................................................................................................................... 26

Chapter 9: Redirection ............................................................................................................................................. 27
Section 9.1: Redirecting standard output .................................................................................................................. 27
Section 9.2: Append vs Truncate ............................................................................................................................... 27
Section 9.3: Redirecting both STDOUT and STDERR .............................................................................................. 28
Section 9.4: Using named pipes ................................................................................................................................. 28
Section 9.5: Redirection to network addresses ........................................................................................................ 30
Section 9.6: Print error messages to stderr .............................................................................................................. 30
Section 9.7: Redirecting multiple commands to the same file ............................................................................... 31
Section 9.8: Redirecting STDIN .................................................................................................................................. 31
Section 9.9: Redirecting STDERR ............................................................................................................................... 32
Section 9.10: STDIN, STDOUT and STDERR explained ............................................................................................ 32

Chapter 10: Control Structures ............................................................................................................................ 34
Section 10.1: Conditional execution of command lists ............................................................................................. 34
Section 10.2: If statement ............................................................................................................................................ 35
Section 10.3: Looping over an array .......................................................................................................................... 36
Section 10.4: Using For Loop to List Iterate Over Numbers .................................................................................... 37
Section 10.5: continue and break ............................................................................................................................... 37
Section 10.6: Loop break ............................................................................................................................................. 37
Section 10.7: While Loop ............................................................................................................................................. 38
Section 10.8: For Loop with C-style syntax ............................................................................................................... 39
Section 10.9: Until Loop ............................................................................................................................................... 39
Section 10.10: Switch statement with case ................................................................................................................ 39
Section 10.11: For Loop without a list-of-words parameter ..................................................................................... 40

Chapter 11: true, false and : commands .......................................................................................................... 41
Section 11.1: Infinite Loop ............................................................................................................................................. 41
Section 11.2: Function Return ...................................................................................................................................... 41
Section 11.3: Code that will always/never be executed ........................................................................................... 41

Chapter 12: Arrays ...................................................................................................................................................... 42
Section 12.1: Array Assignments ................................................................................................................................. 42
Section 12.2: Accessing Array Elements .................................................................................................................... 43
Section 12.3: Array Modification ................................................................................................................................. 43
Section 12.4: Array Iteration ....................................................................................................................................... 44
Section 12.5: Array Length .......................................................................................................................................... 45
Section 12.6: Associative Arrays ................................................................................................................................. 45
Section 12.7: Looping through an array .................................................................................................................... 46
Section 12.8: Destroy, Delete, or Unset an Array ..................................................................................................... 47
Section 12.9: Array from string ................................................................................................................................... 47
Section 12.10: List of initialized indexes ...................................................................................................................... 47
Section 12.11: Reading an entire file into an array .................................................................................................... 48
Section 12.12: Array insert function ............................................................................................................................ 48

Chapter 13: Associative arrays ............................................................................................................................ 50
Section 13.1: Examining assoc arrays ........................................................................................................................ 50

Chapter 14: Functions ............................................................................................................................................... 52
Section 14.1: Functions with arguments ..................................................................................................................... 52
Section 14.2: Simple Function ..................................................................................................................................... 53
Section 14.3: Handling flags and optional parameters ........................................................................................... 53

Section 14.4: Print the function definition .................................................................................................................. 54
Section 14.5: A function that accepts named parameters ...................................................................................... 54
Section 14.6: Return value from a function ............................................................................................................... 55
Section 14.7: The exit code of a function is the exit code of its last command ..................................................... 55

Chapter 15: Bash Parameter Expansion .......................................................................................................... 57
Section 15.1: Modifying the case of alphabetic characters ..................................................................................... 57
Section 15.2: Length of parameter ............................................................................................................................ 57
Section 15.3: Replace pattern in string ...................................................................................................................... 58
Section 15.4: Substrings and subarrays .................................................................................................................... 59
Section 15.5: Delete a pattern from the beginning of a string ................................................................................ 60
Section 15.6: Parameter indirection ........................................................................................................................... 61
Section 15.7: Parameter expansion and filenames .................................................................................................. 61
Section 15.8: Default value substitution ..................................................................................................................... 62
Section 15.9: Delete a pattern from the end of a string .......................................................................................... 62
Section 15.10: Munging during expansion ................................................................................................................. 63
Section 15.11: Error if variable is empty or unset ...................................................................................................... 64

Chapter 16: Copying (cp) ......................................................................................................................................... 65
Section 16.1: Copy a single file .................................................................................................................................... 65
Section 16.2: Copy folders ........................................................................................................................................... 65

Chapter 17: Find ........................................................................................................................................................... 66
Section 17.1: Searching for a file by name or extension .......................................................................................... 66
Section 17.2: Executing commands against a found file ......................................................................................... 66
Section 17.3: Finding file by access / modification time .......................................................................................... 67
Section 17.4: Finding files according to size .............................................................................................................. 68
Section 17.5: Filter the path ......................................................................................................................................... 69
Section 17.6: Finding files by type .............................................................................................................................. 70
Section 17.7: Finding files by specific extension ........................................................................................................ 70

Chapter 18: Using sort .............................................................................................................................................. 71
Section 18.1: Sort command output ........................................................................................................................... 71
Section 18.2: Make output unique .............................................................................................................................. 71
Section 18.3: Numeric sort .......................................................................................................................................... 71
Section 18.4: Sort by keys ........................................................................................................................................... 72

Chapter 19: Sourcing ................................................................................................................................................. 74
Section 19.1: Sourcing a file ......................................................................................................................................... 74
Section 19.2: Sourcing a virtual environment ........................................................................................................... 74

Chapter 20: Here documents and here strings ........................................................................................... 76
Section 20.1: Execute command with here document ............................................................................................. 76
Section 20.2: Indenting here documents .................................................................................................................. 76
Section 20.3: Create a file ........................................................................................................................................... 77
Section 20.4: Here strings ........................................................................................................................................... 77
Section 20.5: Run several commands with sudo ..................................................................................................... 78
Section 20.6: Limit Strings .......................................................................................................................................... 78

Chapter 21: Quoting ................................................................................................................................................... 80
Section 21.1: Double quotes for variable and command substitution .................................................................... 80
Section 21.2: Dierence between double quote and single quote ......................................................................... 80
Section 21.3: Newlines and control characters ......................................................................................................... 81
Section 21.4: Quoting literal text ................................................................................................................................. 81

Chapter 22: Conditional Expressions ................................................................................................................ 83
Section 22.1: File type tests ......................................................................................................................................... 83

Section 22.2: String comparison and matching ....................................................................................................... 83
Section 22.3: Test on exit status of a command ...................................................................................................... 85
Section 22.4: One liner test ......................................................................................................................................... 85
Section 22.5: File comparison .................................................................................................................................... 85
Section 22.6: File access tests .................................................................................................................................... 86
Section 22.7: Numerical comparisons ....................................................................................................................... 86

Chapter 23: Scripting with Parameters ........................................................................................................... 88
Section 23.1: Multiple Parameter Parsing .................................................................................................................. 88
Section 23.2: Argument parsing using a for loop .................................................................................................... 89
Section 23.3: Wrapper script ...................................................................................................................................... 89
Section 23.4: Accessing Parameters ......................................................................................................................... 90
Section 23.5: Split string into an array in Bash ......................................................................................................... 91

Chapter 24: Bash history substitutions ........................................................................................................... 92
Section 24.1: Quick Reference .................................................................................................................................... 92
Section 24.2: Repeat previous command with sudo ............................................................................................... 93
Section 24.3: Search in the command history by pattern ....................................................................................... 93
Section 24.4: Switch to newly created directory with !#:N ...................................................................................... 93
Section 24.5: Using !$ .................................................................................................................................................. 94
Section 24.6: Repeat the previous command with a substitution .......................................................................... 94

Chapter 25: Math ........................................................................................................................................................ 95
Section 25.1: Math using dc ........................................................................................................................................ 95
Section 25.2: Math using bash capabilities ............................................................................................................... 96
Section 25.3: Math using bc ........................................................................................................................................ 96
Section 25.4: Math using expr .................................................................................................................................... 97

Chapter 26: Bash Arithmetic ................................................................................................................................. 98
Section 26.1: Simple arithmetic with (( )) ................................................................................................................... 98
Section 26.2: Arithmetic command ........................................................................................................................... 98
Section 26.3: Simple arithmetic with expr ................................................................................................................. 99

Chapter 27: Scoping ................................................................................................................................................ 100
Section 27.1: Dynamic scoping in action ................................................................................................................. 100

Chapter 28: Process substitution ...................................................................................................................... 101
Section 28.1: Compare two files from the web ....................................................................................................... 101
Section 28.2: Feed a while loop with the output of a command .......................................................................... 101
Section 28.3: Concatenating files ............................................................................................................................ 101
Section 28.4: Stream a file through multiple programs at once .......................................................................... 101
Section 28.5: With paste command ........................................................................................................................ 102
Section 28.6: To avoid usage of a sub-shell ........................................................................................................... 102

Chapter 29: Programmable completion ....................................................................................................... 103
Section 29.1: Simple completion using function ..................................................................................................... 103
Section 29.2: Simple completion for options and filenames ................................................................................ 103

Chapter 30: Customizing PS1 .............................................................................................................................. 104
Section 30.1: Colorize and customize terminal prompt ......................................................................................... 104
Section 30.2: Show git branch name in terminal prompt ..................................................................................... 105
Section 30.3: Show time in terminal prompt ........................................................................................................... 105
Section 30.4: Show a git branch using PROMPT_COMMAND .............................................................................. 106
Section 30.5: Change PS1 prompt ............................................................................................................................ 106
Section 30.6: Show previous command return status and time .......................................................................... 107

Chapter 31: Brace Expansion ............................................................................................................................... 109
Section 31.1: Modifying filename extension ............................................................................................................. 109

Section 31.2: Create directories to group files by month and year ...................................................................... 109
Section 31.3: Create a backup of dotfiles ................................................................................................................ 109
Section 31.4: Use increments .................................................................................................................................... 109
Section 31.5: Using brace expansion to create lists ............................................................................................... 109
Section 31.6: Make Multiple Directories with Sub-Directories ............................................................................... 110

Chapter 32: getopts : smart positional-parameter parsing .............................................................. 111
Section 32.1: pingnmap ............................................................................................................................................. 111

Chapter 33: Debugging .......................................................................................................................................... 113
Section 33.1: Checking the syntax of a script with "-n" .......................................................................................... 113
Section 33.2: Debugging using bashdb .................................................................................................................. 113
Section 33.3: Debugging a bash script with "-x" .................................................................................................... 113

Chapter 34: Pattern matching and regular expressions ..................................................................... 115
Section 34.1: Get captured groups from a regex match against a string ........................................................... 115
Section 34.2: Behaviour when a glob does not match anything ......................................................................... 115
Section 34.3: Check if a string matches a regular expression .............................................................................. 116
Section 34.4: Regex matching .................................................................................................................................. 116
Section 34.5: The * glob ............................................................................................................................................ 116
Section 34.6: The ** glob ........................................................................................................................................... 117
Section 34.7: The ? glob ............................................................................................................................................ 117
Section 34.8: The [ ] glob ......................................................................................................................................... 118
Section 34.9: Matching hidden files ......................................................................................................................... 119
Section 34.10: Case insensitive matching ................................................................................................................ 119
Section 34.11: Extended globbing ............................................................................................................................. 119

Chapter 35: Change shell ...................................................................................................................................... 121
Section 35.1: Find the current shell .......................................................................................................................... 121
Section 35.2: List available shells ............................................................................................................................. 121
Section 35.3: Change the shell ................................................................................................................................. 121

Chapter 36: Internal variables ........................................................................................................................... 122
Section 36.1: Bash internal variables at a glance ................................................................................................... 122
Section 36.2: $@ ........................................................................................................................................................ 123
Section 36.3: $# ......................................................................................................................................................... 123
Section 36.4: $HISTSIZE ............................................................................................................................................ 124
Section 36.5: $FUNCNAME ....................................................................................................................................... 124
Section 36.6: $HOME ................................................................................................................................................. 124
Section 36.7: $IFS ....................................................................................................................................................... 124
Section 36.8: $OLDPWD ............................................................................................................................................ 125
Section 36.9: $PWD ................................................................................................................................................... 125
Section 36.10: $1 $2 $3 etc.. ....................................................................................................................................... 125
Section 36.11: $* .......................................................................................................................................................... 126
Section 36.12: $! .......................................................................................................................................................... 126
Section 36.13: $? ......................................................................................................................................................... 126
Section 36.14: $$ ........................................................................................................................................................ 126
Section 36.15: $RANDOM .......................................................................................................................................... 126
Section 36.16: $BASHPID ........................................................................................................................................... 127
Section 36.17: $BASH_ENV ....................................................................................................................................... 127
Section 36.18: $BASH_VERSINFO ............................................................................................................................ 127
Section 36.19: $BASH_VERSION .............................................................................................................................. 127
Section 36.20: $EDITOR ............................................................................................................................................ 127
Section 36.21: $HOSTNAME ...................................................................................................................................... 127
Section 36.22: $HOSTTYPE ....................................................................................................................................... 127

Section 36.23: $MACHTYPE ...................................................................................................................................... 128
Section 36.24: $OSTYPE ............................................................................................................................................ 128
Section 36.25: $PATH ................................................................................................................................................ 128
Section 36.26: $PPID .................................................................................................................................................. 128
Section 36.27: $SECONDS ......................................................................................................................................... 128
Section 36.28: $SHELLOPTS ..................................................................................................................................... 128
Section 36.29: $_ ....................................................................................................................................................... 129
Section 36.30: $GROUPS ........................................................................................................................................... 129
Section 36.31: $LINENO ............................................................................................................................................. 129
Section 36.32: $SHLVL .............................................................................................................................................. 129
Section 36.33: $UID .................................................................................................................................................... 130

Chapter 37: Job Control ........................................................................................................................................ 131
Section 37.1: List background processes ................................................................................................................ 131
Section 37.2: Bring a background process to the foreground ............................................................................. 131
Section 37.3: Restart stopped background process .............................................................................................. 131
Section 37.4: Run command in background .......................................................................................................... 131
Section 37.5: Stop a foreground process ................................................................................................................ 131

Chapter 38: Case statement ............................................................................................................................... 132
Section 38.1: Simple case statement ....................................................................................................................... 132
Section 38.2: Case statement with fall through ..................................................................................................... 132
Section 38.3: Fall through only if subsequent pattern(s) match .......................................................................... 132

Chapter 39: Read a file (data stream, variable) line-by-line (and/or field-by-field)? ......... 134
Section 39.1: Looping through a file line by line ..................................................................................................... 134
Section 39.2: Looping through the output of a command field by field ............................................................. 134
Section 39.3: Read lines of a file into an array ....................................................................................................... 134
Section 39.4: Read lines of a string into an array .................................................................................................. 134
Section 39.5: Looping through a string line by line ................................................................................................ 135
Section 39.6: Looping through the output of a command line by line ................................................................ 135
Section 39.7: Read a file field by field ..................................................................................................................... 135
Section 39.8: Read a string field by field ................................................................................................................ 136
Section 39.9: Read fields of a file into an array ..................................................................................................... 136
Section 39.10: Read fields of a string into an array ............................................................................................... 136
Section 39.11: Reads file (/etc/passwd) line by line and field by field ................................................................. 137

Chapter 40: File execution sequence ............................................................................................................. 139
Section 40.1: .profile vs .bash_profile (and .bash_login) ....................................................................................... 139

Chapter 41: Splitting Files .................................................................................................................................... 140
Section 41.1: Split a file ............................................................................................................................................... 140

Chapter 42: File Transfer using scp ................................................................................................................ 141
Section 42.1: scp transferring file ............................................................................................................................. 141
Section 42.2: scp transferring multiple files ........................................................................................................... 141
Section 42.3: Downloading file using scp ................................................................................................................ 141

Chapter 43: Pipelines .............................................................................................................................................. 142
Section 43.1: Using |& ................................................................................................................................................. 142
Section 43.2: Show all processes paginated .......................................................................................................... 143
Section 43.3: Modify continuous output of a command ....................................................................................... 143

Chapter 44: Managing PATH environment variable ............................................................................. 144
Section 44.1: Add a path to the PATH environment variable ................................................................................ 144
Section 44.2: Remove a path from the PATH environment variable .................................................................. 144

Chapter 45: Word splitting .................................................................................................................................. 146

Section 45.1: What, when and Why? ........................................................................................................................ 146
Section 45.2: Bad eects of word splitting ............................................................................................................. 146
Section 45.3: Usefulness of word splitting .............................................................................................................. 147
Section 45.4: Splitting by separator changes ......................................................................................................... 148
Section 45.5: Splitting with IFS .................................................................................................................................. 148
Section 45.6: IFS & word splitting ............................................................................................................................. 148

Chapter 46: Avoiding date using printf ........................................................................................................ 150
Section 46.1: Get the current date ............................................................................................................................ 150
Section 46.2: Set variable to current time .............................................................................................................. 150

Chapter 47: Using "trap" to react to signals and system events ................................................... 151
Section 47.1: Introduction: clean up temporary files .............................................................................................. 151
Section 47.2: Catching SIGINT or Ctl+C ................................................................................................................... 151
Section 47.3: Accumulate a list of trap work to run at exit ................................................................................... 152
Section 47.4: Killing Child Processes on Exit ........................................................................................................... 152
Section 47.5: react on change of terminals window size ...................................................................................... 152

Chapter 48: Chain of commands and operations ................................................................................... 154
Section 48.1: Counting a text pattern ocurrence .................................................................................................... 154
Section 48.2: transfer root cmd output to user file ............................................................................................... 154
Section 48.3: logical chaining of commands with && and || ................................................................................. 154
Section 48.4: serial chaining of commands with semicolon ................................................................................. 154
Section 48.5: chaining commands with | ................................................................................................................. 155

Chapter 49: Type of Shells .................................................................................................................................. 156
Section 49.1: Start an interactive shell ..................................................................................................................... 156
Section 49.2: Detect type of shell ............................................................................................................................ 156
Section 49.3: Introduction to dot files ...................................................................................................................... 156

Chapter 50: Color script output (cross-platform) ................................................................................... 158
Section 50.1: color-output.sh .................................................................................................................................... 158

Chapter 51: co-processes ..................................................................................................................................... 159
Section 51.1: Hello World ........................................................................................................................................... 159

Chapter 52: Typing variables ............................................................................................................................. 160
Section 52.1: declare weakly typed variables ........................................................................................................ 160

Chapter 53: Jobs at specific times .................................................................................................................. 161
Section 53.1: Execute job once at specific time ...................................................................................................... 161
Section 53.2: Doing jobs at specified times repeatedly using systemd.timer .................................................... 161

Chapter 54: Handling the system prompt .................................................................................................. 163
Section 54.1: Using the PROMPT_COMMAND envrionment variable .................................................................. 163
Section 54.2: Using PS2 ............................................................................................................................................ 164
Section 54.3: Using PS3 ............................................................................................................................................. 164
Section 54.4: Using PS4 ............................................................................................................................................ 164
Section 54.5: Using PS1 ............................................................................................................................................. 164

Chapter 55: The cut command .......................................................................................................................... 166
Section 55.1: Only one delimiter character ............................................................................................................. 166
Section 55.2: Repeated delimiters are interpreted as empty fields .................................................................... 166
Section 55.3: No quoting ........................................................................................................................................... 166
Section 55.4: Extracting, not manipulating ............................................................................................................. 166

Chapter 56: Bash on Windows 10 ...................................................................................................................... 168
Section 56.1: Readme ................................................................................................................................................ 168

Chapter 57: Cut Command .................................................................................................................................. 169

Section 57.1: Show the first column of a file ........................................................................................................... 169
Section 57.2: Show columns x to y of a file ............................................................................................................ 169

Chapter 58: global and local variables ......................................................................................................... 170
Section 58.1: Global variables ................................................................................................................................... 170
Section 58.2: Local variables .................................................................................................................................... 170
Section 58.3: Mixing the two together ..................................................................................................................... 170

Chapter 59: CGI Scripts .......................................................................................................................................... 172
Section 59.1: Request Method: GET ......................................................................................................................... 172
Section 59.2: Request Method: POST /w JSON ..................................................................................................... 174

Chapter 60: Select keyword ................................................................................................................................ 176
Section 60.1: Select keyword can be used for getting input argument in a menu format ................................ 176

Chapter 61: When to use eval ............................................................................................................................. 177
Section 61.1: Using Eval ............................................................................................................................................. 177
Section 61.2: Using Eval with Getopt ........................................................................................................................ 178

Chapter 62: Networking With Bash ................................................................................................................. 179
Section 62.1: Networking commands ...................................................................................................................... 179

Chapter 63: Parallel ................................................................................................................................................. 181
Section 63.1: Parallelize repetitive tasks on list of files .......................................................................................... 181
Section 63.2: Parallelize STDIN ................................................................................................................................ 181

Chapter 64: Decoding URL .................................................................................................................................. 183
Section 64.1: Simple example ................................................................................................................................... 183
Section 64.2: Using printf to decode a string ......................................................................................................... 183

Chapter 65: Design Patterns ............................................................................................................................... 184
Section 65.1: The Publish/Subscribe (Pub/Sub) Pattern ....................................................................................... 184

Chapter 66: Pitfalls .................................................................................................................................................. 186
Section 66.1: Whitespace When Assigning Variables ............................................................................................ 186
Section 66.2: Failed commands do not stop script execution .............................................................................. 186
Section 66.3: Missing The Last Line in a File ........................................................................................................... 186

Appendix A: Keyboard shortcuts ...................................................................................................................... 187
Section A.1: Editing Shortcuts ................................................................................................................................... 187
Section A.2: Recall Shortcuts ................................................................................................................................... 187
Section A.3: Macros ................................................................................................................................................... 187
Section A.4: Custome Key Bindings ........................................................................................................................ 187
Section A.5: Job Control ........................................................................................................................................... 188

Credits ............................................................................................................................................................................ 189
You may also like ...................................................................................................................................................... 193

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Bash Notes for Professionals

1

Chapter 1: Getting started with Bash
Version Release Date
0.99
1989-06-08
1.01
1989-06-23
2.0
1996-12-31
2.02
1998-04-20
2.03
1999-02-19
2.04
2001-03-21
2.05b 2002-07-17
3.0
2004-08-03
3.1
2005-12-08
3.2
2006-10-11
4.0
2009-02-20
4.1
2009-12-31
4.2
2011-02-13
4.3
2014-02-26
4.4
2016-09-15

Section 1.1: Hello World
Interactive Shell
The Bash shell is commonly used interactively: It lets you enter and edit commands, then executes them when
you press the Return key. Many Unix-based and Unix-like operating systems use Bash as their default shell
(notably Linux and macOS). The terminal automatically enters an interactive Bash shell process on startup.
Output Hello World by typing the following:
echo "Hello World"
#> Hello World # Output Example

Notes
You can change the shell by just typing the name of the shell in terminal. For example: sh, bash, etc.
echo is a Bash builtin command that writes the arguments it receives to the standard output. It appends a

newline to the output, by default.
Non-Interactive Shell
The Bash shell can also be run non-interactively from a script, making the shell require no human interaction.
Interactive behavior and scripted behavior should be identical – an important design consideration of Unix V7
Bourne shell and transitively Bash. Therefore anything that can be done at the command line can be put in a script
file for reuse.
Follow these steps to create a Hello World script:
1. Create a new file called hello-world.sh
touch hello-world.sh

2. Make the script executable by running chmod +x hello-world.sh
3. Add this code:
Bash Notes for Professionals

2

#!/bin/bash
echo "Hello World"

Line 1: The first line of the script must start with the character sequence #!, referred to as shebang1. The
shebang instructs the operating system to run /bin/bash, the Bash shell, passing it the script's path as an
argument.
E.g. /bin/bash hello-world.sh
Line 2: Uses the echo command to write Hello World to the standard output.

4. Execute the hello-world.sh script from the command line using one of the following:
./hello-world.sh – most commonly used, and recommended
/bin/bash hello-world.sh
bash hello-world.sh – assuming /bin is in your $PATH
sh hello-world.sh

For real production use, you would omit the .sh extension (which is misleading anyway, since this is a Bash script,
not a sh script) and perhaps move the file to a directory within your PATH so that it is available to you regardless of
your current working directory, just like a system command such as cat or ls.
Common mistakes include:
1. Forgetting to apply execute permission on the file, i.e., chmod +x hello-world.sh, resulting in the output of
./hello-world.sh: Permission denied.

2. Editing the script on Windows, which produces incorrect line ending characters that Bash cannot handle.
A common symptom is : command not found where the carriage return has forced the cursor to the
beginning of line, overwriting the text before the colon in the error message.
The script can be fixed using the dos2unix program.
An example use: dos2unix hello-world.sh
dos2unix edits the file inline.

3. Using sh ./hello-world.sh, not realizing that bash and sh are distinct shells with distinct features (though
since Bash is backwards-compatible, the opposite mistake is harmless).
Anyway, simply relying on the script's shebang line is vastly preferable to explicitly writing bash or sh (or
python or perl or awk or ruby or...) before each script's file name.

A common shebang line to use in order to make your script more portable is to use #!/usr/bin/env bash
instead of hard-coding a path to Bash. That way, /usr/bin/env has to exist, but beyond that point, bash just
needs to be on your PATH. On many systems, /bin/bash doesn't exist, and you should use
/usr/local/bin/bash or some other absolute path; this change avoids having to figure out the details of

that.

Bash Notes for Professionals

3

1 Also referred to as sha-bang, hashbang, pound-bang, hash-pling.

Section 1.2: Hello World Using Variables
Create a new file called hello.sh with the following content and give it executable permissions with chmod +x
hello.sh.

Execute/Run via: ./hello.sh

#!/usr/bin/env bash
# Note that spaces cannot be used around the `=` assignment operator
whom_variable="World"
# Use printf to safely output the data
printf "Hello, %s\n" "$whom_variable"
#> Hello, World

This will print Hello, World to standard output when executed.
To tell bash where the script is you need to be very specific, by pointing it to the containing directory, normally with
./ if it is your working directory, where . is an alias to the current directory. If you do not specify the directory, bash

tries to locate the script in one of the directories contained in the $PATH environment variable.
The following code accepts an argument $1, which is the first command line argument, and outputs it in a
formatted string, following Hello,.
Execute/Run via: ./hello.sh World

#!/usr/bin/env bash
printf "Hello, %s\n" "$1"
#> Hello, World

It is important to note that $1 has to be quoted in double quote, not single quote. "$1" expands to the first
command line argument, as desired, while '$1' evaluates to literal string $1.
Security Note:
Read Security implications of forgetting to quote a variable in bash shells to understand the
importance of placing the variable text within double quotes.

Section 1.3: Hello World with User Input
The following will prompt a user for input, and then store that input as a string (text) in a variable. The variable is
then used to give a message to the user.
#!/usr/bin/env bash
echo "Who are you?"
read name
echo "Hello, $name."

The command read here reads one line of data from standard input into the variable name. This is then referenced
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4

using $name and printed to standard out using echo.
Example output:
$ ./hello_world.sh
Who are you?
Matt
Hello, Matt.

Here the user entered the name "Matt", and this code was used to say Hello, Matt..
And if you want to append something to the variable value while printing it, use curly brackets around the variable
name as shown in the following example:
#!/usr/bin/env bash
echo "What are you doing?"
read action
echo "You are ${action}ing."

Example output:
$ ./hello_world.sh
What are you doing?
Sleep
You are Sleeping.

Here when user enters an action, "ing" is appended to that action while printing.

Section 1.4: Importance of Quoting in Strings
Quoting is important for string expansion in bash. With these, you can control how the bash parses and expands
your strings.
There are two types of quoting:
Weak: uses double quotes: "
Strong: uses single quotes: '
If you want to bash to expand your argument, you can use Weak Quoting:
#!/usr/bin/env bash
world="World"
echo "Hello $world"
#> Hello World

If you don't want to bash to expand your argument, you can use Strong Quoting:
#!/usr/bin/env bash
world="World"
echo 'Hello $world'
#> Hello $world

You can also use escape to prevent expansion:
#!/usr/bin/env bash
world="World"

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echo "Hello \$world"
#> Hello $world

For more detailed information other than beginner details, you can continue to read it here.

Section 1.5: Viewing information for Bash built-ins
help <command>

This will display the Bash help (manual) page for the specified built-in.
For example, help unset will show:
unset: unset [-f] [-v] [-n] [name ...]
Unset values and attributes of shell variables and functions.
For each NAME, remove the corresponding variable or function.
Options:
-f
treat
-v
treat
-n
treat
rather than

each NAME as
each NAME as
each NAME as
the variable

a shell function
a shell variable
a name reference and unset the variable itself
it references

Without options, unset first tries to unset a variable, and if that fails,
tries to unset a function.
Some variables cannot be unset; also see `readonly'.
Exit Status:
Returns success unless an invalid option is given or a NAME is read-only.

To see a list of all built-ins with a short description, use
help -d

Section 1.6: Hello World in "Debug" mode
$ cat hello.sh
#!/bin/bash
echo "Hello World"
$ bash -x hello.sh
+ echo Hello World
Hello World

The -x argument enables you to walk through each line in the script. One good example is here:
$ cat hello.sh
#!/bin/bash
echo "Hello World\n"
adding_string_to_number="s"
v=$(expr 5 + $adding_string_to_number)
$ ./hello.sh
Hello World

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expr: non-integer argument

The above prompted error is not enough to trace the script; however, using the following way gives you a better
sense where to look for the error in the script.
$ bash -x hello.sh
+ echo Hello World\n
Hello World
+ adding_string_to_number=s
+ expr 5 + s
expr: non-integer argument
+ v=

Section 1.7: Handling Named Arguments
#!/bin/bash
deploy=false
uglify=false
while (( $# > 1 )); do case $1 in
--deploy) deploy="$2";;
--uglify) uglify="$2";;
*) break;
esac; shift 2
done
$deploy && echo "will deploy... deploy = $deploy"
$uglify && echo "will uglify... uglify = $uglify"
# how to run
# chmod +x script.sh
# ./script.sh --deploy true --uglify false

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Chapter 2: Script shebang
Section 2.1: Env shebang
To execute a script file with the bash executable found in the PATH environment variable by using the executable
env, the first line of a script file must indicate the absolute path to the env executable with the argument bash:
#!/usr/bin/env bash

The env path in the shebang is resolved and used only if a script is directly launch like this:
script.sh

The script must have execution permission.
The shebang is ignored when a bash interpreter is explicitly indicated to execute a script:
bash script.sh

Section 2.2: Direct shebang
To execute a script file with the bash interpreter, the first line of a script file must indicate the absolute path to the
bash executable to use:
#!/bin/bash

The bash path in the shebang is resolved and used only if a script is directly launch like this:
./script.sh

The script must have execution permission.
The shebang is ignored when a bash interpreter is explicitly indicated to execute a script:
bash script.sh

Section 2.3: Other shebangs
There are two kinds of programs the kernel knows of. A binary program is identified by it's ELF
(ExtenableLoadableFormat) header, which is usually produced by a compiler. The second one are scripts of any
kind.
If a file starts in the very first line with the sequence #! then the next string has to be a pathname of an interpreter.
If the kernel reads this line, it calls the interpreter named by this pathname and gives all of the following words in
this line as arguments to the interpreter. If there is no file named "something" or "wrong":
#!/bin/bash something wrong
echo "This line never gets printed"

bash tries to execute its argument "something wrong" which doesn't exist. The name of the script file is added too.
To see this clearly use an echo shebang:

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#"/bin/echo something wrong
# and now call this script named "thisscript" like so:
# thisscript one two
# the output will be:
something wrong ./thisscript one two

Some programs like awk use this technique to run longer scripts residing in a disk file.

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Chapter 3: Navigating directories
Section 3.1: Absolute vs relative directories
To change to an absolutely specified directory, use the entire name, starting with a slash /, thus:
cd /home/username/project/abc

If you want to change to a directory near your current on, you can specify a relative location. For example, if you are
already in /home/username/project, you can enter the subdirectory abc thus:
cd abc

If you want to go to the directory above the current directory, you can use the alias ... For example, if you were in
/home/username/project/abc and wanted to go to /home/username/project, then you would do the following:
cd ..

This may also be called going "up" a directory.

Section 3.2: Change to the last directory
For the current shell, this takes you to the previous directory that you were in, no matter where it was.
cd -

Doing it multiple times effectively "toggles" you being in the current directory or the previous one.

Section 3.3: Change to the home directory
The default directory is the home directory ($HOME, typically /home/username), so cd without any directory takes you
there
cd

Or you could be more explicit:
cd $HOME

A shortcut for the home directory is ~, so that could be used as well.
cd ~

Section 3.4: Change to the Directory of the Script
In general, there are two types of Bash scripts:
1. System tools which operate from the current working directory
2. Project tools which modify files relative to their own place in the files system
For the second type of scripts, it is useful to change to the directory where the script is stored. This can be done
with the following command:
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cd "$(dirname "$(readlink -f "$0")")"

This command runs 3 commands:
1. readlink -f "$0" determines the path to the current script ($0)
2. dirname converts the path to script to the path to its directory
3. cd changes the current work directory to the directory it receives from dirname

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Chapter 4: Listing Files
Option

Description
List all entries including ones that start with a dot
-A, --almost-all
List all entries excluding . and ..
-c
Sort files by change time
-d, --directory
List directory entries
-h, --human-readable Show sizes in human readable format (i.e. K, M)
-H
Same as above only with powers of 1000 instead of 1024
-l
Show contents in long-listing format
-o
Long -listing format without group info
-r, --reverse
Show contents in reverse order
-s, --size
Print size of each file in blocks
-S
Sort by file size
--sort=WORD
Sort contents by a word. (i.e size, version, status)
-t
Sort by modification time
-u
Sort by last access time
-v
Sort by version
-1
List one file per line
-a, --all

Section 4.1: List Files in a Long Listing Format
The ls command's -l option prints a specified directory's contents in a long listing format. If no directory is
specified then, by default, the contents of the current directory are listed.
ls -l /etc

Example Output:
total 1204
drwxr-xr-x
-rw-r--r-drwxr-xr-x
...

3 root root
1 root root
2 root root

4096 Apr 21 03:44 acpi
3028 Apr 21 03:38 adduser.conf
4096 Jun 11 20:42 alternatives

The output first displays total, which indicates the total size in blocks of all the files in the listed directory. It then
displays eight columns of information for each file in the listed directory. Below are the details for each column in
the output:
Column No. Example
Description
d
1.1
File type (see table below)
rwxr-xr-x
1.2
Permission string
3
2
Number of hard links
root
3
Owner name
root
4
Owner group
4096
5
File size in bytes
Apr 21 03:44 Modification time
6
acpi
7
File name
File Type
The file type can be one of any of the following characters.
Character

File Type

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b
c
C
d
D
l
M
n
p
P
s
?

Regular file
Block special file
Character special file
High performance ("contiguous data") file
Directory
Door (special IPC file in Solaris 2.5+ only)
Symbolic link
Off-line ("migrated") file (Cray DMF)
Network special file (HP-UX)
FIFO (named pipe)
Port (special system file in Solaris 10+ only)
Socket
Some other file type

Section 4.2: List the Ten Most Recently Modified Files
The following will list up to ten of the most recently modified files in the current directory, using a long listing
format (-l) and sorted by time (-t).
ls -lt | head

Section 4.3: List All Files Including Dotfiles
A dotfile is a file whose names begin with a .. These are normally hidden by ls and not listed unless requested.
For example the following output of ls:
$ ls
bin pki

The -a or --all option will list all files, including dotfiles.
$ ls -a
.
.ansible
.. .bash_history

.bash_logout
.bash_profile

.bashrc
bin

.lesshst
pki

.puppetlabs
.ssh

.viminfo

The -A or --almost-all option will list all files, including dotfiles, but does not list implied . and ... Note that . is
the current directory and .. is the parent directory.
$ ls -A
.ansible
.bash_history

.bash_logout
.bash_profile

.bashrc
bin

.lesshst
pki

.puppetlabs
.ssh

.viminfo

Section 4.4: List Files Without Using `ls`
Use the Bash shell's filename expansion and brace expansion capabilities to obtain the filenames:
# display the files and directories that are in the current directory
printf "%s\n" *
# display only the directories in the current directory
printf "%s\n" */
# display only (some) image files

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printf "%s\n" *.{gif,jpg,png}

To capture a list of files into a variable for processing, it is typically good practice to use a bash array:
files=( * )
# iterate over them
for file in "${files[@]}"; do
echo "$file"
done

Section 4.5: List Files
The ls command lists the contents of a specified directory, excluding dotfiles. If no directory is specified then, by
default, the contents of the current directory are listed.
Listed files are sorted alphabetically, by default, and aligned in columns if they don’t fit on one line.
$ ls
apt configs
bin Desktop

Documents
eclipse

Fonts
git

Music
Pictures

Programming
Public

Templates
Videos

workspace

Section 4.6: List Files in a Tree-Like Format
The tree command lists the contents of a specified directory in a tree-like format. If no directory is specified then,
by default, the contents of the current directory are listed.
Example Output:
$ tree /tmp
/tmp
├── 5037
├── adb.log
└── evince-20965
└── image.FPWTJY.png

Use the tree command's -L option to limit the display depth and the -d option to only list directories.
Example Output:
$ tree -L 1 -d /tmp
/tmp
└── evince-20965

Section 4.7: List Files Sorted by Size
The ls command's -S option sorts the files in descending order of file size.
$ ls -l -S
total 444
-rw-rw-rw-rw-rw-rw-rw-rw-rw-

./Fruits
1 root root 295303 Jul 28 19:19 apples.jpg
1 root root 102283 Jul 28 19:19 kiwis.jpg
1 root root 50197 Jul 28 19:19 bananas.jpg

When used with the -r option the sort order is reversed.

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$ ls -l -S
total 444
-rw-rw-rw-rw-rw-rw-rw-rw-rw-

-r /Fruits
1 root root 50197 Jul 28 19:19 bananas.jpg
1 root root 102283 Jul 28 19:19 kiwis.jpg
1 root root 295303 Jul 28 19:19 apples.jpg

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Chapter 5: Using cat
Option
Details
-n
Print line numbers
-v
Show non-printing characters using ^ and M- notation except LFD and TAB
-T
Show TAB characters as ^I
-E
Show linefeed(LF) characters as $
-e
Same as -vE
-b
Number nonempty output lines, overrides -n
-A
equivalent to -vET
-s
suppress repeated empty output lines, s refers to squeeze

Section 5.1: Concatenate files
This is the primary purpose of cat.
cat file1 file2 file3 > file_all
cat can also be used similarly to concatenate files as part of a pipeline, e.g.
cat file1 file2 file3 | grep foo

Section 5.2: Printing the Contents of a File
cat file.txt

will print the contents of a file.
If the file contains non-ASCII characters, you can display those characters symbolically with cat -v. This can be
quite useful for situations where control characters would otherwise be invisible.
cat -v unicode.txt

Very often, for interactive use, you are better off using an interactive pager like less or more, though. (less is far
more powerful than more and it is advised to use less more often than more.)
less file.txt

To pass the contents of a file as input to a command. An approach usually seen as better (UUOC) is to use
redirection.
tr A-Z a-z <file.txt

# as an alternative to cat file.txt | tr A-Z a-z

In case the content needs to be listed backwards from its end the command tac can be used:
tac file.txt

If you want to print the contents with line numbers, then use -n with cat:
cat -n file.txt

To display the contents of a file in a completely unambiguous byte-by-byte form, a hex dump is the standard
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solution. This is good for very brief snippets of a file, such as when you don't know the precise encoding. The
standard hex dump utility is od -cH, though the representation is slightly cumbersome; common replacements
include xxd and hexdump.
$ printf 'Hëllö wörld' | xxd
0000000: 48c3 ab6c 6cc3 b620 77c3 b672 6c64

H..ll.. w..rld

Section 5.3: Write to a file
cat >file

It will let you write the text on terminal which will be saved in a file named file.
cat >>file

will do the same, except it will append the text to the end of the file.
N.B: Ctrl+D to end writing text on terminal (Linux)
A here document can be used to inline the contents of a file into a command line or a script:
cat <<END >file
Hello, World.
END

The token after the << redirection symbol is an arbitrary string which needs to occur alone on a line (with no leading
or trailing whitespace) to indicate the end of the here document. You can add quoting to prevent the shell from
performing command substitution and variable interpolation:
cat <<'fnord'
Nothing in `here` will be $changed
fnord

(Without the quotes, here would be executed as a command, and $changed would be substituted with the value of
the variable changed -- or nothing, if it was undefined.)

Section 5.4: Show non printable characters
This is useful to see if there are any non-printable characters, or non-ASCII characters.
e.g. If you have copy-pasted the code from web, you may have quotes like ” instead of standard ".
$ cat -v file.txt
$ cat -vE file.txt # Useful in detecting trailing spaces.

e.g.
$ echo '”
M-bM-^@M-^]

' | cat -vE # echo | will be replaced by actual file.
$

You may also want to use cat -A (A for All) that is equivalent to cat -vET. It will display TAB characters (displayed
as ^I), non printable characters and end of each line:
$ echo '” `' | cat -A

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M-bM-^@M-^]^I`$

Section 5.5: Read from standard input
cat < file.txt

Output is same as cat file.txt, but it reads the contents of the file from standard input instead of directly from
the file.
printf "first line\nSecond line\n" | cat -n

The echo command before | outputs two lines. The cat command acts on the output to add line numbers.

Section 5.6: Display line numbers with output
Use the --number flag to print line numbers before each line. Alternatively, -n does the same thing.
$ cat --number file
1
2
3
4
5

line 1
line 2
line 4
line 5

To skip empty lines when counting lines, use the --number-nonblank, or simply -b.
$ cat -b file
1
2

line 1
line 2

3
4

line 4
line 5

Section 5.7: Concatenate gzipped files
Files compressed by gzip can be directly concatenated into larger gzipped files.
cat file1.gz file2.gz file3.gz > combined.gz

This is a property of gzip that is less efficient than concatenating the input files and gzipping the result:
cat file1 file2 file3 | gzip > combined.gz

A complete demonstration:
echo
echo
gzip
gzip

'Hello world!' > hello.txt
'Howdy world!' > howdy.txt
hello.txt
howdy.txt

cat hello.txt.gz howdy.txt.gz > greetings.txt.gz
gunzip greetings.txt.gz

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cat greetings.txt

Which results in
Hello world!
Howdy world!

Notice that greetings.txt.gz is a single file and is decompressed as the single file greeting.txt. Contrast this
with tar -czf hello.txt howdy.txt > greetings.tar.gz, which keeps the files separate inside the tarball.

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Chapter 6: Grep
Section 6.1: How to search a file for a pattern
To find the word foo in the file bar :
grep foo ~/Desktop/bar

To find all lines that do not contain foo in the file bar :
grep –v foo ~/Desktop/bar

To use find all words containing foo in the end (WIldcard Expansion):
grep "*foo" ~/Desktop/bar

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Chapter 7: Aliasing
Shell aliases are a simple way to create new commands or to wrap existing commands with code of your own. They
somewhat overlap with shell functions, which are however more versatile and should therefore often be preferred.

Section 7.1: Bypass an alias
Sometimes you may want to bypass an alias temporarily, without disabling it. To work with a concrete example,
consider this alias:
alias ls='ls --color=auto'

And let's say you want to use the ls command without disabling the alias. You have several options:
Use the command builtin: command ls
Use the full path of the command: /bin/ls
Add a \ anywhere in the command name, for example: \ls, or l\s
Quote the command: "ls" or 'ls'

Section 7.2: Create an Alias
alias word='command'

Invoking word will run command. Any arguments supplied to the alias are simply appended to the target of the alias:
alias myAlias='some command --with --options'
myAlias foo bar baz

The shell will then execute:
some command --with --options foo bar baz

To include multiple commands in the same alias, you can string them together with &&. For example:
alias print_things='echo "foo" && echo "bar" && echo "baz"'

Section 7.3: Remove an alias
To remove an existing alias, use:
unalias {alias_name}

Example:
# create an alias
$ alias now='date'
# preview the alias
$ now
Thu Jul 21 17:11:25 CEST 2016
# remove the alias
$ unalias now

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# test if removed
$ now
-bash: now: command not found

Section 7.4: The BASH_ALIASES is an internal bash assoc
array
Aliases are named shortcuts of commands, one can define and use in interactive bash instances. They are held in
an associative array named BASH_ALIASES. To use this var in a script, it must be run within an interactive shell
#!/bin/bash -li
# note the -li above! -l makes this behave like a login shell
# -i makes it behave like an interactive shell
#
# shopt -s expand_aliases will not work in most cases
echo There are ${#BASH_ALIASES[*]} aliases defined.
for ali in "${!BASH_ALIASES[@]}"; do
printf "alias: %-10s triggers: %s\n" "$ali" "${BASH_ALIASES[$ali]}"
done

Section 7.5: Expand alias
Assuming that bar is an alias for someCommand -flag1.
Type bar on the command line and then press Ctrl + alt + e
you'll get someCommand -flag1 where bar was standing.

Section 7.6: List all Aliases
alias -p

will list all the current aliases.

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Chapter 8: Jobs and Processes
Section 8.1: Job handling
Creating jobs
To create an job, just append a single & after the command:
$ sleep 10 &
[1] 20024

You can also make a running process a job by pressing Ctrl + z :
$ sleep 10
^Z
[1]+ Stopped

sleep 10

Background and foreground a process
To bring the Process to the foreground, the command fg is used together with %
$ sleep 10 &
[1] 20024
$ fg %1
sleep 10

Now you can interact with the process. To bring it back to the background you can use the bg command. Due to the
occupied terminal session, you need to stop the process first by pressing Ctrl + z .
$ sleep 10
^Z
[1]+ Stopped

sleep 10

$ bg %1
[1]+ sleep 10 &

Due to the laziness of some Programmers, all these commands also work with a single % if there is only one
process, or for the first process in the list. For Example:
$ sleep 10 &
[1] 20024
$ fg %
sleep 10

# to bring a process to foreground 'fg %' is also working.

or just
$ %
sleep 10

# laziness knows no boundaries, '%' is also working.

Additionally, just typing fg or bg without any argument handles the last job:
$ sleep 20 &
$ sleep 10 &
$ fg

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sleep 10
^C
$ fg
sleep 20

Killing running jobs
$ sleep 10 &
[1] 20024
$ kill %1
[1]+ Terminated

sleep 10

The sleep process runs in the background with process id (pid) 20024 and job number 1. In order to reference the
process, you can use either the pid or the job number. If you use the job number, you must prefix it with %. The
default kill signal sent by kill is SIGTERM, which allows the target process to exit gracefully.
Some common kill signals are shown below. To see a full list, run kill -l.
Signal name Signal value

Effect

Hangup
Interrupt from keyboard
SIGKILL
9
Kill signal
SIGTERM
15
Termination signal
Start and kill specific processes
SIGHUP

1

SIGINT

2

Probably the easiest way of killing a running process is by selecting it through the process name as in the following
example using pkill command as
pkill -f test.py

(or) a more fool-proof way using pgrep to search for the actual process-id
kill $(pgrep -f 'python test.py')

The same result can be obtained using grep over ps -ef | grep name_of_process then killing the process
associated with the resulting pid (process id). Selecting a process using its name is convinient in a testing
environment but can be really dangerous when the script is used in production: it is virtually impossible to be sure
that the name will match the process you actually want to kill. In those cases, the following approach is actually
much safe.
Start the script that will eventually killed with the following approach. Let's assume that the command you want to
execute and eventually kill is python test.py.
#!/bin/bash
if [[ ! -e /tmp/test.py.pid ]]; then
# Check if the file already exists
python test.py &
#+and if so do not run another process.
echo $! > /tmp/test.py.pid
else
echo -n "ERROR: The process is already running with pid "
cat /tmp/test.py.pid
echo
fi

This will create a file in the /tmp directory containing the pid of the python test.py process. If the file already
exists, we assume that the command is already running and the script return an error.

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24

Then, when you want to kill it use the following script:
#!/bin/bash
if [[ -e /tmp/test.py.pid ]]; then
kill `cat /tmp/test.py.pid`
rm /tmp/test.py.pid
else
echo "test.py is not running"
fi

# If the file do not exists, then the
#+the process is not running. Useless
#+trying to kill it.

that will kill exactly the process associated with your command, without relying on any volatile information (like the
string used to run the command). Even in this case if the file does not exist, the script assume that you want to kill a
non-running process.
This last example can be easily improved for running the same command multiple times (appending to the pid file
instead of overwriting it, for example) and to manage cases where the process dies before being killed.

Section 8.2: Check which process running on specific port
To check which process running on port 8080
lsof -i :8080

Section 8.3: Disowning background job
$ gzip extremelylargefile.txt &
$ bg
$ disown %1

This allows a long running process to continue once your shell (terminal, ssh, etc) is closed.

Section 8.4: List Current Jobs
$ tail -f /var/log/syslog > log.txt
[1]+ Stopped
tail -f /var/log/syslog > log.txt
$ sleep 10 &
$ jobs
[1]+ Stopped
[2]- Running

tail -f /var/log/syslog > log.txt
sleep 10 &

Section 8.5: Finding information about a running process
ps aux | grep <search-term> shows processes matching search-term

Example:
root@server7:~#
root
315
/usr/sbin/nginx
www-data 5647
www-data 5648
root
13134
root@server7:~#

ps aux | grep nginx
0.0 0.3 144392 1020 ?

Ss

May28

0:00 nginx: master process

0.0
0.0
0.0

S
S
S+

Jul18
Jul18
14:33

2:53 nginx: worker process
0:00 nginx: cache manager process
0:00 grep --color=auto nginx

1.1 145124
0.1 144392
0.3
4960

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3048 ?
376 ?
920 pts/0

25

Here, second column is the process id. For example, if you want to kill the nginx process, you can use the command
kill 5647. It is always adviced to use the kill command with SIGTERM rather than SIGKILL.

Section 8.6: List all processes
There are two common ways to list all processes on a system. Both list all processes running by all users, though
they differ in the format they output (the reason for the differences are historical).
ps -ef
ps aux

# lists all processes
# lists all processes in alternative format (BSD)

This can be used to check if a given application is running. For example, to check if the SSH server (sshd) is running:
ps -ef | grep sshd

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Chapter 9: Redirection
Parameter
Details
internal file descriptor
An integer.
One of >, < or <>
direction
external file descriptor or path & followed by an integer for file descriptor or a path.

Section 9.1: Redirecting standard output
> redirect the standard output (aka STDOUT) of the current command into a file or another descriptor.

These examples write the output of the ls command into the file file.txt
ls >file.txt
> file.txt ls

The target file is created if it doesn't exists, otherwise this file is truncated.
The default redirection descriptor is the standard output or 1 when none is specified. This command is equivalent
to the previous examples with the standard output explicitly indicated:
ls 1>file.txt

Note: the redirection is initialized by the executed shell and not by the executed command, therefore it is done
before the command execution.

Section 9.2: Append vs Truncate
Truncate >
1. Create specified file if it does not exist.
2. Truncate (remove file's content)
3. Write to file
$ echo "first line" > /tmp/lines
$ echo "second line" > /tmp/lines
$ cat /tmp/lines
second line

Append >>
1. Create specified file if it does not exist.
2. Append file (writing at end of file).
# Overwrite existing file
$ echo "first line" > /tmp/lines
# Append a second line
$ echo "second line" >> /tmp/lines
$ cat /tmp/lines
first line
second line

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Section 9.3: Redirecting both STDOUT and STDERR
File descriptors like 0 and 1 are pointers. We change what file descriptors point to with redirection. >/dev/null
means 1 points to /dev/null.
First we point 1 (STDOUT) to /dev/null then point 2 (STDERR) to whatever 1 points to.
# STDERR is redirect to STDOUT: redirected to /dev/null,
# effectually redirecting both STDERR and STDOUT to /dev/null
echo 'hello' > /dev/null 2>&1
Version ≥ 4.0

This can be further shortened to the following:
echo 'hello' &> /dev/null

However, this form may be undesirable in production if shell compatibility is a concern as it conflicts with POSIX,
introduces parsing ambiguity, and shells without this feature will misinterpret it:
# Actual code
echo 'hello' &> /dev/null
echo 'hello' &> /dev/null 'goodbye'
# Desired behavior
echo 'hello' > /dev/null 2>&1
echo 'hello' 'goodbye' > /dev/null 2>&1
# Actual behavior
echo 'hello' &
echo 'hello' & goodbye > /dev/null

NOTE: &> is known to work as desired in both Bash and Zsh.

Section 9.4: Using named pipes
Sometimes you may want to output something by one program and input it into another program, but can't use a
standard pipe.
ls -l | grep ".log"

You could simply write to a temporary file:
touch tempFile.txt
ls -l > tempFile.txt
grep ".log" < tempFile.txt

This works fine for most applications, however, nobody will know what tempFile does and someone might remove
it if it contains the output of ls -l in that directory. This is where a named pipe comes into play:
mkfifo myPipe
ls -l > myPipe
grep ".log" < myPipe
myPipe is technically a file (everything is in Linux), so let's do ls -l in an empty directory that we just created a pipe

in:
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mkdir pipeFolder
cd pipeFolder
mkfifo myPipe
ls -l

The output is:
prw-r--r-- 1 root root 0 Jul 25 11:20 myPipe

Notice the first character in the permissions, it's listed as a pipe, not a file.
Now let's do something cool.
Open one terminal, and make note of the directory (or create one so that cleanup is easy), and make a pipe.
mkfifo myPipe

Now let's put something in the pipe.
echo "Hello from the other side" > myPipe

You'll notice this hangs, the other side of the pipe is still closed. Let's open up the other side of the pipe and let that
stuff through.
Open another terminal and go to the directory that the pipe is in (or if you know it, prepend it to the pipe):
cat < myPipe

You'll notice that after hello from the other side is output, the program in the first terminal finishes, as does
that in the second terminal.
Now run the commands in reverse. Start with cat < myPipe and then echo something into it. It still works, because
a program will wait until something is put into the pipe before terminating, because it knows it has to get
something.
Named pipes can be useful for moving information between terminals or between programs.
Pipes are small. Once full, the writer blocks until some reader reads the contents, so you need to either run the
reader and writer in different terminals or run one or the other in the background:
ls -l /tmp > myPipe &
cat < myPipe

More examples using named pipes:
Example 1 - all commands on the same terminal / same shell
$ { ls -l && cat file3; } >mypipe &
$ cat <mypipe
# Output: Prints ls -l data and then prints file3 contents on screen

Example 2 - all commands on the same terminal / same shell
$ ls -l >mypipe &
$ cat file3 >mypipe &

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$ cat <mypipe
#Output: This prints on screen the contents of mypipe.

Mind that first contents of file3 are displayed and then the ls -l data is displayed (LIFO configuration).
Example 3 - all commands on the same terminal / same shell
$ { pipedata=$(<mypipe) && echo "$pipedata"; } &
$ ls >mypipe
# Output: Prints the output of ls directly on screen

Mind that the variable $pipedata is not available for usage in the main terminal / main shell since the use of
& invokes a subshell and $pipedata was only available in this subshell.

Example 4 - all commands on the same terminal / same shell
$ export pipedata
$ pipedata=$(<mypipe) &
$ ls -l *.sh >mypipe
$ echo "$pipedata"
#Output : Prints correctly the contents of mypipe

This prints correctly the value of $pipedata variable in the main shell due to the export declaration of the
variable. The main terminal/main shell is not hanging due to the invocation of a background shell (&).

Section 9.5: Redirection to network addresses
Version ≥ 2.04

Bash treats some paths as special and can do some network communication by writing to
/dev/{udp|tcp}/host/port. Bash cannot setup a listening server, but can initiate a connection, and for TCP can

read the results at least.
For example, to send a simple web request one could do:
exec 3</dev/tcp/www.google.com/80
printf 'GET / HTTP/1.0\r\n\r\n' >&3
cat <&3

and the results of www.google.com's default web page will be printed to stdout.
Similarly
printf 'HI\n' >/dev/udp/192.168.1.1/6666

would send a UDP message containing HI\n to a listener on 192.168.1.1:6666

Section 9.6: Print error messages to stderr
Error messages are generally included in a script for debugging purposes or for providing rich user experience.
Simply writing error message like this:

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30

cmd || echo 'cmd failed'

may work for simple cases but it's not the usual way. In this example, the error message will pollute the actual
output of the script by mixing both errors and successful output in stdout.
In short, error message should go to stderr not stdout. It's pretty simple:
cmd || echo 'cmd failed' >/dev/stderr

Another example:
if cmd; then
echo 'success'
else
echo 'cmd failed' >/dev/stderr
fi

In the above example, the success message will be printed on stdout while the error message will be printed on
stderr.

A better way to print error message is to define a function:
err(){
echo "E: $*" >>/dev/stderr
}

Now, when you have to print an error:
err "My error message"

Section 9.7: Redirecting multiple commands to the same file
{
echo "contents of home directory"
ls ~
} > output.txt

Section 9.8: Redirecting STDIN
< reads from its right argument and writes to its left argument.

To write a file into STDIN we should read /tmp/a_file and write into STDIN i.e 0</tmp/a_file
Note: Internal file descriptor defaults to 0 (STDIN) for <
$
$
$
$
a
b
c

echo
echo
echo
sort

"b" > /tmp/list.txt
"a" >> /tmp/list.txt
"c" >> /tmp/list.txt
< /tmp/list.txt

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Section 9.9: Redirecting STDERR
2 is STDERR.
$ echo_to_stderr 2>/dev/null # echos nothing

Definitions:
echo_to_stderr is a command that writes "stderr" to STDERR
echo_to_stderr () {
echo stderr >&2
}
$ echo_to_stderr
stderr

Section 9.10: STDIN, STDOUT and STDERR explained
Commands have one input (STDIN) and two kinds of outputs, standard output (STDOUT) and standard error
(STDERR).
For example:
STDIN
root@server~# read
Type some text here

Standard input is used to provide input to a program. (Here we're using the read builtin to read a line from STDIN.)
STDOUT
root@server~# ls file
file

Standard output is generally used for "normal" output from a command. For example, ls lists files, so the files are
sent to STDOUT.
STDERR
root@server~# ls anotherfile
ls: cannot access 'anotherfile': No such file or directory

Standard error is (as the name implies) used for error messages. Because this message is not a list of files, it is sent
to STDERR.
STDIN, STDOUT and STDERR are the three standard streams. They are identified to the shell by a number rather
than a name:
0 = Standard in
1 = Standard out
2 = Standard error
By default, STDIN is attached to the keyboard, and both STDOUT and STDERR appear in the terminal. However, we
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32

can redirect either STDOUT or STDERR to whatever we need. For example, let's say that you only need the standard
out and all error messages printed on standard error should be suppressed. That's when we use the descriptors 1
and 2.
Redirecting STDERR to /dev/null
Taking the previous example,
root@server~# ls anotherfile 2>/dev/null
root@server~#

In this case, if there is any STDERR, it will be redirected to /dev/null (a special file which ignores anything put into it),
so you won't get any error output on the shell.

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33

Chapter 10: Control Structures
Parameter to [ or test
Details
File Operators
Details
-e "$file"
Returns true if the file exists.
-d "$file"
Returns true if the file exists and is a directory
-f "$file"
Returns true if the file exists and is a regular file
-h "$file"
Returns true if the file exists and is a symbolic link
String Comparators
Details
-z "$str"
True if length of string is zero
-n "$str
True if length of string is non-zero
True if string $str is equal to string $str2. Not best for integers. It may work but will be
"$str" = "$str2"
inconsitent
"$str" != "$str2"
True if the strings are not equal
Integer Comparators
Details
"$int1" -eq "$int2"
True if the integers are equal
"$int1" -ne "$int2"
True if the integers are not equals
"$int1" -gt "$int2"
True if int1 is greater than int 2
"$int1" -ge "$int2"
True if int1 is greater than or equal to int2
"$int1" -lt "$int2"
True if int1 is less than int 2
"$int1" -le "$int2"
True if int1 is less than or equal to int2

Section 10.1: Conditional execution of command lists
How to use conditional execution of command lists
Any builtin command, expression, or function, as well as any external command or script can be executed
conditionally using the &&(and) and ||(or) operators.
For example, this will only print the current directory if the cd command was successful.
cd my_directory && pwd

Likewise, this will exit if the cd command fails, preventing catastrophe:
cd my_directory || exit
rm -rf *

When combining multiple statements in this manner, it's important to remember that (unlike many C-style
languages) these operators have no precedence and are left-associative.
Thus, this statement will work as expected...
cd my_directory && pwd || echo "No such directory"

If the cd succeeds, the && pwd executes and the current working directory name is printed. Unless pwd fails (a
rarity) the || echo ... will not be executed.
If the cd fails, the && pwd will be skipped and the || echo ... will run.
But this will not (if you're thinking if...then...else)...
cd my_directory && ls || echo "No such directory"

If the cd fails, the && ls is skipped and the || echo ... is executed.
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If the cd succeeds, the && ls is executed.
If the ls succeeds, the || echo ... is ignored. (so far so good)
BUT... if the ls fails, the || echo ... will also be executed.
It is the ls, not the cd, that is the previous command.

Why use conditional execution of command lists
Conditional execution is a hair faster than if...then but its main advantage is allowing functions and scripts to exit
early, or "short circuit".
Unlike many languages like C where memory is explicitly allocated for structs and variables and such (and thus
must be deallocated), bash handles this under the covers. In most cases, we don't have to clean up anything before
leaving the function. A return statement will deallocate everything local to the function and pickup execution at the
return address on the stack.
Returning from functions or exiting scripts as soon as possible can thus significantly improve performance and
reduce system load by avoiding the unnecessary execution of code. For example...
my_function () {
### ALWAYS CHECK THE RETURN CODE
# one argument required. "" evaluates to false(1)
[[ "$1" ]]
|| return 1
# work with the argument. exit on failure
do_something_with "$1" || return 1
do_something_else
|| return 1
# Success! no failures detected, or we wouldn't be here
return 0
}

Section 10.2: If statement
if [[ $1 -eq 1 ]]; then
echo "1 was passed in the first parameter"
elif [[ $1 -gt 2 ]]; then
echo "2 was not passed in the first parameter"
else
echo "The first parameter was not 1 and is not more than 2."
fi

The closing fi is necessary, but the elif and/or the else clauses can be omitted.
The semicolons before then are standard syntax for combining two commands on a single line; they can be omitted
only if then is moved to the next line.
It's important to understand that the brackets [[ are not part of the syntax, but are treated as a command; it is the
exit code from this command that is being tested. Therefore, you must always include spaces around the brackets.
This also means that the result of any command can be tested. If the exit code from the command is a zero, the
statement is considered true.
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35

if grep "foo" bar.txt; then
echo "foo was found"
else
echo "foo was not found"
fi

Mathematical expressions, when placed inside double parentheses, also return 0 or 1 in the same way, and can
also be tested:
if (( $1 + 5 > 91 )); then
echo "$1 is greater than 86"
fi

You may also come across if statements with single brackets. These are defined in the POSIX standard and are
guaranteed to work in all POSIX-compliant shells including Bash. The syntax is very similar to that in Bash:
if [ "$1" -eq 1 ]; then
echo "1 was passed in the first parameter"
elif [ "$1" -gt 2 ]; then
echo "2 was not passed in the first parameter"
else
echo "The first parameter was not 1 and is not more than 2."
fi

Section 10.3: Looping over an array
for loop:
arr=(a b c d e f)
for i in "${arr[@]}";do
echo "$i"
done

Or
for ((i=0;i<${#arr[@]};i++));do
echo "${arr[$i]}"
done
while loop:
i=0
while [ $i -lt ${#arr[@]} ];do
echo "${arr[$i]}"
i=$(expr $i + 1)
done

Or
i=0
while (( $i < ${#arr[@]} ));do
echo "${arr[$i]}"
((i++))
done

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36

Section 10.4: Using For Loop to List Iterate Over Numbers
#! /bin/bash
for i in {1..10}; do # {1..10} expands to "1 2 3 4 5 6 7 8 9 10"
echo $i
done

This outputs the following:
1
2
3
4
5
6
7
8
8
10

Section 10.5: continue and break
Example for continue
for i in [series]
do
command 1
command 2
if (condition) # Condition to jump over command 3
continue # skip to the next value in "series"
fi
command 3
done

Example for break
for i in [series]
do
command 4
if (condition) # Condition to break the loop
then
command 5 # Command if the loop needs to be broken
break
fi
command 6 # Command to run if the "condition" is never true
done

Section 10.6: Loop break
Break multiple loop:
arr=(a b c d e f)
for i in "${arr[@]}";do
echo "$i"
for j in "${arr[@]}";do
echo "$j"
break 2

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37

done
done

Output:
a
a

Break single loop:
arr=(a b c d e f)
for i in "${arr[@]}";do
echo "$i"
for j in "${arr[@]}";do
echo "$j"
break
done
done

Output:
a
a
b
a
c
a
d
a
e
a
f
a

Section 10.7: While Loop
#! /bin/bash
i=0
while [ $i -lt 5 ] #While i is less than 5
do
echo "i is currently $i"
i=$[$i+1] #Not the lack of spaces around the brackets. This makes it a not a test expression
done #ends the loop

Watch that there are spaces around the brackets during the test (after the while statement). These spaces are
necessary.
This loop outputs:
i
i
i
i
i

is
is
is
is
is

currently
currently
currently
currently
currently

0
1
2
3
4

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38

Section 10.8: For Loop with C-style syntax
The basic format of C-style for loop is:
for (( variable assignment; condition; iteration process ))

Notes:
The assignment of the variable inside C-style for loop can contain spaces unlike the usual assignment
Variables inside C-style for loop aren't preceded with $.
Example:
for (( i = 0; i < 10; i++ ))
do
echo "The iteration number is $i"
done

Also we can process multiple variables inside C-style for loop:
for (( i = 0, j = 0; i < 10; i++, j = i * i ))
do
echo "The square of $i is equal to $j"
done

Section 10.9: Until Loop
Until loop executes until condition is true
i=5
until [[ i -eq 10 ]]; do #Checks if i=10
echo "i=$i" #Print the value of i
i=$((i+1)) #Increment i by 1
done

Output:
i=5
i=6
i=7
i=8
i=9

When i reaches 10 the condition in until loop becomes true and the loop ends.

Section 10.10: Switch statement with case
With the case statement you can match values against one variable.
The argument passed to case is expanded and try to match against each patterns.
If a match is found, the commands upto ;; are executed.
case "$BASH_VERSION" in
[34]*)

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echo {1..4}
;;
*)
seq -s" " 1 4
esac

Pattern are not regular expressions but shell pattern matching (aka globs).

Section 10.11: For Loop without a list-of-words parameter
for arg; do
echo arg=$arg
done

A for loop without a list of words parameter will iterate over the positional parameters instead. In other words, the
above example is equivalent to this code:
for arg in "$@"; do
echo arg=$arg
done

In other words, if you catch yourself writing for i in "$@"; do ...; done, just drop the in part, and write simply
for i; do ...; done.

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40

Chapter 11: true, false and : commands
Section 11.1: Infinite Loop
while true; do
echo ok
done

or
while :; do
echo ok
done

or
until false; do
echo ok
done

Section 11.2: Function Return
function positive() {
return 0
}
function negative() {
return 1
}

Section 11.3: Code that will always/never be executed
if true; then
echo Always executed
fi
if false; then
echo Never executed
fi

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41

Chapter 12: Arrays
Section 12.1: Array Assignments
List Assignment
If you are familiar with Perl, C, or Java, you might think that Bash would use commas to separate array elements,
however this is not the case; instead, Bash uses spaces:
# Array in Perl
my @array = (1, 2, 3, 4);
# Array in Bash
array=(1 2 3 4)

Create an array with new elements:
array=('first element' 'second element' 'third element')

Subscript Assignment
Create an array with explicit element indices:
array=([3]='fourth element' [4]='fifth element')

Assignment by index
array[0]='first element'
array[1]='second element'

Assignment by name (associative array)
Version ≥ 4.0

declare -A array
array[first]='First element'
array[second]='Second element'

Dynamic Assignment
Create an array from the output of other command, for example use seq to get a range from 1 to 10:
array=(`seq 1 10`)

Assignment from script's input arguments:
array=("$@")

Assignment within loops:
while read -r; do
#array+=("$REPLY")
# Array append
array[$i]="$REPLY"
# Assignment by index
let i++
# Increment index
done < <(seq 1 10) # command substitution

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echo ${array[@]}

# output: 1 2 3 4 5 6 7 8 9 10

where $REPLY is always the current input

Section 12.2: Accessing Array Elements
Print element at index 0
echo "${array[0]}"
Version < 4.3

Print last element using substring expansion syntax
echo "${arr[@]: -1 }"
Version ≥ 4.3

Print last element using subscript syntax
echo "${array[-1]}"

Print all elements, each quoted separately
echo "${array[@]}"

Print all elements as a single quoted string
echo "${array[*]}"

Print all elements from index 1, each quoted separately
echo "${array[@]:1}"

Print 3 elements from index 1, each quoted separately
echo "${array[@]:1:3}"

String Operations
If referring to a single element, string operations are permitted:
array=(zero one two)
echo "${array[0]:0:3}" # gives out zer (chars at position 0, 1 and 2 in the string zero)
echo "${array[0]:1:3}" # gives out ero (chars at position 1, 2 and 3 in the string zero)

so ${array[$i]:N:M} gives out a string from the Nth position (starting from 0) in the string ${array[$i]} with M
following chars.

Section 12.3: Array Modification
Change Index
Initialize or update a particular element in the array
array[10]="elevenths element"

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# because it's starting with 0

43

Version ≥ 3.1

Append
Modify array, adding elements to the end if no subscript is specified.
array+=('fourth element' 'fifth element')

Replace the entire array with a new parameter list.
array=("${array[@]}" "fourth element" "fifth element")

Add an element at the beginning:
array=("new element" "${array[@]}")

Insert
Insert an element at a given index:
arr=(a b c d)
# insert an element at index 2
i=2
arr=("${arr[@]:0:$i}" 'new' "${arr[@]:$i}")
echo "${arr[2]}" #output: new

Delete
Delete array indexes using the unset builtin:
arr=(a b c)
echo "${arr[@]}"
echo "${!arr[@]}"
unset -v 'arr[1]'
echo "${arr[@]}"
echo "${!arr[@]}"

# outputs: a b c
# outputs: 0 1 2
# outputs: a c
# outputs: 0 2

Merge
array3=("${array1[@]}" "${array2[@]}")

This works for sparse arrays as well.
Re-indexing an array
This can be useful if elements have been removed from an array, or if you're unsure whether there are gaps in the
array. To recreate the indices without gaps:
array=("${array[@]}")

Section 12.4: Array Iteration
Array iteration comes in two flavors, foreach and the classic for-loop:
a=(1 2 3 4)
# foreach loop

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for y in "${a[@]}"; do
# act on $y
echo "$y"
done
# classic for-loop
for ((idx=0; idx < ${#a[@]}; ++idx)); do
# act on ${a[$idx]}
echo "${a[$idx]}"
done

You can also iterate over the output of a command:
a=($(tr ',' ' ' <<<"a,b,c,d")) # tr can transform one character to another
for y in "${a[@]}"; do
echo "$y"
done

Section 12.5: Array Length
${#array[@]} gives the length of the array ${array[@]}:
array=('first element' 'second element' 'third element')
echo "${#array[@]}" # gives out a length of 3

This works also with Strings in single elements:
echo "${#array[0]}"

# gives out the lenght of the string at element 0: 13

Section 12.6: Associative Arrays
Version ≥ 4.0

Declare an associative array
declare -A aa

Declaring an associative array before initialization or use is mandatory.
Initialize elements
You can initialize elements one at a time as follows:
aa[hello]=world
aa[ab]=cd
aa["key with space"]="hello world"

You can also initialize an entire associative array in a single statement:
aa=([hello]=world [ab]=cd ["key with space"]="hello world")

Access an associative array element
echo ${aa[hello]}
# Out: world

Listing associative array keys

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45

echo "${!aa[@]}"
#Out: hello ab key with space

Listing associative array values
echo "${aa[@]}"
#Out: world cd hello world

Iterate over associative array keys and values
for key in "${!aa[@]}"; do
echo "Key:
${key}"
echo "Value: ${array[$key]}"
done
#
#
#
#
#
#
#

Out:
Key:
Value:
Key:
Value:
Key:
Value:

hello
world
ab
cd
key with space
hello world

Count associative array elements
echo "${#aa[@]}"
# Out: 3

Section 12.7: Looping through an array
Our example array:
arr=(a b c d e f)

Using a for..in loop:
for i in "${arr[@]}"; do
echo "$i"
done
Version ≥ 2.04

Using C-style for loop:
for ((i=0;i<${#arr[@]};i++)); do
echo "${arr[$i]}"
done

Using while loop:
i=0
while [ $i -lt ${#arr[@]} ]; do
echo "${arr[$i]}"
i=$((i + 1))
done
Version ≥ 2.04

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46

Using while loop with numerical conditional:
i=0
while (( $i < ${#arr[@]} )); do
echo "${arr[$i]}"
((i++))
done

Using an until loop:
i=0
until [ $i -ge ${#arr[@]} ]; do
echo "${arr[$i]}"
i=$((i + 1))
done
Version ≥ 2.04

Using an until loop with numerical conditional:
i=0
until (( $i >= ${#arr[@]} )); do
echo "${arr[$i]}"
((i++))
done

Section 12.8: Destroy, Delete, or Unset an Array
To destroy, delete, or unset an array:
unset array

To destroy, delete, or unset a single array element:
unset array[10]

Section 12.9: Array from string
stringVar="Apple Orange Banana Mango"
arrayVar=(${stringVar// / })

Each space in the string denotes a new item in the resulting array.
echo ${arrayVar[0]} # will print Apple
echo ${arrayVar[3]} # will print Mango

Similarly, other characters can be used for the delimiter.
stringVar="Apple+Orange+Banana+Mango"
arrayVar=(${stringVar//+/ })
echo ${arrayVar[0]} # will print Apple
echo ${arrayVar[2]} # will print Banana

Section 12.10: List of initialized indexes
Get the list of inialized indexes in an array
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47

$
$
$
$
2

arr[2]='second'
arr[10]='tenth'
arr[25]='twenty five'
echo ${!arr[@]}
10 25

Section 12.11: Reading an entire file into an array
Reading in a single step:
IFS=$'\n' read -r -a arr < file

Reading in a loop:
arr=()
while IFS= read -r line; do
arr+=("$line")
done
Version ≥ 4.0

Using mapfile or readarray (which are synonymous):
mapfile -t arr < file
readarray -t arr < file

Section 12.12: Array insert function
This function will insert an element into an array at a given index:
insert(){
h='
################## insert ########################
# Usage:
#
insert arr_name index element
#
#
Parameters:
#
arr_name
: Name of the array variable
#
index
: Index to insert at
#
element
: Element to insert
##################################################
'
[[ $1 = -h ]] && { echo "$h" >/dev/stderr; return 1; }
declare -n __arr__=$1
# reference to the array variable
i=$2
# index to insert at
el="$3"
# element to insert
# handle errors
[[ ! "$i" =~ ^[0-9]+$ ]] && { echo "E: insert: index must be a valid integer" >/dev/stderr;
return 1; }
(( $1 < 0 )) && { echo "E: insert: index can not be negative" >/dev/stderr; return 1; }
# Now insert $el at $i
__arr__=("${__arr__[@]:0:$i}" "$el" "${__arr__[@]:$i}")
}

Usage:
insert array_variable_name index element

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Example:
arr=(a b c d)
echo "${arr[2]}" # output: c
# Now call the insert function and pass the array variable name,
# index to insert at
# and the element to insert
insert arr 2 'New Element'
# 'New Element' was inserted at index 2 in arr, now print them
echo "${arr[2]}" # output: New Element
echo "${arr[3]}" # output: c

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Chapter 13: Associative arrays
Section 13.1: Examining assoc arrays
All needed usage shown with this snippet:
#!/usr/bin/env bash
declare -A assoc_array=([key_string]=value
\
[one]="something"
\
[two]="another thing"
\
[ three ]='mind the blanks!'
\
[ " four" ]='count the blanks of this key later!'
\
[IMPORTANT]='SPACES DO ADD UP!!!'
\
[1]='there are no integers!'
\
[info]="to avoid history expansion "
\
[info2]="quote exclamation mark with single quotes" \
)
echo # just a blank line
echo now here are the values of assoc_array:
echo ${assoc_array[@]}
echo not that useful,
echo # just a blank line
echo this is better:
declare -p assoc_array

# -p == print

echo have a close look at the spaces above\!\!\!
echo # just a blank line
echo
echo
echo
echo

accessing the keys
the keys in assoc_array are ${!assoc_array[*]}
mind the use of indirection operator \!
# just a blank line

echo
echo
echo
echo

now we loop over the assoc_array line by line
note the \! indirection operator which works differently,
if used with assoc_array.
# just a blank line

for key in "${!assoc_array[@]}"; do # accessing keys using ! indirection!!!!
printf "key: \"%s\"\nvalue: \"%s\"\n\n" "$key" "${assoc_array[$key]}"
done

echo have a close look at the spaces in entries with keys two, three and four above\!\!\!
echo # just a blank line
echo # just another blank line
echo
i=1
echo
echo
echo
echo
echo
echo
echo

there is a difference using integers as keys\!\!\!
declaring an integer var i=1
# just a blank line
Within an integer_array bash recognizes artithmetic context.
Within an assoc_array bash DOES NOT recognize artithmetic context.
# just a blank line
this works: \${assoc_array[\$i]}: ${assoc_array[$i]}
this NOT!!: \${assoc_array[i]}: ${assoc_array[i]}

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echo # just a blank line
echo # just a blank line
echo an \${assoc_array[i]} has a string context within braces in contrast to an integer_array
declare -i integer_array=( one two three )
echo "doing a: declare -i integer_array=( one two three )"
echo # just a blank line
echo both forms do work: \${integer_array[i]} : ${integer_array[i]}
echo and this too: \${integer_array[\$i]} : ${integer_array[$i]}

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51

Chapter 14: Functions
Section 14.1: Functions with arguments
In helloJohn.sh:
#!/bin/bash
greet() {
local name="$1"
echo "Hello, $name"
}
greet "John Doe"
# running above script
$ bash helloJohn.sh
Hello, John Doe

1. If you don't modify the argument in any way, there is no need to copy it to a local variable - simply echo
"Hello, $1".

2. You can use $1, $2, $3 and so on to access the arguments inside the function.

Note: for arguments more than 9 $10 won't work (bash will read it as $10), you need to do ${10},
${11} and so on.

3. $@ refers to all arguments of a function:
#!/bin/bash
foo() {
echo "$@"
}
foo 1 2 3 # output => 1 2 3

Note: You should practically always use double quotes around "$@", like here.
Omitting the quotes will cause the shell to expand wildcards (even when the user specifically quoted them in
order to avoid that) and generally introduce unwelcome behavior and potentially even security problems.
foo "string with spaces;" '$HOME' "*"
# output => string with spaces; $HOME *

4. for default arguments use ${1:-default_val}. Eg:
#!/bin/bash
foo() {
local val=${1:-25}
echo "$val"
}

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52

foo
foo 30

# output => 25
# output => 30

5. to require an argument use ${var:?error message}
foo() {
local val=${1:?Must provide an argument}
echo "$val"
}

Section 14.2: Simple Function
In helloWorld.sh
#!/bin/bash
# Define a function greet
greet ()
{
echo "Hello World!"
}
# Call the function greet
greet

In running the script, we see our message
$ bash helloWorld.sh
Hello World!

Note that sourcing a file with functions makes them available in your current bash session.
$ source helloWorld.sh
$ greet
Hello World!

# or, more portably, ". helloWorld.sh"

You can export a function in some shells, so that it is exposed to child processes.
bash -c 'greet'
export -f greet
bash -c 'greet'

# fails
# export function; note -f
# success

Section 14.3: Handling flags and optional parameters
The getopts builtin can be used inside functions to write functions that accommodate flags and optional
parameters. This presents no special difficulty but one has to handle appropriately the values touched by getopts.
As an example, we define a failwith function that writes a message on stderr and exits with code 1 or an arbitrary
code supplied as parameter to the -x option:
# failwith [-x STATUS] PRINTF-LIKE-ARGV
# Fail with the given diagnostic message
#
# The -x flag can be used to convey a custom exit status, instead of
# the value 1. A newline is automatically added to the output.
failwith()

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53

{
local OPTIND OPTION OPTARG status
status=1
OPTIND=1
while getopts 'x:' OPTION; do
case ${OPTION} in
x)
status="${OPTARG}";;
*)
1>&2 printf 'failwith: %s: Unsupported option.\n' "${OPTION}";;
esac
done
shift $(( OPTIND - 1 ))
{
printf 'Failure: '
printf "$@"
printf '\n'
} 1>&2
exit "${status}"
}

This function can be used as follows:
failwith '%s: File not found.' "${filename}"
failwith -x 70 'General internal error.'

and so on.
Note that as for printf, variables should not be used as first argument. If the message to print consists of the
content of a variable, one should use the %s specifier to print it, like in
failwith '%s' "${message}"

Section 14.4: Print the function definition
getfunc() {
declare -f "$@"
}
function func(){
echo "I am a sample function"
}
funcd="$(getfunc func)"
getfunc func # or echo "$funcd"

Output:
func ()
{
echo "I am a sample function"
}

Section 14.5: A function that accepts named parameters
foo() {
while [[ "$#" -gt 0 ]]

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54

do
case $1 in
-f|--follow)
local FOLLOW="following"
;;
-t|--tail)
local TAIL="tail=$2"
;;
esac
shift
done
echo "FOLLOW: $FOLLOW"
echo "TAIL: $TAIL"
}

Example usage:
foo
foo
foo
foo

-f
-t 10
-f --tail 10
--follow --tail 10

Section 14.6: Return value from a function
The return statement in Bash doesn't return a value like C-functions, instead it exits the function with a return
status. You can think of it as the exit status of that function.
If you want to return a value from the function then send the value to stdout like this:
fun() {
local var="Sample value to be returned"
echo "$var"
#printf "%s\n" "$var"
}

Now, if you do:
var="$(fun)"

the output of fun will be stored in $var.

Section 14.7: The exit code of a function is the exit code of its
last command
Consider this example function to check if a host is up:
is_alive() {
ping -c1 "$1" &> /dev/null
}

This function sends a single ping to the host specified by the first function parameter. The output and error output
of ping are both redirected to /dev/null, so the function will never output anything. But the ping command will
have exit code 0 on success, and non-zero on failure. As this is the last (and in this example, the only) command of
the function, the exit code of ping will be reused for the exit code of the function itself.

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55

This fact is very useful in conditional statements.
For example, if host graucho is up, then connect to it with ssh:
if is_alive graucho; then
ssh graucho
fi

Another example: repeatedly check until host graucho is up, and then connect to it with ssh:
while ! is_alive graucho; do
sleep 5
done
ssh graucho

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56

Chapter 15: Bash Parameter Expansion
The $ character introduces parameter expansion, command substitution, or arithmetic expansion. The parameter
name or symbol to be expanded may be enclosed in braces, which are optional but serve to protect the variable to
be expanded from characters immediately following it which could be interpreted as part of the name.
Read more in the Bash User Manual.

Section 15.1: Modifying the case of alphabetic characters
Version ≥ 4.0

To uppercase
$ v="hello"
# Just the first character
$ printf '%s\n' "${v^}"
Hello
# All characters
$ printf '%s\n' "${v^^}"
HELLO
# Alternative
$ v="hello world"
$ declare -u string="$v"
$ echo "$string"
HELLO WORLD

To lowercase
$ v="BYE"
# Just the first character
$ printf '%s\n' "${v,}"
bYE
# All characters
$ printf '%s\n' "${v,,}"
bye
# Alternative
$ v="HELLO WORLD"
$ declare -l string="$v"
$ echo "$string"
hello world

Toggle Case
$ v="Hello World"
# All chars
$ echo "${v~~}"
hELLO wORLD
$ echo "${v~}"
# Just the first char
hello World

Section 15.2: Length of parameter
# Length of a string
$ var='12345'
$ echo "${#var}"

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57

5

Note that it's the length in number of characters which is not necessarily the same as the number of bytes (like in
UTF-8 where most characters are encoded in more than one byte), nor the number of glyphs/graphemes (some of
which are combinations of characters), nor is it necessarily the same as the display width.
# Number of array elements
$ myarr=(1 2 3)
$ echo "${#myarr[@]}"
3
# Works for positional parameters as well
$ set -- 1 2 3 4
$ echo "${#@}"
4
# But more commonly (and portably to other shells), one would use
$ echo "$#"
4

Section 15.3: Replace pattern in string
First match:
$ a='I am a string'
$ echo "${a/a/A}"
I Am a string

All matches:
$ echo "${a//a/A}"
I Am A string

Match at the beginning:
$ echo "${a/#I/y}"
y am a string

Match at the end:
$ echo "${a/%g/N}"
I am a strinN

Replace a pattern with nothing:
$ echo "${a/g/}"
I am a strin

Add prefix to array items:
$ A=(hello world)
$ echo "${A[@]/#/R}"
Rhello Rworld

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58

Section 15.4: Substrings and subarrays
var='0123456789abcdef'
# Define a zero-based offset
$ printf '%s\n' "${var:3}"
3456789abcdef
# Offset and length of substring
$ printf '%s\n' "${var:3:4}"
3456
Version ≥ 4.2

# Negative length counts from the end of the string
$ printf '%s\n' "${var:3:-5}"
3456789a
# Negative offset counts from the end
# Needs a space to avoid confusion with ${var:-6}
$ printf '%s\n' "${var: -6}"
abcdef
# Alternative: parentheses
$ printf '%s\n' "${var:(-6)}"
abcdef
# Negative offset and negative length
$ printf '%s\n' "${var: -6:-5}"
a

The same expansions apply if the parameter is a positional parameter or the element of a subscripted array:
# Set positional parameter $1
set -- 0123456789abcdef
# Define offset
$ printf '%s\n' "${1:5}"
56789abcdef
# Assign to array element
myarr[0]='0123456789abcdef'
# Define offset and length
$ printf '%s\n' "${myarr[0]:7:3}"
789

Analogous expansions apply to positional parameters, where offsets are one-based:
# Set positional parameters $1, $2, ...
$ set -- 1 2 3 4 5 6 7 8 9 0 a b c d e f
# Define an offset (beware $0 (not a positional parameter)
# is being considered here as well)
$ printf '%s\n' "${@:10}"
0
a
b
c
d
e
f

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59

# Define an offset and a length
$ printf '%s\n' "${@:10:3}"
0
a
b
# No negative lengths allowed for positional parameters
$ printf '%s\n' "${@:10:-2}"
bash: -2: substring expression < 0
# Negative offset counts from the end
# Needs a space to avoid confusion with ${@:-10:2}
$ printf '%s\n' "${@: -10:2}"
7
8
# ${@:0} is $0 which is not otherwise a positional parameters or part
# of $@
$ printf '%s\n' "${@:0:2}"
/usr/bin/bash
1

Substring expansion can be used with indexed arrays:
# Create array (zero-based indices)
$ myarr=(0 1 2 3 4 5 6 7 8 9 a b c d e f)
# Elements with index 5 and higher
$ printf '%s\n' "${myarr[@]:12}"
c
d
e
f
# 3 elements, starting with index 5
$ printf '%s\n' "${myarr[@]:5:3}"
5
6
7
# The last element of the array
$ printf '%s\n' "${myarr[@]: -1}"
f

Section 15.5: Delete a pattern from the beginning of a string
Shortest match:
$ a='I am a string'
$ echo "${a#*a}"
m a string

Longest match:
$ echo "${a##*a}"
string

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60

Section 15.6: Parameter indirection
Bash indirection permits to get the value of a variable whose name is contained in another variable. Variables

example:
$ red="the color red"
$ green="the color green"
$ color=red
$ echo "${!color}"
the color red
$ color=green
$ echo "${!color}"
the color green

Some more examples that demonstrate the indirect expansion usage:
$ foo=10
$ x=foo
$ echo ${x}
foo
$ foo=10
$ x=foo
$ echo ${!x}
10

#Classic variable print

#Indirect expansion

One more example:
$ argtester () { for (( i=1; i<="$#"; i++ )); do echo "${i}";done; }; argtester -ab -cd -ef
1
#i expanded to 1
2
#i expanded to 2
3
#i expanded to 3
$ argtester () { for (( i=1; i<="$#"; i++ ));
-ab
# i=1 --> expanded to $1 ---> expanded
-cd
# i=2 --> expanded to $2 ---> expanded
-ef
# i=3 --> expanded to $3 ---> expanded

do
to
to
to

echo "${!i}";done; }; argtester -ab -cd -ef
first argument sent to function
second argument sent to function
third argument sent to function

Section 15.7: Parameter expansion and filenames
You can use Bash Parameter Expansion to emulate common filename-processing operations like basename and
dirname.

We will use this as our example path:
FILENAME="/tmp/example/myfile.txt"

To emulate dirname and return the directory name of a file path:
echo "${FILENAME%/*}"
#Out: /tmp/example

To emulate basename $FILENAME and return the filename of a file path:
echo "${FILENAME##*/}"

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61

#Out: myfile.txt

To emulate basename $FILENAME .txt and return the filename without the .txt. extension:
BASENAME="${FILENAME##*/}"
echo "${BASENAME%%.txt}"
#Out: myfile

Section 15.8: Default value substitution
${parameter:-word}

If parameter is unset or null, the expansion of word is substituted. Otherwise, the value of parameter is
substituted.

$ unset var
$ echo "${var:-XX}"
XX
$ var=""
$ echo "${var:-XX}"
XX
$ var=23
$ echo "${var:-XX}"
23

# Parameter is unset -> expansion XX occurs
# Parameter is null -> expansion XX occurs

# Parameter is not null -> original expansion occurs

${parameter:=word}

If parameter is unset or null, the expansion of word is assigned to parameter. The value of parameter is
then substituted. Positional parameters and special parameters may not be assigned to in this way.

$ unset var
$ echo "${var:=XX}"
XX
$ echo "$var"
XX
$ var=""
$ echo "${var:=XX}"
XX
$ echo "$var"
XX
$ var=23
$ echo "${var:=XX}"
23
$ echo "$var"
23

# Parameter is unset -> word is assigned to XX

# Parameter is null -> word is assigned to XX

# Parameter is not null -> no assignment occurs

Section 15.9: Delete a pattern from the end of a string
Shortest match:
$ a='I am a string'
$ echo "${a%a*}"
I am

Longest match:
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62

$ echo "${a%%a*}"
I

Section 15.10: Munging during expansion
Variables don't necessarily have to expand to their values - substrings can be extracted during expansion, which
can be useful for extracting file extensions or parts of paths. Globbing characters keep their usual meanings, so .*
refers to a literal dot, followed by any sequence of characters; it's not a regular expression.
$ v=foo-bar-baz
$ echo ${v%%-*}
foo
$ echo ${v%-*}
foo-bar
$ echo ${v##*-}
baz
$ echo ${v#*-}
bar-baz

It's also possible to expand a variable using a default value - say I want to invoke the user's editor, but if they've not
set one I'd like to give them vim.
$
$
#
$
$
#

EDITOR=nano
${EDITOR:-vim} /tmp/some_file
opens nano
unset EDITOR
$ ${EDITOR:-vim} /tmp/some_file
opens vim

There are two different ways of performing this expansion, which differ in whether the relevant variable is empty or
unset. Using :- will use the default if the variable is either unset or empty, whilst - only uses the default if the
variable is unset, but will use the variable if it is set to the empty string:
$ a="set"
$ b=""
$ unset c
$ echo ${a:-default_a} ${b:-default_b} ${c:-default_c}
set default_b default_c
$ echo ${a-default_a} ${b-default_b} ${c-default_c}
set default_c

Similar to defaults, alternatives can be given; where a default is used if a particular variable isn't available, an
alternative is used if the variable is available.
$ a="set"
$ b=""
$ echo ${a:+alternative_a} ${b:+alternative_b}
alternative_a

Noting that these expansions can be nested, using alternatives becomes particularly useful when supplying
arguments to command line flags;
$
$
#
$

output_file=/tmp/foo
wget ${output_file:+"-o ${output_file}"} www.stackexchange.com
expands to wget -o /tmp/foo www.stackexchange.com
unset output_file

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63

$ wget ${output_file:+"-o ${output_file}"} www.stackexchange.com
# expands to wget www.stackexchange.com

Section 15.11: Error if variable is empty or unset
The semantics for this are similar to that of default value substitution, but instead of substituting a default value, it
errors out with the provided error message. The forms are ${VARNAME?ERRMSG} and ${VARNAME:?ERRMSG}. The form
with : will error our if the variable is unset or empty, whereas the form without will only error out if the variable is
unset. If an error is thrown, the ERRMSG is output and the exit code is set to 1.
#!/bin/bash
FOO=
# ./script.sh: line 4: FOO: EMPTY
echo "FOO is ${FOO:?EMPTY}"
# FOO is
echo "FOO is ${FOO?UNSET}"
# ./script.sh: line 8: BAR: EMPTY
echo "BAR is ${BAR:?EMPTY}"
# ./script.sh: line 10: BAR: UNSET
echo "BAR is ${BAR?UNSET}"

The run the full example above each of the erroring echo statements needs to be commented out to proceed.

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64

Chapter 16: Copying (cp)
Option

Description
Combines the d, p and r options
-b, -backup
Before removal, makes a backup
-d, --no-deference Preserves links
-f, --force
Remove existing destinations without prompting user
-i, --interactive Show prompt before overwriting
-l, --link
Instead of copying, link files instead
-p, --preserve
Preserve file attributes when possible
-R, --recursive
Recursively copy directories
-a,-archive

Section 16.1: Copy a single file
Copy foo.txt from /path/to/source/ to /path/to/target/folder/
cp /path/to/source/foo.txt /path/to/target/folder/

Copy foo.txt from /path/to/source/ to /path/to/target/folder/ into a file called bar.txt
cp /path/to/source/foo.txt /path/to/target/folder/bar.txt

Section 16.2: Copy folders
copy folder foo into folder bar
cp -r /path/to/foo /path/to/bar

if folder bar exists before issuing the command, then foo and its content will be copied into the folder bar.
However, if bar does not exist before issuing the command, then the folder bar will be created and the content of
foo will be placed into bar

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65

Chapter 17: Find
find is a command to recursively search a directory for files(or directories) that match a criteria, and then perform
some action on the selected files.
find search_path selection_criteria action

Section 17.1: Searching for a file by name or extension
To find files/directories with a specific name, relative to pwd:
$ find . -name "myFile.txt"
./myFile.txt

To find files/directories with a specific extension, use a wildcard:
$ find . -name "*.txt"
./myFile.txt
./myFile2.txt

To find files/directories matching one of many extensions, use the or flag:
$ find . -name "*.txt" -o -name "*.sh"

To find files/directories which name begin with abc and end with one alpha character following a one digit:
$ find . -name "abc[a-z][0-9]"

To find all files/directories located in a specific directory
$ find /opt

To search for files only (not directories), use -type f:
find /opt -type f

To search for directories only (not regular files), use -type d:
find /opt -type d

Section 17.2: Executing commands against a found file
Sometimes we will need to run commands against a lot of files. This can be done using xargs.
find . -type d -print | xargs -r chmod 770

The above command will recursively find all directories (-type d) relative to . (which is your current working
directory), and execute chmod 770 on them. The -r option specifies to xargs to not run chmod if find did not find
any files.
If your files names or directories have a space character in them, this command may choke; a solution is to use the
following

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find . -type d -print0 | xargs -r -0 chmod 770

In the above example, the -print0 and -0 flags specify that the file names will be separated using a null byte, and
allows the use of special characters, like spaces, in the file names. This is a GNU extension, and may not work in
other versions of find and xargs.
The preferred way to do this is to skip the xargs command and let find call the subprocess itself:
find . -type d -exec chmod 770 {} \;

Here, the {} is a placeholder indicating that you want to use the file name at that point. find will execute chmod on
each file individually.
You can alternatively pass all file names to a single call of chmod, by using
find . -type d -exec chmod 770 {} +

This is also the behaviour of the above xargs snippets. (To call on each file individually, you can use xargs -n1).
A third option is to let bash loop over the list of filenames find outputs:
find . -type d | while read -r d; do chmod 770 "$d"; done

This is syntactically the most clunky, but convenient when you want to run multiple commands on each found file.
However, this is unsafe in the face of file names with odd names.
find . -type f | while read -r d; do mv "$d" "${d// /_}"; done

which will replace all spaces in file names with underscores.(This example also won't work if there are spaces in
leading directory names.)
The problem with the above is that while read -r expects one entry per line, but file names can contain newlines
(and also, read -r will lose any trailing whitespace). You can fix this by turning things around:
find . -type d -exec bash -c 'for f; do mv "$f" "${f// /_}"; done' _ {} +

This way, the -exec receives the file names in a form which is completely correct and portable; the bash -c receives
them as a number of arguments, which will be found in $@, correctly quoted etc. (The script will need to handle
these names correctly, of course; every variable which contains a file name needs to be in double quotes.)
The mysterious _ is necessary because the first argument to bash -c 'script' is used to populate $0.

Section 17.3: Finding file by access / modification time
On an ext filesystem, each file has a stored Access, Modification, and (Status) Change time associated with it - to
view this information you can use stat myFile.txt; using flags within find, we can search for files that were
modified within a certain time range.
To find files that have been modified within the last 2 hours:
$ find . -mmin -120

To find files that have not been modified within the last 2 hours:

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$ find . -mmin +120

The above example are searching only on the modified time - to search on access times, or changed times, use a, or
c accordingly.
$ find . -amin -120
$ find . -cmin +120

General format:
-mmin n : File was modified n minutes ago
-mmin -n : File was modified less than n minutes ago
-mmin +n : File was modified more than n minutes ago

Find files that have been modified within the last 2 days:
find . -mtime -2

Find files that have not been modified within the last 2 days
find . -mtime +2

Use -atime and -ctime for access time and status change time respectively.
General format:
-mtime n : File was modified nx24 hours ago
-mtime -n : File was modified less than nx24 hours ago
-mtime +n : File was modified more than nx24 hours ago

Find files modified in a range of dates, from 2007-06-07 to 2007-06-08:
find . -type f -newermt 2007-06-07 ! -newermt 2007-06-08

Find files accessed in a range of timestamps (using files as timestamp), from 1 hour ago to 10 minutes ago:
touch -t $(date -d '1 HOUR AGO' +%Y%m%d%H%M.%S) start_date
touch -t $(date -d '10 MINUTE AGO' +%Y%m%d%H%M.%S) end_date
timeout 10 find "$LOCAL_FOLDER" -newerat "start_date" ! -newerat "end_date" -print

General format:
-newerXY reference : Compares the timestamp of the current file with reference. XY could have one of the

following values: at (access time), mt (modification time), ct (change time) and more. reference is the name of a file
whe want to compare the timestamp specified (access, modification, change) or a string describing an absolute
time.

Section 17.4: Finding files according to size
Find files larger than 15MB:
find -type f -size +15M

Find files less than 12KB:
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find -type f -size -12k

Find files exactly of 12KB size:
find -type f -size 12k

Or
find -type f -size 12288c

Or
find -type f -size 24b

Or
find -type f -size 24

General format:
find [options] -size n[cwbkMG]

Find files of n-block size, where +n means more than n-block, -n means less than n-block and n (without
any sign) means exactly n-block
Block size:
1. c: bytes
2. w: 2 bytes
3. b: 512 bytes (default)
4. k: 1 KB
5. M: 1 MB
6. G: 1 GB

Section 17.5: Filter the path
The -path parameter allows to specify a pattern to match the path of the result. The pattern can match also the
name itself.
To find only files containing log anywhere in their path (folder or name):
find . -type f -path '*log*'

To find only files within a folder called log (on any level):
find . -type f -path '*/log/*'

To find only files within a folder called log or data:
find . -type f -path '*/log/*' -o -path '*/data/*'

To find all files except the ones contained in a folder called bin:
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find . -type f -not -path '*/bin/*'

To find all file all files except the ones contained in a folder called bin or log files:
find . -type f -not -path '*log' -not -path '*/bin/*'

Section 17.6: Finding files by type
To find files, use the -type f flag
$ find . -type f

To find directories, use the -type d flag
$ find . -type d

To find block devices, use the -type b flag
$ find /dev -type b

To find symlinks, use the -type l flag
$ find . -type l

Section 17.7: Finding files by specific extension
To find all the files of a certain extension within the current path you can use the following find syntax. It works by
making use of bash's built-in glob construct to match all the names having the .extension.
find /directory/to/search -maxdepth 1 -type f -name "*.extension"

To find all files of type .txt from the current directory alone, do
find . -maxdepth 1 -type f -name "*.txt"

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Chapter 18: Using sort
Option
Meaning
-u
Make each lines of output unique
sort is a Unix command to order data in file(s) in a sequence.

Section 18.1: Sort command output
sort command is used to sort a list of lines.

Input from a file
sort file.txt

Input from a command
You can sort any output command. In the example a list of file following a pattern.
find * -name pattern | sort

Section 18.2: Make output unique
If each lines of the output need to be unique, add -u option.
To display owner of files in folder
ls -l | awk '{print $3}' | sort -u

Section 18.3: Numeric sort
Suppose we have this file:
test>>cat file
10.Gryffindor
4.Hogwarts
2.Harry
3.Dumbledore
1.The sorting hat

To sort this file numerically, use sort with -n option:
test>>sort -n file

This should sort the file as below:
1.The sorting hat
2.Harry
3.Dumbledore
4.Hogwarts
10.Gryffindor

Reversing sort order: To reverse the order of the sort use the -r option

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To reverse the sort order of the above file use:
sort -rn file

This should sort the file as below:
10.Gryffindor
4.Hogwarts
3.Dumbledore
2.Harry
1.The sorting hat

Section 18.4: Sort by keys
Suppose we have this file:
test>>cat Hogwarts
Harry
Malfoy
Gryffindor
Slytherin
Hermione
Goyle
Ron
Snape
Ron
Goyle

Rowena
Ravenclaw
Lockhart
Olivander
Flitwick

Helga
Hufflepuff
Tonks
Newt
Sprout

To sort this file using a column as key use the k option:
test>>sort -k 2 Hogwarts

This will sort the file with column 2 as the key:
Ron
Hermione
Harry
Gryffindor
Ron

Goyle
Goyle
Malfoy
Slytherin
Snape

Flitwick
Lockhart
Rowena
Ravenclaw
Olivander

Sprout
Tonks
Helga
Hufflepuff
Newt

Now if we have to sort the file with a secondary key along with the primary key use:
sort -k 2,2 -k 1,1 Hogwarts

This will first sort the file with column 2 as primary key, and then sort the file with column 1 as secondary key:
Hermione
Ron
Harry
Gryffindor
Ron

Goyle
Goyle
Malfoy
Slytherin
Snape

Lockhart
Flitwick
Rowena
Ravenclaw
Olivander

Tonks
Sprout
Helga
Hufflepuff
Newt

If we need to sort a file with more than 1 key , then for every -k option we need to specify where the sort ends. So k1,1 means start the sort at the first column and end sort at first column.
-t option
In the previous example the file had the default delimeter - tab. In case of sorting a file that has non-default
delimeter we need the -t option to specify the delimeter. Suppose we have the file as below:
test>>cat file

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5.|Gryffindor
4.|Hogwarts
2.|Harry
3.|Dumbledore
1.|The sorting hat

To sort this file as per the second column, use:
test>>sort -t "|" -k 2 file

This will sort the file as below:
3.|Dumbledore
5.|Gryffindor
2.|Harry
4.|Hogwarts
1.|The sorting hat

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Chapter 19: Sourcing
Section 19.1: Sourcing a file
Sourcing a file is different from execution, in that all commands are evaluated within the context of the current
bash session - this means that any variables, function, or aliases defined will persist throughout your session.
Create the file you wish to source sourceme.sh
#!/bin/bash
export A="hello_world"
alias sayHi="echo Hi"
sayHello() {
echo Hello
}

From your session, source the file
$ source sourceme.sh

From hencefourth, you have all the resources of the sourced file available
$ echo $A
hello_world
$ sayHi
Hi
$ sayHello
Hello

Note that the command . is synonymous to source, such that you can simply use
$ . sourceme.sh

Section 19.2: Sourcing a virtual environment
When developing several applications on one machine, it becomes useful to separate out dependencies into virtual
environments.
With the use of virtualenv, these environments are sourced into your shell so that when you run a command, it
comes from that virtual environment.
This is most commonly installed using pip.
pip install https://github.com/pypa/virtualenv/tarball/15.0.2

Create a new environment
virtualenv --python=python3.5 my_env

Activate the environment

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source my_env/bin/activate

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75

Chapter 20: Here documents and here
strings
Section 20.1: Execute command with here document
ssh -p 21 example@example.com <<EOF
echo 'printing pwd'
echo "\$(pwd)"
ls -a
find '*.txt'
EOF
$ is escaped because we do not want it to be expanded by the current shell i.e $(pwd) is to be executed on the

remote shell.
Another way:
ssh -p 21 example@example.com <<'EOF'
echo 'printing pwd'
echo "$(pwd)"
ls -a
find '*.txt'
EOF

Note: The closing EOF should be at the beginning of the line (No whitespaces before). If indentation is required,
tabs may be used if you start your heredoc with <<-. See the Indenting here documents and Limit Strings examples
for more information.

Section 20.2: Indenting here documents
You can indent the text inside here documents with tabs, you need to use the <<- redirection operator instead of
<<:
$ cat <<- EOF
This is some content indented with tabs `\t`.
You cannot indent with spaces you __have__ to use tabs.
Bash will remove empty space before these lines.
__Note__: Be sure to replace spaces with tabs when copying this example.
EOF
This is some content indented with tabs _\t_.
You cannot indent with spaces you __have__ to use tabs.
Bash will remove empty space before these lines.
__Note__: Be sure to replace spaces with tabs when copying this example.

One practical use case of this (as mentioned in man bash) is in shell scripts, for example:
if cond; then
cat <<- EOF
hello
there
EOF
fi

It is customary to indent the lines within code blocks as in this if statement, for better readability. Without the <<Bash Notes for Professionals

76

operator syntax, we would be forced to write the above code like this:
if cond; then
cat << EOF
hello
there
EOF
fi

That's very unpleasant to read, and it gets much worse in a more complex realistic script.

Section 20.3: Create a file
A classic use of here documents is to create a file by typing its content:
cat > fruits.txt << EOF
apple
orange
lemon
EOF

The here-document is the lines between the << EOF and EOF.
This here document becomes the input of the cat command. The cat command simply outputs its input, and using
the output redirection operator > we redirect to a file fruits.txt.
As a result, the fruits.txt file will contain the lines:
apple
orange
lemon

The usual rules of output redirection apply: if fruits.txt did not exist before, it will be created. If it existed before,
it will be truncated.

Section 20.4: Here strings
Version ≥ 2.05b

You can feed a command using here strings like this:
$ awk '{print $2}' <<< "hello world - how are you?"
world
$ awk '{print $1}' <<< "hello how are you
> she is fine"
hello
she

You can also feed a while loop with a here string:
$ while IFS=" " read -r word1 word2 rest
> do
> echo "$word1"
> done <<< "hello how are you - i am fine"
hello

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Section 20.5: Run several commands with sudo
sudo -s <<EOF
a='var'
echo 'Running serveral commands with sudo'
mktemp -d
echo "\$a"
EOF
$a needs to be escaped to prevent it to be expanded by the current shell

Or
sudo -s <<'EOF'
a='var'
echo 'Running serveral commands with sudo'
mktemp -d
echo "$a"
EOF

Section 20.6: Limit Strings
A heredoc uses the limitstring to determine when to stop consuming input. The terminating limitstring must
Be at the start of a line.
Be the only text on the line Note: If you use <<- the limitstring can be prefixed with tabs \t
Correct:
cat <<limitstring
line 1
line 2
limitstring

This will output:
line 1
line 2

Incorrect use:
cat <<limitstring
line 1
line 2
limitstring

Since limitstring on the last line is not exactly at the start of the line, the shell will continue to wait for further
input, until it sees a line that starts with limitstring and doesn't contain anything else. Only then it will stop
waiting for input, and proceed to pass the here-document to the cat command.
Note that when you prefix the initial limitstring with a hyphen, any tabs at the start of the line are removed before
parsing, so the data and the limit string can be indented with tabs (for ease of reading in shell scripts).
cat <<-limitstring
line 1
has a tab each before the words line and has
line 2 has two leading tabs

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limitstring

will produce
line 1
has a tab each before the words line and has
line 2 has two leading tabs

with the leading tabs (but not the internal tabs) removed.

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Chapter 21: Quoting
Section 21.1: Double quotes for variable and command
substitution
Variable substitutions should only be used inside double quotes.
calculation='2 * 3'
echo "$calculation"
echo $calculation
echo "$(($calculation))"

# prints 2 * 3
# prints 2, the list of files in the current directory, and 3
# prints 6

Outside of double quotes, $var takes the value of var, splits it into whitespace-delimited parts, and interprets each
part as a glob (wildcard) pattern. Unless you want this behavior, always put $var inside double quotes: "$var".
The same applies to command substitutions: "$(mycommand)" is the output of mycommand, $(mycommand) is the
result of split+glob on the output.
echo "$var"
echo "$(mycommand)"
another=$var
make -D THING=$var
make -D THING="$var"
make -D "THING=$var"

#
#
#
#
#
#

good
good
also works, assignment is implicitly double-quoted
BAD! This is not a bash assignment.
good
also good

Command substitutions get their own quoting contexts. Writing arbitrarily nested substitutions is easy because the
parser will keep track of nesting depth instead of greedily searching for the first " character. The StackOverflow
syntax highlighter parses this wrong, however. For example:
echo "formatted text: $(printf "a + b = %04d" "${c}")" # “formatted text: a + b = 0000”

Variable arguments to a command substitution should be double-quoted inside the expansions as well:
echo "$(mycommand "$arg1" "$arg2")"

Section 21.2: Dierence between double quote and single
quote
Double quote
Single quote
Allows variable expansion
Prevents variable expansion
Allows history expansion if enabled
Prevents history expansion
Allows command substitution
Prevents command substitution
* and @ can have special meaning
* and @ are always literals
Can contain both single quote or double quote
Single quote is not allowed inside single quote
$, `, ", \ can be escaped with \ to prevent their special meaning All of them are literals
Properties that are common to both:
Prevents globbing
Prevents word splitting
Examples:
$ echo "!cat"

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echo "cat file"
cat file
$ echo '!cat'
!cat
echo "\"'\""
"'"
$ a='var'
$ echo '$a'
$a
$ echo "$a"
var

Section 21.3: Newlines and control characters
A newline can be included in a single-quoted string or double-quoted string. Note that backslash-newline does not
result in a newline, the line break is ignored.
newline1='
'
newline2="
"
newline3=$'\n'
empty=\
echo
echo
echo
echo

"Line${newline1}break"
"Line${newline2}break"
"Line${newline3}break"
"No line break${empty} here"

Inside dollar-quote strings, backslash-letter or backslash-octal can be used to insert control characters, like in many
other programming languages.
echo
echo
echo
echo

$'Tab: [\t]'
$'Tab again: [\009]'
$'Form feed: [\f]'
$'Line\nbreak'

Section 21.4: Quoting literal text
All the examples in this paragraph print the line
!"#$&'()*;<=>?

@[\]^`{|}~

A backslash quotes the next character, i.e. the next character is interpreted literally. The one exception is a newline:
backslash-newline expands to the empty string.
echo \!\"\#\$\&\'\(\)\*\;\<\=\>\?\ \ \@\[\\\]\^\`\{\|\}\~

All text between single quotes (forward quotes ', also known as apostrophe) is printed literally. Even backslash
stands for itself, and it's impossible to include a single quote; instead, you can stop the literal string, include a literal
single quote with a backslash, and start the literal string again. Thus the 4-character sequence '\'' effectively allow
to include a single quote in a literal string.
echo '!"#$&'\''()*;<=>?
#
^^^^

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@[\]^`{|}~'

81

Dollar-single-quote starts a string literal $'…' like many other programming languages, where backslash quotes the
next character.
echo $'!"#$&\'()*;<=>?
#
^^

@[\\]^`{|}~'
^^

Double quotes " delimit semi-literal strings where only the characters " \ $ and ` retain their special meaning.
These characters need a backslash before them (note that if backslash is followed by some other character, the
backslash remains). Double quotes are mostly useful when including a variable or a command substitution.
echo "!\"#\$&'()*;<=>?
#
^^
echo "!\"#\$&'()*;<=>?
#
^^

@[\\]^\`{|}~"
^^ ^^
@[\]^\`{|}~"
^ ^^
\[ prints \[

Interactively, beware that ! triggers history expansion inside double quotes: "!oops" looks for an older command
containing oops; "\!oops" doesn't do history expansion but keeps the backslash. This does not happen in scripts.

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82

Chapter 22: Conditional Expressions
Section 22.1: File type tests
The -e conditional operator tests whether a file exists (including all file types: directories, etc.).
if [[ -e $filename ]]; then
echo "$filename exists"
fi

There are tests for specific file types as well.
if [[ -f $filename ]]; then
echo "$filename is a regular file"
elif [[ -d $filename ]]; then
echo "$filename is a directory"
elif [[ -p $filename ]]; then
echo "$filename is a named pipe"
elif [[ -S $filename ]]; then
echo "$filename is a named socket"
elif [[ -b $filename ]]; then
echo "$filename is a block device"
elif [[ -c $filename ]]; then
echo "$filename is a character device"
fi
if [[ -L $filename ]]; then
echo "$filename is a symbolic link (to any file type)"
fi

For a symbolic link, apart from -L, these tests apply to the target, and return false for a broken link.
if [[ -L $filename || -e $filename ]]; then
echo "$filename exists (but may be a broken symbolic link)"
fi
if [[ -L $filename && ! -e $filename ]]; then
echo "$filename is a broken symbolic link"
fi

Section 22.2: String comparison and matching
String comparison uses the == operator between quoted strings. The != operator negates the comparison.
if [[ "$string1" == "$string2" ]]; then
echo "\$string1 and \$string2 are identical"
fi
if [[ "$string1" != "$string2" ]]; then
echo "\$string1 and \$string2 are not identical"
fi

If the right-hand side is not quoted then it is a wildcard pattern that $string1 is matched against.
string='abc'
pattern1='a*'
pattern2='x*'
if [[ "$string" == $pattern1 ]]; then
# the test is true

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echo "The string $string matches the pattern $pattern"
fi
if [[ "$string" != $pattern2 ]]; then
# the test is false
echo "The string $string does not match the pattern $pattern"
fi

The < and > operators compare the strings in lexicographic order (there are no less-or-equal or greater-or-equal
operators for strings).
There are unary tests for the empty string.
if [[ -n "$string" ]]; then
echo "$string is non-empty"
fi
if [[ -z "${string// }" ]]; then
echo "$string is empty or contains only spaces"
fi
if [[ -z "$string" ]]; then
echo "$string is empty"
fi

Above, the -z check may mean $string is unset, or it is set to an empty string. To distinguish between empty and
unset, use:
if [[ -n
echo
fi
if [[ -n
echo
fi
if [[ -z
echo
fi
if [[ -z
echo
fi

"${string+x}" ]]; then
"$string is set, possibly to the empty string"
"${string-x}" ]]; then
"$string is either unset or set to a non-empty string"
"${string+x}" ]]; then
"$string is unset"
"${string-x}" ]]; then
"$string is set to an empty string"

where x is arbitrary. Or in table form:
+-------+-------+-----------+
$string is: | unset | empty | non-empty |
+-----------------------+-------+-------+-----------+
| [[ -z ${string} ]]
| true | true | false
|
| [[ -z ${string+x} ]] | true | false | false
|
| [[ -z ${string-x} ]] | false | true | false
|
| [[ -n ${string} ]]
| false | false | true
|
| [[ -n ${string+x} ]] | false | true | true
|
| [[ -n ${string-x} ]] | true | false | true
|
+-----------------------+-------+-------+-----------+

Alternatively, the state can be checked in a case statement:
case ${var+x$var} in
(x) echo empty;;
("") echo unset;;
(x*[![:blank:]]*) echo non-blank;;
(*) echo blank

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esac

Where [:blank:] is locale specific horizontal spacing characters (tab, space, etc).

Section 22.3: Test on exit status of a command
Exit status 0: success
Exit status other than 0: failure
To test on the exit status of a command:
if command;then
echo 'success'
else
echo 'failure'
fi

Section 22.4: One liner test
You can do things like this:
[[
[[
[[
[[
((

$s
$s
$s
$s
$s

= 'something' ]] && echo 'matched' || echo "didn't match"
== 'something' ]] && echo 'matched' || echo "didn't match"
!= 'something' ]] && echo "didn't match" || echo "matched"
-eq 10 ]] && echo 'equal' || echo "not equal"
== 10 )) && echo 'equal' || echo 'not equal'

One liner test for exit status:
command && echo 'exited with 0' || echo 'non 0 exit'
cmd && cmd1 && echo 'previous cmds were successful' || echo 'one of them failed'
cmd || cmd1 #If cmd fails try cmd1

Section 22.5: File comparison
if [[ $file1 -ef $file2 ]]; then
echo "$file1 and $file2 are the same file"
fi

“Same file” means that modifying one of the files in place affects the other. Two files can be the same even if they
have different names, for example if they are hard links, or if they are symbolic links with the same target, or if one
is a symbolic link pointing to the other.
If two files have the same content, but they are distinct files (so that modifying one does not affect the other), then
-ef reports them as different. If you want to compare two files byte by byte, use the cmp utility.
if cmp -s -- "$file1" "$file2"; then
echo "$file1 and $file2 have identical contents"
else
echo "$file1 and $file2 differ"
fi

To produce a human-readable list of differences between text files, use the diff utility.
if diff -u "$file1" "$file2"; then

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echo "$file1 and $file2 have identical contents"
else
: # the differences between the files have been listed
fi

Section 22.6: File access tests
if [[ -r $filename ]]; then
echo "$filename is a readable file"
fi
if [[ -w $filename ]]; then
echo "$filename is a writable file"
fi
if [[ -x $filename ]]; then
echo "$filename is an executable file"
fi

These tests take permissions and ownership into account to determine whether the script (or programs launched
from the script) can access the file.
Beware of race conditions (TOCTOU): just because the test succeeds now doesn't mean that it's still valid on the
next line. It's usually better to try to access a file, and handle the error, rather than test first and then have to
handle the error anyway in case the file has changed in the meantime.

Section 22.7: Numerical comparisons
Numerical comparisons use the -eq operators and friends
if [[ $num1 -eq $num2 ]]; then
echo "$num1 == $num2"
fi
if [[ $num1 -le $num2 ]]; then
echo "$num1 <= $num2"
fi

There are six numeric operators:
-eq equal
-ne not equal
-le less or equal
-lt less than
-ge greater or equal
-gt greater than

Note that the < and > operators inside [[ … ]] compare strings, not numbers.
if [[ 9 -lt 10 ]]; then
echo "9 is before 10 in numeric order"
fi
if [[ 9 > 10 ]]; then
echo "9 is after 10 in lexicographic order"
fi

The two sides must be numbers written in decimal (or in octal with a leading zero). Alternatively, use the ((…))
arithmetic expression syntax, which performs integer calculations in a C/Java/…-like syntax.

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x=2
if ((2*x == 4)); then
echo "2 times 2 is 4"
fi
((x += 1))
echo "2 plus 1 is $x"

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Chapter 23: Scripting with Parameters
Section 23.1: Multiple Parameter Parsing
To parse lots of parameters, the preferred way of doing this is using a while loop, a case statement, and shift.
shift is used to pop the first parameter in the series, making what used to be $2, now be $1. This is useful for

processing arguments one at a time.
#!/bin/bash
# Load the user defined parameters
while [[ $# > 0 ]]
do
case "$1" in
-a|--valueA)
valA="$2"
shift
;;
-b|--valueB)
valB="$2"
shift
;;
--help|*)
echo
echo
echo
echo
exit
;;

"Usage:"
"
--valueA \"value\""
"
--valueB \"value\""
"
--help"
1

esac
shift
done
echo "A: $valA"
echo "B: $valB"

Inputs and Outputs
$ ./multipleParams.sh --help
Usage:
--valueA "value"
--valueB "value"
--help
$ ./multipleParams.sh
A:
B:
$ ./multipleParams.sh --valueB 2
A:
B: 2
$ ./multipleParams.sh --valueB 2 --valueA "hello world"
A: hello world

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B: 2

Section 23.2: Argument parsing using a for loop
A simple example which provides the options:
Opt

Details
Show help
-v
--version
Show version info
-dr path --doc-root path An option which takes a secondary parameter (a path)
-i
--install
A boolean option (true/false)
-*
-Invalid option
-h

Alt. Opt

--help

#!/bin/bash
dr=''
install=false
skip=false
for op in "$@";do
if $skip;then skip=false;continue;fi
case "$op" in
-v|--version)
echo "$ver_info"
shift
exit 0
;;
-h|--help)
echo "$help"
shift
exit 0
;;
-dr|--doc-root)
shift
if [[ "$1" != "" ]]; then
dr="${1/%\//}"
shift
skip=true
else
echo "E: Arg missing for -dr option"
exit 1
fi
;;
-i|--install)
install=true
shift
;;
-*)
echo "E: Invalid option: $1"
shift
exit 1
;;
esac
done

Section 23.3: Wrapper script
Wrapper script is a script that wraps another script or command to provide extra functionalities or just to make
something less tedious.
For example, the actual egrep in new GNU/Linux system is being replaced by a wrapper script named egrep. This is
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how it looks:
#!/bin/sh
exec grep -E "$@"

So, when you run egrep in such systems, you are actually running grep -E with all the arguments forwarded.
In general case, if you want to run an example script/command exmp with another script mexmp then the wrapper
mexmp script will look like:
#!/bin/sh
exmp "$@" # Add other options before "$@"
# or
#full/path/to/exmp "$@"

Section 23.4: Accessing Parameters
When executing a Bash script, parameters passed into the script are named in accordance to their position: $1 is
the name of the first parameter, $2 is the name of the second parameter, and so on.
A missing parameter simply evaluates to an empty string. Checking for the existence of a parameter can be done as
follows:
if [ -z "$1" ]; then
echo "No argument supplied"
fi

Getting all the parameters
$@ and $* are ways of interacting with all the script parameters. Referencing the Bash man page, we see that:
$*: Expands to the positional parameters, starting from one. When the expansion occurs within double

quotes, it expands to a single word with the value of each parameter separated by the first character of the
IFS special variable.
$@: Expands to the positional parameters, starting from one. When the expansion occurs within double

quotes, each parameter expands to a separate word.
Getting the number of parameters
$# gets the number of parameters passed into a script. A typical use case would be to check if the appropriate

number of arguments are passed:
if [ $# -eq 0 ]; then
echo "No arguments supplied"
fi

Example 1
Loop through all arguments and check if they are files:
for item in "$@"
do
if [[ -f $item ]]; then
echo "$item is a file"
fi
done

Example 2
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90

Loop through all arguments and check if they are files:
for (( i = 1; i <= $#; ++ i ))
do
item=${@:$i:1}
if [[ -f $item ]]; then
echo "$item is a file"
fi
done

Section 23.5: Split string into an array in Bash
Let's say we have a String parameter and we want to split it by comma
my_param="foo,bar,bash"

To split this string by comma we can use;
IFS=',' read -r -a array <<< "$my_param"

Here, IFS is a special variable called Internal field separator which defines the character or characters used to
separate a pattern into tokens for some operations.
To access an individual element:
echo "${array[0]}"

To iterate over the elements:
for element in "${array[@]}"
do
echo "$element"
done

To get both the index and the value:
for index in "${!array[@]}"
do
echo "$index ${array[index]}"
done

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91

Chapter 24: Bash history substitutions
Section 24.1: Quick Reference
Interaction with the history
# List all previous commands
history
# Clear the history, useful if you entered a password by accident
history -c

Event designators
# Expands to line n of bash history
!n
# Expands to last command
!!
# Expands to last command starting with "text"
!text
# Expands to last command containing "text"
!?text
# Expands to command n lines ago
!-n
# Expands to last command with first occurrence of "foo" replaced by "bar"
^foo^bar^
# Expands to the current command
!#

Word designators
These are separated by : from the event designator they refer to. The colon can be omitted if the word designator
doesn't start with a number: !^ is the same as !:^.
# Expands to the first argument of the most recent command
!^
# Expands to the last argument of the most recent command (short for !!:$)
!$
# Expands to the third argument of the most recent command
!:3
# Expands to arguments x through y (inclusive) of the last command
# x and y can be numbers or the anchor characters ^ $
!:x-y
# Expands to all words of the last command except the 0th
# Equivalent to :^-$
!*

Modifiers
These modify the preceding event or word designator.
# Replacement in the expansion using sed syntax

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# Allows flags before the s and alternate separators
:s/foo/bar/ #substitutes bar for first occurrence of foo
:gs|foo|bar| #substitutes bar for all foo
# Remove leading path from last argument ("tail")
:t
# Remove trailing path from last argument ("head")
:h
# Remove file extension from last argument
:r

If the Bash variable HISTCONTROL contains either ignorespace or ignoreboth (or, alternatively, HISTIGNORE contains
the pattern [ ]*), you can prevent your commands from being stored in Bash history by prepending them with a
space:
# This command won't be saved in the history
foo
# This command will be saved
bar

Section 24.2: Repeat previous command with sudo
$ apt-get install r-base
E: Could not open lock file /var/lib/dpkg/lock - open (13: Permission denied)
E: Unable to lock the administration directory (/var/lib/dpkg/), are you root?
$ sudo !!
sudo apt-get install r-base
[sudo] password for <user>:

Section 24.3: Search in the command history by pattern
Press control

r and type a pattern.

For example, if you recently executed man 5 crontab, you can find it quickly by starting to type "crontab". The
prompt will change like this:
(reverse-i-search)`cr': man 5 crontab

The `cr' there is the string I typed so far. This is an incremental search, so as you continue typing, the search result
gets updated to match the most recent command that contained the pattern.
Press the left or right arrow keys to edit the matched command before running it, or the enter key to run the
command.
By default the search finds the most recently executed command matching the pattern. To go further back in the
history press control

r again. You may press it repeatedly until you find the desired command.

Section 24.4: Switch to newly created directory with !#:N
$ mkdir backup_download_directory && cd !#:1
mkdir backup_download_directory && cd backup_download_directory

This will substitute the Nth argument of the current command. In the example !#:1 is replaced with the first
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argument, i.e. backup_download_directory.

Section 24.5: Using !$
You can use the !$ to reduce repetition when using the command line:
$ echo ping
ping
$ echo !$
ping

You can also build upon the repetition
$ echo !$ pong
ping pong
$ echo !$, a great game
pong, a great game

Notice that in the last example we did not get ping pong, a great game because the last argument passed to the
previous command was pong, we can avoid issue like this by adding quotes. Continuing with the example, our last
argument was game:
$ echo "it is !$ time"
it is game time
$ echo "hooray, !$!"
hooray, it is game time!

Section 24.6: Repeat the previous command with a
substitution
$ mplayer Lecture_video_part1.mkv
$ ^1^2^
mplayer Lecture_video_part2.mkv

This command will replace 1 with 2 in the previously executed command. It will only replace the first occurrence of
the string and is equivalent to !!:s/1/2/.
If you want to replace all occurrences, you have to use !!:gs/1/2/ or !!:as/1/2/.

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Chapter 25: Math
Section 25.1: Math using dc
dc is one of the oldest programs on Unix.

It uses reverse polish notation, which means that you first stack numbers, then operations. For example 1+1 is
written as 1 1+.
To print an element from the top of the stack use command p
echo '2 3 + p' | dc
5
or
dc <<< '2 3 + p'
5

You can print the top element many times
dc <<< '1 1 + p 2 + p'
2
4

For negative numbers use _ prefix
dc <<< '_1 p'
-1

You can also use capital letters from A to F for numbers between 10 and 15 and . as a decimal point
dc <<< 'A.4 p'
10.4
dc is using abitrary precision which means that the precision is limited only by the available memory. By default the

precision is set to 0 decimals
dc <<< '4 3 / p'
1

We can increase the precision using command k. 2k will use
dc <<< '2k 4 3 / p'
1.33
dc <<< '4k 4 3 / p'
1.3333

You can also use it over multiple lines
dc << EOF
1 1 +
3 *
p

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EOF
6

bc is a preprocessor for dc.

Section 25.2: Math using bash capabilities
Arithmetic computation can be also done without involving any other programs like this:
Multiplication:
echo $((5 * 2))
10

Division:
echo $((5 / 2))
2

Modulo:
echo $((5 % 2))
1

Exponentiation:
echo $((5 ** 2))
25

Section 25.3: Math using bc
bc is an arbitrary precision calculator language. It could be used interactively or be executed from command line.

For example, it can print out the result of an expression:
echo '2 + 3' | bc
5
echo '12 / 5' | bc
2

For floating-post arithmetic, you can import standard library bc -l:
echo '12 / 5' | bc -l
2.40000000000000000000

It can be used for comparing expressions:
echo '8 > 5' | bc
1
echo '10 == 11' | bc
0

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echo '10 == 10 && 8 > 3' | bc
1

Section 25.4: Math using expr
expr or Evaluate expressions evaluates an expression and writes the result on standard output

Basic arithmetics
expr 2 + 3
5

When multiplying, you need to escape the * sign
expr 2 \* 3
6

You can also use variables
a=2
expr $a + 3
5

Keep in mind that it only supports integers, so expression like this
expr 3.0 / 2

will throw an error expr: not a decimal number: '3.0'.
It supports regular expression to match patterns
expr 'Hello World' : 'Hell\(.*\)rld'
o Wo

Or find the index of the first char in the search string
This will throw expr: syntax error on Mac OS X, because it uses BSD expr which does not have the
index command, while expr on Linux is generally GNU expr

expr index hello l
3
expr index 'hello' 'lo'
3

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Chapter 26: Bash Arithmetic
Parameter
Details
EXPRESSION Expression to evaluate

Section 26.1: Simple arithmetic with (( ))
#!/bin/bash
echo $(( 1 + 2 ))

Output: 3
# Using variables
#!/bin/bash
var1=4
var2=5
((output=$var1 * $var2))
printf "%d\n" "$output"

Output: 20

Section 26.2: Arithmetic command
let
let
let
let
let

num=1+2
num="1+2"
'num= 1 + 2'
num=1 num+=2

You need quotes if there are spaces or globbing characters. So those will get error:
let
let
let
let

num = 1 + 2
'num = 1 + 2'
a[1] = 1 + 1
'a[1] = 1 + 1'

#wrong
#right
#wrong
#right

(( ))
((a=$a+1))
((a = a + 1))
((a += 1))

#add 1 to a
#like above
#like above

We can use (()) in if. Some Example:
if (( a > 1 )); then echo "a is greater than 1"; fi

The output of (()) can be assigned to a variable:
result=$((a + 1))

Or used directly in output:
echo "The result of a + 1 is $((a + 1))"

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Section 26.3: Simple arithmetic with expr
#!/bin/bash
expr 1 + 2

Output: 3

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Chapter 27: Scoping
Section 27.1: Dynamic scoping in action
Dynamic scoping means that variable lookups occur in the scope where a function is called, not where it is defined.
$ x=3
$ func1 () { echo "in func1: $x"; }
$ func2 () { local x=9; func1; }
$ func2
in func1: 9
$ func1
in func1: 3

In a lexically scoped language, func1 would always look in the global scope for the value of x, because func1 is
defined in the local scope.
In a dynamically scoped language, func1 looks in the scope where it is called. When it is called from within func2, it
first looks in the body of func2 for a value of x. If it weren't defined there, it would look in the global scope, where
func2 was called from.

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Chapter 28: Process substitution
Section 28.1: Compare two files from the web
The following compares two files with diff using process substitution instead of creating temporary files.
diff <(curl http://www.example.com/page1) <(curl http://www.example.com/page2)

Section 28.2: Feed a while loop with the output of a command
This feeds a while loop with the output of a grep command:
while IFS=":" read -r user _
do
# "$user" holds the username in /etc/passwd
done < <(grep "hello" /etc/passwd)

Section 28.3: Concatenating files
It is well known that you cannot use the same file for input and output in the same command. For instance,
$ cat header.txt body.txt >body.txt

doesn’t do what you want. By the time cat reads body.txt, it has already been truncated by the redirection and it is
empty. The final result will be that body.txt will hold the contents of header.txt only.
One might think to avoid this with process substitution, that is, that the command
$ cat header.txt <(cat body.txt) > body.txt

will force the original contents of body.txt to be somehow saved in some buffer somewhere before the file is
truncated by the redirection. It doesn’t work. The cat in parentheses begins reading the file only after all file
descriptors have been set up, just like the outer one. There is no point in trying to use process substitution in this
case.
The only way to prepend a file to another file is to create an intermediate one:
$ cat header.txt body.txt >body.txt.new
$ mv body.txt.new body.txt

which is what sed or perl or similar programs do under the carpet when called with an edit-in-place option (usually
-i).

Section 28.4: Stream a file through multiple programs at
once
This counts the number of lines in a big file with wc -l while simultaneously compressing it with gzip. Both run
concurrently.
tee >(wc -l >&2) < bigfile | gzip > bigfile.gz

Normally tee writes its input to one or more files (and stdout). We can write to commands instead of files with tee
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>(command).

Here the command wc -l >&2 counts the lines read from tee (which in turn is reading from bigfile). (The line
count is sent to stderr (>&2) to avoid mixing with the input to gzip.) The stdout of tee is simultaneously fed into
gzip.

Section 28.5: With paste command
# Process substitution with paste command is common
# To compare the contents of two directories
paste <( ls /path/to/directory1 ) <( ls /path/to/directory2 )

Section 28.6: To avoid usage of a sub-shell
One major aspect of process substitution is that it lets us avoid usage of a sub-shell when piping commands from
the shell.
This can be demonstrated with a simple example below. I have the following files in my current folder:
$ find . -maxdepth 1 -type f -print
foo bar zoo foobar foozoo barzoo

If I pipe to a while/read loop that increments a counter as follows:
count=0
find . -maxdepth 1 -type f -print | while IFS= read -r _; do
((count++))
done
$count now does not contain 6, because it was modified in the sub-shell context. Any of the commands shown

below are run in a sub-shell context and the scope of the variables used within are lost after the sub-shell
terminates.
command &
command | command
( command )

Process substitution will solve the problem by avoiding use the of pipe | operator as in
count=0
while IFS= read -r _; do
((count++))
done < <(find . -maxdepth 1 -type f -print)

This will retain the count variable value as no sub-shells are invoked.

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Chapter 29: Programmable completion
Section 29.1: Simple completion using function
_mycompletion() {
local command_name="$1" # not used in this example
local current_word="$2"
local previous_word="$3" # not used in this example
# COMPREPLY is an array which has to be filled with the possible completions
# compgen is used to filter matching completions
COMPREPLY=( $(compgen -W 'hello world' -- "$current_word") )
}
complete -F _mycompletion mycommand

Usage Example:
$ mycommand [TAB][TAB]
hello world
$ mycommand h[TAB][TAB]
$ mycommand hello

Section 29.2: Simple completion for options and filenames
# The following shell function will be used to generate completions for
# the "nuance_tune" command.
_nuance_tune_opts ()
{
local curr_arg prev_arg
curr_arg=${COMP_WORDS[COMP_CWORD]}
prev_arg=${COMP_WORDS[COMP_CWORD-1]}
# The "config" option takes a file arg, so get a list of the files in the
# current dir. A case statement is probably unnecessary here, but leaves
# room to customize the parameters for other flags.
case "$prev_arg" in
-config)
COMPREPLY=( $( /bin/ls -1 ) )
return 0
;;
esac
# Use compgen to provide completions for all known options.
COMPREPLY=( $(compgen -W '-analyze -experiment -generate_groups -compute_thresh -config -output
-help -usage -force -lang -grammar_overrides -begin_date -end_date -group -dataset -multiparses dump_records -no_index -confidencelevel -nrecs -dry_run -rec_scripts_only -save_temp -full_trc single_session -verbose -ep -unsupervised -write_manifest -remap -noreparse -upload -reference target -use_only_matching -histogram -stepsize' -- $curr_arg ) );
}
# The -o parameter tells Bash to process completions as filenames, where applicable.

complete -o filenames -F _nuance_tune_opts nuance_tune

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Chapter 30: Customizing PS1
Section 30.1: Colorize and customize terminal prompt
This is how the author sets their personal PS1 variable:
gitPS1(){
gitps1=$(git branch 2>/dev/null | grep '*')
gitps1="${gitps1:+ (${gitps1/#\* /})}"
echo "$gitps1"
}
#Please use the below function if you are a mac user
gitPS1ForMac(){
git branch 2> /dev/null | sed -e '/^[^*]/d' -e 's/* \(.*\)/ (\1)/'
}
timeNow(){
echo "$(date +%r)"
}
if [ "$color_prompt" = yes ]; then
if [ x$EUID = x0 ]; then
PS1='\[\033[1;38m\][$(timeNow)]\[\033[00m\]
\[\033[1;31m\]\u\[\033[00m\]\[\033[1;37m\]@\[\033[00m\]\[\033[1;33m\]\h\[\033[00m\]
\[\033[1;34m\]\w\[\033[00m\]\[\033[1;36m\]$(gitPS1)\[\033[00m\] \[\033[1;31m\]:/#\[\033[00m\] '
else
PS1='\[\033[1;38m\][$(timeNow)]\[\033[00m\]
\[\033[1;32m\]\u\[\033[00m\]\[\033[1;37m\]@\[\033[00m\]\[\033[1;33m\]\h\[\033[00m\]
\[\033[1;34m\]\w\[\033[00m\]\[\033[1;36m\]$(gitPS1)\[\033[00m\] \[\033[1;32m\]:/$\[\033[00m\] '
fi
else
PS1='[$(timeNow)] \u@\h \w$(gitPS1) :/$ '
fi

And this is how my prompt looks like:

Color reference:
# Colors
txtblk='\e[0;30m'
txtred='\e[0;31m'
txtgrn='\e[0;32m'
txtylw='\e[0;33m'
txtblu='\e[0;34m'
txtpur='\e[0;35m'
txtcyn='\e[0;36m'
txtwht='\e[0;37m'
bldblk='\e[1;30m'
bldred='\e[1;31m'
bldgrn='\e[1;32m'
bldylw='\e[1;33m'
bldblu='\e[1;34m'
bldpur='\e[1;35m'
bldcyn='\e[1;36m'

#
#
#
#
#
#
#
#
#
#
#
#
#
#
#

Black - Regular
Red
Green
Yellow
Blue
Purple
Cyan
White
Black - Bold
Red
Green
Yellow
Blue
Purple
Cyan

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bldwht='\e[1;37m'
unkblk='\e[4;30m'
undred='\e[4;31m'
undgrn='\e[4;32m'
undylw='\e[4;33m'
undblu='\e[4;34m'
undpur='\e[4;35m'
undcyn='\e[4;36m'
undwht='\e[4;37m'
bakblk='\e[40m'
bakred='\e[41m'
badgrn='\e[42m'
bakylw='\e[43m'
bakblu='\e[44m'
bakpur='\e[45m'
bakcyn='\e[46m'
bakwht='\e[47m'
txtrst='\e[0m'

#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#

White
Black - Underline
Red
Green
Yellow
Blue
Purple
Cyan
White
Black - Background
Red
Green
Yellow
Blue
Purple
Cyan
White
Text Reset

Notes:
Make the changes in ~/.bashrc or /etc/bashrc or ~/.bash_profile or ~./profile file (depending on the
OS) and save it.
For root you might also need to edit the /etc/bash.bashrc or /root/.bashrc file
Run source ~/.bashrc (distro specific) after saving the file.
Note: if you have saved the changes in ~/.bashrc, then remember to add source ~/.bashrc in your
~/.bash_profile so that this change in PS1 will be recorded every time the Terminal application starts.

Section 30.2: Show git branch name in terminal prompt
You can have functions in the PS1 variable, just make sure to single quote it or use escape for special chars:
gitPS1(){
gitps1=$(git branch 2>/dev/null | grep '*')
gitps1="${gitps1:+ (${gitps1/#\* /})}"
echo "$gitps1"
}
PS1='\u@\h:\w$(gitPS1)$ '

It will give you a prompt like this:
user@Host:/path (master)$

Notes:
Make the changes in ~/.bashrc or /etc/bashrc or ~/.bash_profile or ~./profile file (depending on the
OS) and save it.
Run source ~/.bashrc (distro specific) after saving the file.

Section 30.3: Show time in terminal prompt
timeNow(){
echo "$(date +%r)"
}

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105

PS1='[$(timeNow)] \u@\h:\w$ '

It will give you a prompt like this:
[05:34:37 PM] user@Host:/path$

Notes:
Make the changes in ~/.bashrc or /etc/bashrc or ~/.bash_profile or ~./profile file (depending on the
OS) and save it.
Run source ~/.bashrc (distro specific) after saving the file.

Section 30.4: Show a git branch using PROMPT_COMMAND
If you are inside a folder of a git repository it might be nice to show the current branch you are on. In ~/.bashrc or
/etc/bashrc add the following (git is required for this to work):
function prompt_command {
# Check if we are inside a git repository
if git status > /dev/null 2>&1; then
# Only get the name of the branch
export GIT_STATUS=$(git status | grep 'On branch' | cut -b 10-)
else
export GIT_STATUS=""
fi
}
# This function gets called every time PS1 is shown
PROMPT_COMMAND=prompt_command
PS1="\$GIT_STATUS \u@\h:\w\$ "

If we are in a folder inside a git repository this will output:
branch user@machine:~$
And if we are inside a normal folder:
user@machine:~$

Section 30.5: Change PS1 prompt
To change PS1, you just have to change the value of PS1 shell variable. The value can be set in ~/.bashrc or
/etc/bashrc file, depending on the distro. PS1 can be changed to any plain text like:
PS1="hello "

Besides the plain text, a number of backslash-escaped special characters are supported:
Format

Action

an ASCII bell character (07)
the date in “Weekday Month Date” format (e.g., “Tue May 26”)
the format is passed to strftime(3) and the result is inserted into the prompt string; an empty format
\D{format}
results in a locale-specific time representation. The braces are required
\a
\d

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\e
\h
\H
\j
\l
\n
\r
\s
\t
\T
\@
\A
\u
\v
\V
\w
\W
\!
\#
\$
\nnn*
\
\[
\]

an ASCII escape character (033)
the hostname up to the first ‘.’
the hostname
the number of jobs currently managed by the shell
the basename of the shell’s terminal device name
newline
carriage return
the name of the shell, the basename of $0 (the portion following the final slash)
the current time in 24-hour HH:MM:SS format
the current time in 12-hour HH:MM:SS format
the current time in 12-hour am/pm format
the current time in 24-hour HH:MM format
the username of the current user
the version of bash (e.g., 2.00)
the release of bash, version + patch level (e.g., 2.00.0)
the current working directory, with $HOME abbreviated with a tilde
the basename of the current working directory, with $HOME abbreviated with a tilde
the history number of this command
the command number of this command
if the effective UID is 0, a #, otherwise a $
the character corresponding to the octal number nnn
a backslash
begin a sequence of non-printing characters, which could be used to embed a terminal control
sequence into the prompt
end a sequence of non-printing characters

So for example, we can set PS1 to:
PS1="\u@\h:\w\$ "

And it will output:
user@machine:~$

Section 30.6: Show previous command return status and time
Sometimes we need a visual hint to indicate the return status of previous command. The following snippet make
put it at the head of the PS1.
Note that the __stat() function should be called every time a new PS1 is generated, or else it would stick to the
return status of last command of your .bashrc or .bash_profile.
# -ANSI-COLOR-CODES- #
Color_Off="\033[0m"
###-Regular-###
Red="\033[0;31m"
Green="\033[0;32m"
Yellow="\033[0;33m"
####-Bold-####
function __stat() {
if [ $? -eq 0 ]; then
echo -en "$Green ? $Color_Off "
else
echo -en "$Red ? $Color_Off "

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fi
}
PS1='$(__stat)'
PS1+="[\t] "
PS1+="\e[0;33m\u@\h\e[0m:\e[1;34m\w\e[0m \n$ "
export PS1

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Chapter 31: Brace Expansion
Section 31.1: Modifying filename extension
$ mv filename.{jar,zip}

This expands into mv filename.jar filename.zip .

Section 31.2: Create directories to group files by month and
year
$ mkdir 20{09..11}-{01..12}

Entering the ls command will show that the following directories were created:
2009-01 2009-04 2009-07 2009-10 2010-01 2010-04 2010-07 2010-10 2011-01 2011-04 2011-07 2011-10
2009-02 2009-05 2009-08 2009-11 2010-02 2010-05 2010-08 2010-11 2011-02 2011-05 2011-08 2011-11
2009-03 2009-06 2009-09 2009-12 2010-03 2010-06 2010-09 2010-12 2011-03 2011-06 2011-09 2011-12

Putting a 0 in front of 9 in the example ensures the numbers are padded with a single 0. You can also pad numbers
with multiple zeros, for example:
$ echo {001..10}
001 002 003 004 005 006 007 008 009 010

Section 31.3: Create a backup of dotfiles
$ cp .vimrc{,.bak}

This expands into the command cp .vimrc .vimrc.bak.

Section 31.4: Use increments
$ echo {0..10..2}
0 2 4 6 8 10

A third parameter to specify an increment, i.e. {start..end..increment}
Using increments is not constrained to just numbers
$ for c in {a..z..5}; do echo -n $c; done
afkpuz

Section 31.5: Using brace expansion to create lists
Bash can easily create lists from alphanumeric characters.
# list from a to z
$ echo {a..z}
a b c d e f g h i j k l m n o p q r s t u v w x y z
# reverse from z to a
$ echo {z..a}

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z y x w v u t s r q p o n m l k j i h g f e d c b a
# digits
$ echo {1..20}
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
# with leading zeros
$ echo {01..20}
01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20
# reverse digit
$ echo {20..1}
20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
# reversed with leading zeros
$ echo {20..01}
20 19 18 17 16 15 14 13 12 11 10 09 08 07 06 05 04 03 02 01
# combining multiple braces
$ echo {a..d}{1..3}
a1 a2 a3 b1 b2 b3 c1 c2 c3 d1 d2 d3

Brace expansion is the very first expansion that takes place, so it cannot be combined with any other expansions.
Only chars and digits can be used.
This won't work: echo {$(date +$H)..24}

Section 31.6: Make Multiple Directories with Sub-Directories
mkdir -p toplevel/sublevel_{01..09}/{child1,child2,child3}

This will create a top level folder called toplevel, nine folders inside of toplevel named sublevel_01, sublevel_02,
etc. Then inside of those sublevels: child1, child2, child3 folders, giving you:
toplevel/sublevel_01/child1
toplevel/sublevel_01/child2
toplevel/sublevel_01/child3
toplevel/sublevel_02/child1

and so on. I find this very useful for creating multiple folders and sub folders for my specific purposes, with one
bash command. Substitute variables to help automate/parse information given to the script.

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Chapter 32: getopts : smart positionalparameter parsing
Parameter
Detail
optstring The option characters to be recognized
name
Then name where parsed option is stored

Section 32.1: pingnmap
#!/bin/bash
# Script name : pingnmap
# Scenario : The systems admin in company X is tired of the monotonous job
# of pinging and nmapping, so he decided to simplify the job using a script.
# The tasks he wish to achieve is
# 1. Ping - with a max count of 5 -the given IP address/domain. AND/OR
# 2. Check if a particular port is open with a given IP address/domain.
# And getopts is for her rescue.
# A brief overview of the options
# n : meant for nmap
# t : meant for ping
# i : The option to enter the IP address
# p : The option to enter the port
# v : The option to get the script version

while getopts ':nti:p:v' opt
#putting : in the beginnnig suppresses the errors for invalid options
do
case "$opt" in
'i')ip="${OPTARG}"
;;
'p')port="${OPTARG}"
;;
'n')nmap_yes=1;
;;
't')ping_yes=1;
;;
'v')echo "pingnmap version 1.0.0"
;;
*) echo "Invalid option $opt"
echo "Usage : "
echo "pingmap -[n|t[i|p]|v]"
;;
esac
done
if [ ! -z "$nmap_yes" ] && [ "$nmap_yes" -eq "1" ]
then
if [ ! -z "$ip" ] && [ ! -z "$port" ]
then
nmap -p "$port" "$ip"
fi
fi
if [ ! -z "$ping_yes" ] && [ "$ping_yes" -eq "1" ]
then
if [ ! -z "$ip" ]
then
ping -c 5 "$ip"
fi

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111

fi
shift $(( OPTIND - 1 )) # Processing additional arguments
if [ ! -z "$@" ]
then
echo "Bogus arguments at the end : $@"
fi

Output
$ ./pingnmap -nt -i google.com -p 80
Starting Nmap 6.40 ( http://nmap.org ) at 2016-07-23 14:31 IST
Nmap scan report for google.com (216.58.197.78)
Host is up (0.034s latency).
rDNS record for 216.58.197.78: maa03s21-in-f14.1e100.net
PORT
STATE SERVICE
80/tcp open http
Nmap done: 1 IP address (1 host up) scanned in 0.22 seconds
PING google.com (216.58.197.78) 56(84) bytes of data.
64 bytes from maa03s21-in-f14.1e100.net (216.58.197.78): icmp_seq=1
64 bytes from maa03s21-in-f14.1e100.net (216.58.197.78): icmp_seq=2
64 bytes from maa03s21-in-f14.1e100.net (216.58.197.78): icmp_seq=3
64 bytes from maa03s21-in-f14.1e100.net (216.58.197.78): icmp_seq=4
64 bytes from maa03s21-in-f14.1e100.net (216.58.197.78): icmp_seq=5

ttl=57
ttl=57
ttl=57
ttl=57
ttl=57

time=29.3
time=30.9
time=34.7
time=39.6
time=32.7

ms
ms
ms
ms
ms

--- google.com ping statistics --5 packets transmitted, 5 received, 0% packet loss, time 4007ms
rtt min/avg/max/mdev = 29.342/33.481/39.631/3.576 ms
$ ./pingnmap -v
pingnmap version 1.0.0
$ ./pingnmap -h
Invalid option ?
Usage :
pingmap -[n|t[i|p]|v]
$ ./pingnmap -v
pingnmap version 1.0.0
$ ./pingnmap -h
Invalid option ?
Usage :
pingmap -[n|t[i|p]|v]

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112

Chapter 33: Debugging
Section 33.1: Checking the syntax of a script with "-n"
The -n flag enables you to check the syntax of a script without having to execute it:
~> $ bash -n testscript.sh
testscript.sh: line 128: unexpected EOF while looking for matching `"'
testscript.sh: line 130: syntax error: unexpected end of file

Section 33.2: Debugging using bashdb
Bashdb is a utility that is similar to gdb, in that you can do things like set breakpoints at a line or at a function, print
content of variables, you can restart script execution and more.
You can normally install it via your package manager, for example on Fedora:
sudo dnf install bashdb

Or get it from the homepage. Then you can run it with your script as a paramater:
bashdb <YOUR SCRIPT>

Here are a few commands to get you started:
l - show local lines, press l again to scroll down
s - step to next line
print $VAR - echo out content of variable
restart - reruns bashscript, it re-loads it prior to execution.
eval - evaluate some custom command, ex: eval echo hi
b set breakpoint on some line
c - continue till some breakpoint
i b - info on break points
d - delete breakpoint at line #
shell - launch a sub-shell in the middle of execution, this is handy for manipulating variables

For more information, I recommend consulting the manual:
http://www.rodericksmith.plus.com/outlines/manuals/bashdbOutline.html
See also homepage:
http://bashdb.sourceforge.net/

Section 33.3: Debugging a bash script with "-x"
Use "-x" to enable debug output of executed lines. It can be run on an entire session or script, or enabled
programmatically within a script.
Run a script with debug output enabled:
$ bash -x myscript.sh

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113

Or
$ bash --debug myscript.sh

Turn on debugging within a bash script. It may optionally be turned back on, though debug output is automatically
reset when the script exits.
#!/bin/bash
set -x
# Enable debugging
# some code here
set +x
# Disable debugging output.

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114

Chapter 34: Pattern matching and regular
expressions
Section 34.1: Get captured groups from a regex match
against a string
a='I am a simple string with digits 1234'
pat='(.*) ([0-9]+)'
[[ "$a" =~ $pat ]]
echo "${BASH_REMATCH[0]}"
echo "${BASH_REMATCH[1]}"
echo "${BASH_REMATCH[2]}"

Output:
I am a simple string with digits 1234
I am a simple string with digits
1234

Section 34.2: Behaviour when a glob does not match anything
Preparation
$ mkdir globbing
$ cd globbing
$ mkdir -p folder/{sub,another}folder/content/deepfolder/
touch macy stacy tracy "file with space" folder/{sub,another}folder/content/deepfolder/file
.hiddenfile
$ shopt -u nullglob
$ shopt -u failglob
$ shopt -u dotglob
$ shopt -u nocaseglob
$ shopt -u extglob
$ shopt -u globstar

In case the glob does not match anything the result is determined by the options nullglob and failglob. If neither
of them are set, Bash will return the glob itself if nothing is matched
$ echo no*match
no*match

If nullglob is activated then nothing (null) is returned:
$ shopt -s nullglob
$ echo no*match
$

If failglob is activated then an error message is returned:
$ shopt -s failglob
$ echo no*match
bash: no match: no*match
$

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115

Notice, that the failglob option supersedes the nullglob option, i.e., if nullglob and failglob are both set, then in case of no match - an error is returned.

Section 34.3: Check if a string matches a regular expression
Version ≥ 3.0

Check if a string consists in exactly 8 digits:
$ date=20150624
$ [[ $date =~ ^[0-9]{8}$ ]] && echo "yes" || echo "no"
yes
$ date=hello
$ [[ $date =~ ^[0-9]{8}$ ]] && echo "yes" || echo "no"
no

Section 34.4: Regex matching
pat='[^0-9]+([0-9]+)'
s='I am a string with some digits 1024'
[[ $s =~ $pat ]] # $pat must be unquoted
echo "${BASH_REMATCH[0]}"
echo "${BASH_REMATCH[1]}"

Output:
I am a string with some digits 1024
1024

Instead of assigning the regex to a variable ($pat) we could also do:
[[ $s =~ [^0-9]+([0-9]+) ]]

Explanation
The [[ $s =~ $pat ]] construct performs the regex matching
The captured groups i.e the match results are available in an array named BASH_REMATCH
The 0th index in the BASH_REMATCH array is the total match
The i'th index in the BASH_REMATCH array is the i'th captured group, where i = 1, 2, 3 ...

Section 34.5: The * glob
Preparation
$ mkdir globbing
$ cd globbing
$ mkdir -p folder/{sub,another}folder/content/deepfolder/
touch macy stacy tracy "file with space" folder/{sub,another}folder/content/deepfolder/file
.hiddenfile
$ shopt -u nullglob
$ shopt -u failglob
$ shopt -u dotglob
$ shopt -u nocaseglob
$ shopt -u extglob
$ shopt -u globstar

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116

The asterisk * is probably the most commonly used glob. It simply matches any String
$ echo *acy
macy stacy tracy

A single * will not match files and folders that reside in subfolders
$ echo *
emptyfolder folder macy stacy tracy
$ echo folder/*
folder/anotherfolder folder/subfolder

Section 34.6: The ** glob
Version ≥ 4.0

Preparation
$ mkdir globbing
$ cd globbing
$ mkdir -p folder/{sub,another}folder/content/deepfolder/
touch macy stacy tracy "file with space" folder/{sub,another}folder/content/deepfolder/file
.hiddenfile
$ shopt -u nullglob
$ shopt -u failglob
$ shopt -u dotglob
$ shopt -u nocaseglob
$ shopt -u extglob
$ shopt -s globstar

Bash is able to interpret two adjacent asterisks as a single glob. With the globstar option activated this can be used
to match folders that reside deeper in the directory structure
echo **
emptyfolder folder folder/anotherfolder folder/anotherfolder/content
folder/anotherfolder/content/deepfolder folder/anotherfolder/content/deepfolder/file
folder/subfolder folder/subfolder/content folder/subfolder/content/deepfolder
folder/subfolder/content/deepfolder/file macy stacy tracy

The ** can be thought of a path expansion, no matter how deep the path is. This example matches any file or
folder that starts with deep, regardless of how deep it is nested:
$ echo **/deep*
folder/anotherfolder/content/deepfolder folder/subfolder/content/deepfolder

Section 34.7: The ? glob
Preparation
$ mkdir globbing
$ cd globbing
$ mkdir -p folder/{sub,another}folder/content/deepfolder/
touch macy stacy tracy "file with space" folder/{sub,another}folder/content/deepfolder/file
.hiddenfile
$ shopt -u nullglob
$ shopt -u failglob
$ shopt -u dotglob

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117

$ shopt -u nocaseglob
$ shopt -u extglob
$ shopt -u globstar

The ? simply matches exactly one character
$ echo ?acy
macy
$ echo ??acy
stacy tracy

Section 34.8: The [ ] glob
Preparation
$ mkdir globbing
$ cd globbing
$ mkdir -p folder/{sub,another}folder/content/deepfolder/
touch macy stacy tracy "file with space" folder/{sub,another}folder/content/deepfolder/file
.hiddenfile
$ shopt -u nullglob
$ shopt -u failglob
$ shopt -u dotglob
$ shopt -u nocaseglob
$ shopt -u extglob
$ shopt -u globstar

If there is a need to match specific characters then '[]' can be used. Any character inside '[]' will be matched exactly
once.
$ echo [m]acy
macy
$ echo [st][tr]acy
stacy tracy

The [] glob, however, is more versatile than just that. It also allows for a negative match and even matching ranges
of characters and character classes. A negative match is achieved by using ! or ^ as the first character following [.
We can match stacy by
$ echo [!t][^r]acy
stacy

Here we are telling bash the we want to match only files which do not not start with a t and the second letter is not
an r and the file ends in acy.
Ranges can be matched by seperating a pair of characters with a hyphen (-). Any character that falls between those
two enclosing characters - inclusive - will be matched. E.g., [r-t] is equivalent to [rst]
$ echo [r-t][r-t]acy
stacy tracy

Character classes can be matched by [:class:], e.g., in order to match files that contain a whitespace
$ echo *[[:blank:]]*
file with space

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118

Section 34.9: Matching hidden files
Preparation
$ mkdir globbing
$ cd globbing
$ mkdir -p folder/{sub,another}folder/content/deepfolder/
touch macy stacy tracy "file with space" folder/{sub,another}folder/content/deepfolder/file
.hiddenfile
$ shopt -u nullglob
$ shopt -u failglob
$ shopt -u dotglob
$ shopt -u nocaseglob
$ shopt -u extglob
$ shopt -u globstar

The Bash built-in option dotglob allows to match hidden files and folders, i.e., files and folders that start with a .
$ shopt -s dotglob
$ echo *
file with space folder .hiddenfile macy stacy tracy

Section 34.10: Case insensitive matching
Preparation
$ mkdir globbing
$ cd globbing
$ mkdir -p folder/{sub,another}folder/content/deepfolder/
touch macy stacy tracy "file with space" folder/{sub,another}folder/content/deepfolder/file
.hiddenfile
$ shopt -u nullglob
$ shopt -u failglob
$ shopt -u dotglob
$ shopt -u nocaseglob
$ shopt -u extglob
$ shopt -u globstar

Setting the option nocaseglob will match the glob in a case insensitive manner
$ echo M*
M*
$ shopt -s nocaseglob
$ echo M*
macy

Section 34.11: Extended globbing
Version ≥ 2.02

Preparation
$ mkdir globbing
$ cd globbing
$ mkdir -p folder/{sub,another}folder/content/deepfolder/
touch macy stacy tracy "file with space" folder/{sub,another}folder/content/deepfolder/file
.hiddenfile
$ shopt -u nullglob

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119

$
$
$
$
$

shopt
shopt
shopt
shopt
shopt

-u
-u
-u
-u
-u

failglob
dotglob
nocaseglob
extglob
globstar

Bash's built-in extglob option can extend a glob's matching capabilities
shopt -s extglob

The following sub-patterns comprise valid extended globs:
?(pattern-list) – Matches zero or one occurrence of the given patterns
*(pattern-list) – Matches zero or more occurrences of the given patterns
+(pattern-list) – Matches one or more occurrences of the given patterns
@(pattern-list) – Matches one of the given patterns
!(pattern-list) – Matches anything except one of the given patterns

The pattern-list is a list of globs separated by |.
$ echo *([r-t])acy
stacy tracy
$ echo *([r-t]|m)acy
macy stacy tracy
$ echo ?([a-z])acy
macy

The pattern-list itself can be another, nested extended glob. In the above example we have seen that we can
match tracy and stacy with *(r-t). This extended glob itself can be used inside the negated extended glob
!(pattern-list) in order to match macy
$ echo !(*([r-t]))acy
macy

It matches anything that does not start with zero or more occurrences of the letters r, s and t, which leaves only
macy as possible match.

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120

Chapter 35: Change shell
Section 35.1: Find the current shell
There are a few ways to determine the current shell
echo $0
ps -p $$
echo $SHELL

Section 35.2: List available shells
To list available login shells :
cat /etc/shells

Example:
$ cat /etc/shells
# /etc/shells: valid login shells
/bin/sh
/bin/dash
/bin/bash
/bin/rbash

Section 35.3: Change the shell
To change the current bash run these commands
export SHELL=/bin/bash
exec /bin/bash

to change the bash that opens on startup edit .profile and add those lines

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121

Chapter 36: Internal variables
An overview of Bash's internal variables, where, how, and when to use them.

Section 36.1: Bash internal variables at a glance
Variable

$* / $@

Details
Function/script positional parameters (arguments). Expand as follows:
$* and $@ are the same as $1 $2 ... (note that it generally makes no sense to leave those

unquoted)
"$*" is the same as "$1 $2 ..." 1
"$@" is the same as "$1" "$2" ...
1. Arguments are separated by the first character of $IFS, which does not have to be a space.
$#
Number of positional parameters passed to the script or function
Process ID of the last (righ-most for pipelines) command in the most recently job put into the
$!
background (note that it's not necessarily the same as the job's process group ID when job control
is enabled)
$$
ID of the process that executed bash
$?
Exit status of the last command
$n
Positional parameters, where n=1, 2, 3, ..., 9
${n}
Positional parameters (same as above), but n can be > 9
In scripts, path with which the script was invoked; with bash -c 'printf "%s\n" "$0"' name
$0
args': name (the first argument after the inline script), otherwise, the argv[0] that bash received.
$_
Last field of the last command
$IFS
Internal field separator
$PATH
PATH environment variable used to look-up executables
$OLDPWD
Previous working directory
$PWD
Present working directory
$FUNCNAME
Array of function names in the execution call stack
$BASH_SOURCE Array containing source paths for elements in FUNCNAME array. Can be used to get the script path.
$BASH_ALIASES Associative array containing all currently defined aliases
$BASH_REMATCH Array of matches from the last regex match
$BASH_VERSION Bash version string
$BASH_VERSINFO An array of 6 elements with Bash version information
Absolute path to the currently executing Bash shell itself (heuristically determined by bash based
$BASH
on argv[0] and the value of $PATH; may be wrong in corner cases)
$BASH_SUBSHELL Bash subshell level
$UID
Real (not effective if different) User ID of the process running bash
$PS1
Primary command line prompt; see Using the PS* Variables
$PS2
Secondary command line prompt (used for additional input)
$PS3
Tertiary command line prompt (used in select loop)
$PS4
Quaternary command line prompt (used to append info with verbose output)
$RANDOM
A pseudo random integer between 0 and 32767
Variable used by read by default when no variable is specified. Also used by SELECT to return the
$REPLY
user-supplied value
Array variable that holds the exit status values of each command in the most recently executed
$PIPESTATUS
foreground pipeline.
Variable Assignment must have no space before and after. a=123 not a = 123. The latter (an equal sign
surrounded by spaces) in isolation means run the command a with the arguments = and 123, though it is
also seen in the string comparison operator (which syntactically is an argument to [ or [[ or whichever
test you are using).

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122

Section 36.2: $@
"$@" expands to all of the command line arguments as separate words. It is different from "$*", which expands to

all of the arguments as a single word.
"$@" is especially useful for looping through arguments and handling arguments with spaces.

Consider we are in a script that we invoked with two arguments, like so:
$ ./script.sh "␣1␣2␣" "␣3␣␣4␣"

The variables $* or $@ will expand into $1␣$2, which in turn expand into 1␣2␣3␣4 so the loop below:
for var in $*; do # same for var in $@; do
echo \\<"$var"\\>
done

will print for both
<1>
<2>
<3>
<4>

While "$*" will be expanded into "$1␣$2" which will in turn expand into "␣1␣2␣␣␣3␣␣4␣" and so the loop:
for var in "$*"; do
echo \\<"$var"\\>
done

will only invoke echo once and will print
<␣1␣2␣␣␣3␣␣4␣>

And finally "$@" will expand into "$1" "$2", which will expand into "␣1␣2␣" "␣3␣␣4␣" and so the loop
for var in "$@"; do
echo \\<"$var"\\>
done

will print
<␣1␣2␣>
<␣3␣␣4␣>

thereby preserving both the internal spacing in the arguments and the arguments separation. Note that the
construction for var in "$@"; do ... is so common and idiomatic that it is the default for a for loop and can be
shortened to for var; do ....

Section 36.3: $#
To get the number of command line arguments or positional parameters - type:
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#!/bin/bash
echo "$#"

When run with three arguments the example above will result with the output:
~> $ ./testscript.sh firstarg secondarg thirdarg
3

Section 36.4: $HISTSIZE
The maximum number of remembered commands:
~> $ echo $HISTSIZE
1000

Section 36.5: $FUNCNAME
To get the name of the current function - type:
my_function()
{
echo "This function is $FUNCNAME"
}

# This will output "This function is my_function"

This instruction will return nothing if you type it outside the function:
my_function
echo "This function is $FUNCNAME"

# This will output "This function is"

Section 36.6: $HOME
The home directory of the user
~> $ echo $HOME
/home/user

Section 36.7: $IFS
Contains the Internal Field Separator string that bash uses to split strings when looping etc. The default is the white
space characters: \n (newline), \t (tab) and space. Changing this to something else allows you to split strings using
different characters:
IFS=","
INPUTSTR="a,b,c,d"
for field in ${INPUTSTR}; do
echo $field
done

The output of the above is:
a
b
c

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d

Notes:
This is responsible for the phenomenon known as word splitting.

Section 36.8: $OLDPWD
OLDPWD (OLDPrintWorkingDirectory) contains directory before the last cd command:
~> $ cd directory
directory> $ echo $OLDPWD
/home/user

Section 36.9: $PWD
PWD (PrintWorkingDirectory) The current working directory you are in at the moment:
~> $ echo $PWD
/home/user
~> $ cd directory
directory> $ echo $PWD
/home/user/directory

Section 36.10: $1 $2 $3 etc..
Positional parameters passed to the script from either the command line or a function:
#!/bin/bash
# $n is the n'th positional parameter
echo "$1"
echo "$2"
echo "$3"

The output of the above is:
~> $ ./testscript.sh firstarg secondarg thirdarg
firstarg
secondarg
thirdarg

If number of positional argument is greater than nine, curly braces must be used.
# "set -- " sets positional parameters
set -- 1 2 3 4 5 6 7 8 nine ten eleven twelve
# the following line will output 10 not 1 as the value of $1 the digit 1
# will be concatenated with the following 0
echo $10
# outputs 1
echo ${10} # outputs ten
# to show this clearly:
set -- arg{1..12}
echo $10
echo ${10}

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Section 36.11: $*
Will return all of the positional parameters in a single string.
testscript.sh:
#!/bin/bash
echo "$*"

Run the script with several arguments:
./testscript.sh firstarg secondarg thirdarg

Output:
firstarg secondarg thirdarg

Section 36.12: $!
The Process ID (pid) of the last job run in the background:
~> $ ls &
testfile1 testfile2
[1]+ Done
~> $ echo $!
21715

ls

Section 36.13: $?
The exit status of the last executed function or command. Usually 0 will mean OK anything else will indicate a
failure:
~> $ ls *.blah;echo $?
ls: cannot access *.blah: No such file or directory
2
~> $ ls;echo $?
testfile1 testfile2
0

Section 36.14: $$
The Process ID (pid) of the current process:
~> $ echo $$
13246

Section 36.15: $RANDOM
Each time this parameter is referenced, a random integer between 0 and 32767 is generated. Assigning a value to
this variable seeds the random number generator (source).
~> $ echo $RANDOM
27119
~> $ echo $RANDOM

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1349

Section 36.16: $BASHPID
Process ID (pid) of the current instance of Bash. This is not the same as the $$ variable, but it often gives the same
result. This is new in Bash 4 and doesn't work in Bash 3.
~> $ echo "\$\$ pid = $$ BASHPID = $BASHPID"
$$ pid = 9265 BASHPID = 9265

Section 36.17: $BASH_ENV
An environment variable pointing to the Bash startup file which is read when a script is invoked.

Section 36.18: $BASH_VERSINFO
An array containing the full version information split into elements, much more convenient than $BASH_VERSION if
you're just looking for the major version:
~> $ for ((i=0; i<=5; i++)); do echo "BASH_VERSINFO[$i] = ${BASH_VERSINFO[$i]}"; done
BASH_VERSINFO[0] = 3
BASH_VERSINFO[1] = 2
BASH_VERSINFO[2] = 25
BASH_VERSINFO[3] = 1
BASH_VERSINFO[4] = release
BASH_VERSINFO[5] = x86_64-redhat-linux-gnu

Section 36.19: $BASH_VERSION
Shows the version of bash that is running, this allows you to decide whether you can use any advanced features:
~> $ echo $BASH_VERSION
4.1.2(1)-release

Section 36.20: $EDITOR
The default editor that will be involked by any scripts or programs, usually vi or emacs.
~> $ echo $EDITOR
vi

Section 36.21: $HOSTNAME
The hostname assigned to the system during startup.
~> $ echo $HOSTNAME
mybox.mydomain.com

Section 36.22: $HOSTTYPE
This variable identifies the hardware, it can be useful in determining which binaries to execute:
~> $ echo $HOSTTYPE

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x86_64

Section 36.23: $MACHTYPE
Similar to $HOSTTYPE above, this also includes information about the OS as well as hardware
~> $ echo $MACHTYPE
x86_64-redhat-linux-gnu

Section 36.24: $OSTYPE
Returns information about the type of OS running on the machine, eg.
~> $ echo $OSTYPE
linux-gnu

Section 36.25: $PATH
The search path for finding binaries for commands. Common examples include /usr/bin and /usr/local/bin.
When a user or script attempts to run a command, the paths in $PATH are searched in order to find a matching file
with execute permission.
The directories in $PATH are separated by a : character.
~> $ echo "$PATH"
/usr/kerberos/bin:/usr/local/bin:/bin:/usr/bin

So, for example, given the above $PATH, if you type lss at the prompt, the shell will look for
/usr/kerberos/bin/lss, then /usr/local/bin/lss, then /bin/lss, then /usr/bin/lss, in this order, before

concluding that there is no such command.

Section 36.26: $PPID
The Process ID (pid) of the script or shell's parent, meaning the process than invoked the current script or shell.
~> $ echo $$
13016
~> $ echo $PPID
13015

Section 36.27: $SECONDS
The number of seconds a script has been running. This can get quite large if shown in the shell:
~> $ echo $SECONDS
98834

Section 36.28: $SHELLOPTS
A readonly list of the options bash is supplied on startup to control its behaviour:
~> $ echo $SHELLOPTS

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braceexpand:emacs:hashall:histexpand:history:interactive-comments:monitor

Section 36.29: $_
Outputs the last field from the last command executed, useful to get something to pass onwards to another
command:
~> $ ls *.sh;echo $_
testscript1.sh testscript2.sh
testscript2.sh

It gives the script path if used before any other commands:
test.sh:
#!/bin/bash
echo "$_"

Output:
~> $ ./test.sh # running test.sh
./test.sh

Note: This is not a foolproof way to get the script path

Section 36.30: $GROUPS
An array containing the numbers of groups the user is in:
#!/usr/bin/env bash
echo You are assigned to the following groups:
for group in ${GROUPS[@]}; do
IFS=: read -r name dummy number members < <(getent group $group )
printf "name: %-10s number: %-15s members: %s\n" "$name" "$number" "$members"
done

Section 36.31: $LINENO
Outputs the line number in the current script. Mostly useful when debugging scripts.
#!/bin/bash
# this is line 2
echo something # this is line 3
echo $LINENO # Will output 4

Section 36.32: $SHLVL
When the bash command is executed a new shell is opened. The $SHLVL environment variable holds the number of
shell levels the current shell is running on top of.
In a new terminal window, executing the following command will produce different results based on the Linux
distribution in use.
echo $SHLVL

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129

Using Fedora 25, the output is "3". This indicates, that when opening a new shell, an initial bash command executes
and performs a task. The initial bash command executes a child process (another bash command) which, in turn,
executes a final bash command to open the new shell. When the new shell opens, it is running as a child process of
2 other shell processes, hence the output of "3".
In the following example (given the user is running Fedora 25), the output of $SHLVL in a new shell will be set to "3".
As each bash command is executed, $SHLVL increments by one.
~>
3
~>
~>
4
~>
~>
5

$ echo $SHLVL
$ bash
$ echo $SHLVL
$ bash
$ echo $SHLVL

One can see that executing the 'bash' command (or executing a bash script) opens a new shell. In comparison,
sourcing a script runs the code in the current shell.
test1.sh
#!/usr/bin/env bash
echo "Hello from test1.sh. My shell level is $SHLVL"
source "test2.sh"

test2.sh
#!/usr/bin/env bash
echo "Hello from test2.sh. My shell level is $SHLVL"

run.sh
#!/usr/bin/env bash
echo "Hello from run.sh. My shell level is $SHLVL"
./test1.sh

Execute:
chmod +x test1.sh && chmod +x run.sh
./run.sh

Output:
Hello from run.sh. My shell level is 4
Hello from test1.sh. My shell level is 5
Hello from test2.sh. My shell level is 5

Section 36.33: $UID
A read only variable that stores the users' ID number:
~> $ echo $UID
12345

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Chapter 37: Job Control
Section 37.1: List background processes
$ jobs
[1]
Running
[2]- Running
[3]+ Running

sleep 500 & (wd: ~)
sleep 600 & (wd: ~)
./Fritzing &

First field shows the job ids. The + and - sign that follows the job id for two jobs denote the default job and next
candidate default job when the current default job ends respectively. The default job is used when the fg or bg
commands are used without any argument.
Second field gives the status of the job. Third field is the command used to start the process.
The last field (wd: ~) says that the sleep commands were started from the working directory ~ (Home).

Section 37.2: Bring a background process to the foreground
$ fg %2
sleep 600

%2 specifies job no. 2. If fg is used without any arguments if brings the last process put in background to the
foreground.
$ fg %?sle
sleep 500
?sle refers to the baground process command containing "sle". If multiple background commands contain the

string, it will produce an error.

Section 37.3: Restart stopped background process
$ bg
[8]+ sleep 600 &

Section 37.4: Run command in background
$ sleep 500 &
[1] 7582

Puts the sleep command in background. 7582 is the process id of the background process.

Section 37.5: Stop a foreground process
Press Ctrl + Z to stop a foreground process and put it in background
$ sleep 600
^Z
[8]+ Stopped

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131

Chapter 38: Case statement
Section 38.1: Simple case statement
In its simplest form supported by all versions of bash, case statement executes the case that matches the pattern.
;; operator breaks after the first match, if any.
#!/bin/bash
var=1
case $var in
1)
echo "Antartica"
;;
2)
echo "Brazil"
;;
3)
echo "Cat"
;;
esac

Outputs:
Antartica

Section 38.2: Case statement with fall through
Version ≥ 4.0

Since bash 4.0, a new operator ;& was introduced which provides fall through mechanism.
#!/bin/bash
var=1
case $var in
1)
echo "Antartica"
;&
2)
echo "Brazil"
;&
3)
echo "Cat"
;&
esac

Outputs:
Antartica
Brazil
Cat

Section 38.3: Fall through only if subsequent pattern(s) match
Version ≥ 4.0

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132

Since Bash 4.0, another operator ;;& was introduced which also provides fall through only if the patterns in
subsequent case statement(s), if any, match.
#!/bin/bash
var=abc
case $var in
a*)
echo "Antartica"
;;&
xyz)
echo "Brazil"
;;&
*b*)
echo "Cat"
;;&
esac

Outputs:
Antartica
Cat

In the below example, the abc matches both first and third case but not the second case. So, second case is not
executed.

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Chapter 39: Read a file (data stream,
variable) line-by-line (and/or field-byfield)?
Parameter
Details
IFS
Internal field separator
file
A file name/path
-r
Prevents backslash interpretation when used with read
-t
Removes a trailing newline from each line read by readarray
-d DELIM Continue until the first character of DELIM is read (with read), rather than newline

Section 39.1: Looping through a file line by line
while IFS= read -r line; do
echo "$line"
done <file

If file may not include a newline at the end, then:
while IFS= read -r line || [ -n "$line" ]; do
echo "$line"
done <file

Section 39.2: Looping through the output of a command field
by field
Let's assume that the field separator is :
while IFS= read -d : -r field || [ -n "$field" ];do
echo "**$field**"
done < <(ping google.com)

Or with a pipe:
ping google.com | while IFS= read -d : -r field || [ -n "$field" ];do
echo "**$field**"
done

Section 39.3: Read lines of a file into an array
readarray -t arr <file

Or with a loop:
arr=()
while IFS= read -r line; do
arr+=("$line")
done <file

Section 39.4: Read lines of a string into an array
var='line 1

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134

line 2
line3'
readarray -t arr <<< "$var"

or with a loop:
arr=()
while IFS= read -r line; do
arr+=("$line")
done <<< "$var"

Section 39.5: Looping through a string line by line
var='line 1
line 2
line3'
while IFS= read -r line; do
echo "-$line-"
done <<< "$var"

or
readarray -t arr <<< "$var"
for i in "${arr[@]}";do
echo "-$i-"
done

Section 39.6: Looping through the output of a command line
by line
while IFS= read -r line;do
echo "**$line**"
done < <(ping google.com)

or with a pipe:
ping google.com |
while IFS= read -r line;do
echo "**$line**"
done

Section 39.7: Read a file field by field
Let's assume that the field separator is : (colon) in the file file.
while IFS= read -d : -r field || [ -n "$field" ]; do
echo "$field"
done <file

For a content:
first : se
con
d:
Thi rd:

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Fourth

The output is:
**first **
** se
con
d**
**
Thi rd**
**
Fourth
**

Section 39.8: Read a string field by field
Let's assume that the field separator is :
var='line: 1
line: 2
line3'
while IFS= read -d : -r field || [ -n "$field" ]; do
echo "-$field-"
done <<< "$var"

Output:
-line- 1
line- 2
line3
-

Section 39.9: Read fields of a file into an array
Let's assume that the field separator is :
arr=()
while IFS= read -d : -r field || [ -n "$field" ]; do
arr+=("$field")
done <file

Section 39.10: Read fields of a string into an array
Let's assume that the field separator is :
var='1:2:3:4:
newline'
arr=()
while IFS= read -d : -r field || [ -n "$field" ]; do
arr+=("$field")
done <<< "$var"
echo "${arr[4]}"

Output:
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newline

Section 39.11: Reads file (/etc/passwd) line by line and field by
field
#!/bin/bash
FILENAME="/etc/passwd"
while IFS=: read -r username password userid groupid comment homedir cmdshell
do
echo "$username, $userid, $comment $homedir"
done < $FILENAME

In unix password file, user information is stored line by line, each line consisting of information for a user separated
by colon (:) character. In this example while reading the file line by line, the line is also split into fields using colon
character as delimiter which is indicated by the value given for IFS.
Sample input
mysql:x:27:27:MySQL Server:/var/lib/mysql:/bin/bash
pulse:x:497:495:PulseAudio System Daemon:/var/run/pulse:/sbin/nologin
sshd:x:74:74:Privilege-separated SSH:/var/empty/sshd:/sbin/nologin
tomcat:x:91:91:Apache Tomcat:/usr/share/tomcat6:/sbin/nologin
webalizer:x:67:67:Webalizer:/var/www/usage:/sbin/nologin

Sample Output
mysql, 27, MySQL Server /var/lib/mysql
pulse, 497, PulseAudio System Daemon /var/run/pulse
sshd, 74, Privilege-separated SSH /var/empty/sshd
tomcat, 91, Apache Tomcat /usr/share/tomcat6
webalizer, 67, Webalizer /var/www/usage

To read line by line and have the entire line assigned to variable, following is a modified version of the example.
Note that we have only one variable by name line mentioned here.
#!/bin/bash
FILENAME="/etc/passwd"
while IFS= read -r line
do
echo "$line"
done < $FILENAME

Sample Input
mysql:x:27:27:MySQL Server:/var/lib/mysql:/bin/bash
pulse:x:497:495:PulseAudio System Daemon:/var/run/pulse:/sbin/nologin
sshd:x:74:74:Privilege-separated SSH:/var/empty/sshd:/sbin/nologin
tomcat:x:91:91:Apache Tomcat:/usr/share/tomcat6:/sbin/nologin
webalizer:x:67:67:Webalizer:/var/www/usage:/sbin/nologin

Sample Output
mysql:x:27:27:MySQL Server:/var/lib/mysql:/bin/bash
pulse:x:497:495:PulseAudio System Daemon:/var/run/pulse:/sbin/nologin
sshd:x:74:74:Privilege-separated SSH:/var/empty/sshd:/sbin/nologin
tomcat:x:91:91:Apache Tomcat:/usr/share/tomcat6:/sbin/nologin

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webalizer:x:67:67:Webalizer:/var/www/usage:/sbin/nologin

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138

Chapter 40: File execution sequence
.bash_profile, .bash_login, .bashrc, and .profile all do pretty much the same thing: set up and define

functions, variables, and the sorts.
The main difference is that .bashrc is called at the opening of a non-login but interactive window, and
.bash_profile and the others are called for a login shell. Many people have their .bash_profile or similar call
.bashrc anyway.

Section 40.1: .profile vs .bash_profile (and .bash_login)
.profile is read by most shells on startup, including bash. However, .bash_profile is used for configurations

specific to bash. For general initialization code, put it in .profile. If it's specific to bash, use .bash_profile.
.profile isn't actually designed for bash specifically, .bash_profile is though instead. (.profile is for Bourne and

other similar shells, which bash is based off) Bash will fall back to .profile if .bash_profile isn't found.
.bash_login is a fallback for .bash_profile, if it isn't found. Generally best to use .bash_profile or .profile

instead.

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139

Chapter 41: Splitting Files
Sometimes it's useful to split a file into multiple separate files. If you have large files, it might be a good idea to
break it into smaller chunks

Section 41.1: Split a file
Running the split command without any options will split a file into 1 or more separate files containing up to 1000
lines each.
split file

This will create files named xaa, xab, xac, etc, each containing up to 1000 lines. As you can see, all of them are
prefixed with the letter x by default. If the initial file was less than 1000 lines, only one such file would be created.
To change the prefix, add your desired prefix to the end of the command line
split file customprefix

Now files named customprefixaa, customprefixab, customprefixac etc. will be created
To specify the number of lines to output per file, use the -l option. The following will split a file into a maximum of
5000 lines
split -l5000 file

OR
split --lines=5000 file

Alternatively, you can specify a maximum number of bytes instead of lines. This is done by using the -b or --bytes
options. For example, to allow a maximum of 1MB
split --bytes=1MB file

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140

Chapter 42: File Transfer using scp
Section 42.1: scp transferring file
To transfer a file securely to another machine - type:
scp file1.txt tom@server2:$HOME

This example presents transferring file1.txt from our host to server2's user tom's home directory.

Section 42.2: scp transferring multiple files
scp can also be used to transfer multiple files from one server to another. Below is example of transferring all files

from my_folder directory with extension .txt to server2. In Below example all files will be transferred to user tom
home directory.
scp /my_folder/*.txt tom@server2:$HOME

Section 42.3: Downloading file using scp
To download a file from remote server to the local machine - type:
scp tom@server2:$HOME/file.txt /local/machine/path/

This example shows how to download the file named file.txt from user tom's home directory to our local
machine's current directory.

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141

Chapter 43: Pipelines
Section 43.1: Using |&
|& connects standard output and standard error of the first command to the second one while | only connects

standard output of the first command to the second command.
In this example, the page is downloaded via curl. with -v option curl writes some info on stderr including , the
downloaded page is written on stdout. Title of page can be found between <title> and </title>.
curl -vs 'http://www.google.com/' |& awk '/Host:/{print}
/<title>/{match($0,/<title>(.*)<\/title>/,a);print a[1]}'

Output is:
> Host: www.google.com
Google

But with | a lot more information will be printed, i.e. those that are sent to stderr because only stdout is piped to
the next command. In this example all lines except the last line (Google) were sent to stderr by curl:
* Hostname was NOT found in DNS cache
*
Trying 172.217.20.228...
* Connected to www.google.com (172.217.20.228) port 80 (#0)
> GET / HTTP/1.1
> User-Agent: curl/7.35.0
> Host: www.google.com
> Accept: */*
>
* HTTP 1.0, assume close after body
< HTTP/1.0 200 OK
< Date: Sun, 24 Jul 2016 19:04:59 GMT
< Expires: -1
< Cache-Control: private, max-age=0
< Content-Type: text/html; charset=ISO-8859-1
< P3P: CP="This is not a P3P policy! See
https://www.google.com/support/accounts/answer/151657?hl=en for more info."
< Server: gws
< X-XSS-Protection: 1; mode=block
< X-Frame-Options: SAMEORIGIN
< Set-Cookie: NID=82=jX0yZLPPUE7u13kKNevUCDg8yG9Ze_C03o0IMEopOSKL0mMITEagIE816G55L2wrTlQwgXkhq4ApFvvYEoaWFoEoq2T0sBTuQVdsIFULj9b2O8X35O0sAgUnc3a3JnTRBqelMcuS9QkQA; expires=Mon, 23-Jan-2017 19:04:59 GMT;
path=/; domain=.google.com; HttpOnly
< Accept-Ranges: none
< Vary: Accept-Encoding
< X-Cache: MISS from jetsib_appliance
< X-Loop-Control: 5.202.190.157 81E4F9836653D5812995BA53992F8065
< Connection: close
<
{ [data not shown]
* Closing connection 0
Google

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142

Section 43.2: Show all processes paginated
ps -e | less
ps -e shows all the processes, its output is connected to the input of more via |, less paginates the results.

Section 43.3: Modify continuous output of a command
~$ ping -c 1 google.com # unmodified output
PING google.com (16.58.209.174) 56(84) bytes of data.
64 bytes from wk-in-f100.1e100.net (16.58.209.174): icmp_seq=1 ttl=53 time=47.4 ms
~$ ping google.com | grep -o '^[0-9]\+[^()]\+' # modified output
64 bytes from wk-in-f100.1e100.net
64 bytes from wk-in-f100.1e100.net
64 bytes from wk-in-f100.1e100.net
64 bytes from wk-in-f100.1e100.net
64 bytes from wk-in-f100.1e100.net
64 bytes from wk-in-f100.1e100.net
64 bytes from wk-in-f100.1e100.net
64 bytes from wk-in-f100.1e100.net
64 bytes from wk-in-f100.1e100.net
64 bytes from wk-in-f100.1e100.net
...

The pipe (|) connects the stdout of ping to the stdin of grep, which processes it immediately. Some other
commands like sed default to buffering their stdin, which means that it has to receive enough data, before it will
print anything, potentially causing delays in further processing.

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Chapter 44: Managing PATH environment
variable
Parameter
Details
PATH
Path environment variable

Section 44.1: Add a path to the PATH environment variable
The PATH environment variable is generally defined in ~/.bashrc or ~/.bash_profile or /etc/profile or ~/.profile or
/etc/bash.bashrc (distro specific Bash configuration file)
$ echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin:
/usr/lib/jvm/jdk1.8.0_92/bin:/usr/lib/jvm/jdk1.8.0_92/db/bin:/usr/lib/jvm/jdk1.8.0_92/jre/bin

Now, if we want to add a path (e.g ~/bin) to the PATH variable:
PATH=~/bin:$PATH
# or
PATH=$PATH:~/bin

But this will modify the PATH only in the current shell (and its subshell). Once you exit the shell, this modification
will be gone.
To make it permanent, we need to add that bit of code to the ~/.bashrc (or whatever) file and reload the file.
If you run the following code (in terminal), it will add ~/bin to the PATH permanently:
echo 'PATH=~/bin:$PATH' >> ~/.bashrc && source ~/.bashrc

Explanation:
echo 'PATH=~/bin:$PATH' >> ~/.bashrc adds the line PATH=~/bin:$PATH at the end of ~/.bashrc file (you

could do it with a text editor)
source ~/.bashrc reloads the ~/.bashrc file

This is a bit of code (run in terminal) that will check if a path is already included and add the path only if not:
path=~/bin
# path to be included
bashrc=~/.bashrc
# bash file to be written and reloaded
# run the following code unmodified
echo $PATH | grep -q "\(^\|:\)$path\(:\|/\{0,1\}$\)" || echo "PATH=\$PATH:$path" >> "$bashrc";
source "$bashrc"

Section 44.2: Remove a path from the PATH environment
variable
To remove a PATH from a PATH environment variable, you need to edit ~/.bashrc or ~/.bash_profile or /etc/profile
or ~/.profile or /etc/bash.bashrc (distro specific) file and remove the assignment for that particular path.
Instead of finding the exact assignment, you could just do a replacement in the $PATH in its final stage.
The following will safely remove $path from $PATH:

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path=~/bin
PATH="$(echo "$PATH" |sed -e "s#\(^\|:\)$(echo "$path" |sed -e 's/[^^]/[&]/g' -e
's/\^/\\^/g')\(:\|/\{0,1\}$\)#\1\2#" -e 's#:\+#:#g' -e 's#^:\|:$##g')"

To make it permanent, you will need to add it at the end of your bash configuration file.
You can do it in a functional way:
rpath(){
for path in "$@";do
PATH="$(echo "$PATH" |sed -e "s#\(^\|:\)$(echo "$path" |sed -e 's/[^^]/[&]/g' -e
's/\^/\\^/g')\(:\|/\{0,1\}$\)#\1\2#" -e 's#:\+#:#g' -e 's#^:\|:$##g')"
done
echo "$PATH"
}
PATH="$(rpath ~/bin /usr/local/sbin /usr/local/bin)"
PATH="$(rpath /usr/games)"
# etc ...

This will make it easier to handle multiple paths.
Notes:
You will need to add these codes in the Bash configuration file (~/.bashrc or whatever).
Run source ~/.bashrc to reload the Bash configuration (~/.bashrc) file.

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Chapter 45: Word splitting
Parameter
Details
IFS
Internal field separator
-x
Print commands and their arguments as they are executed (Shell option)

Section 45.1: What, when and Why?
When the shell performs parameter expansion, command substitution, variable or arithmetic expansion, it scans for
word boundaries in the result. If any word boundary is found, then the result is split into multiple words at that
position. The word boundary is defined by a shell variable IFS (Internal Field Separator). The default value for IFS
are space, tab and newline, i.e. word splitting will occur on these three white space characters if not prevented
explicitly.
set -x
var='I am
a
multiline string'
fun() {
echo "-$1-"
echo "*$2*"
echo ".$3."
}
fun $var

In the above example this is how the fun function is being executed:
fun I am a multiline string

$var is split into 5 args, only I, am and a will be printed.

Section 45.2: Bad eects of word splitting
$ a='I am a string with spaces'
$ [ $a = $a ] || echo "didn't match"
bash: [: too many arguments
didn't match

[ $a = $a ] was interpreted as [ I am a string with spaces = I am a string with spaces ]. [ is

the test command for which I am a string with spaces is not a single argument, rather it's 6
arguments!!

$ [ $a = something ] || echo "didn't match"
bash: [: too many arguments
didn't match

[ $a = something ] was interpreted as [ I am a string with spaces = something ]

$ [ $(grep . file) = 'something' ]

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bash: [: too many arguments

The grep command returns a multiline string with spaces, so you can just imagine how many arguments
are there...:D
See what, when and why for the basics.

Section 45.3: Usefulness of word splitting
There are some cases where word splitting can be useful:
Filling up array:
arr=($(grep -o '[0-9]\+' file))

This will fill up arr with all numeric values found in file
Looping through space separated words:
words='foo bar baz'
for w in $words;do
echo "W: $w"
done

Output:
W: foo
W: bar
W: baz

Passing space separated parameters which don't contain white spaces:
packs='apache2 php php-mbstring php-mysql'
sudo apt-get install $packs

or
packs='
apache2
php
php-mbstring
php-mysql
'
sudo apt-get install $packs

This will install the packages. If you double quote the $packs then it will throw an error.

Unquoetd $packs is sending all the space separated package names as arguments to apt-get, while
quoting it will send the $packs string as a single argument and then apt-get will try to install a package
named apache2 php php-mbstring php-mysql (for the first one) which obviously doesn't exist
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See what, when and why for the basics.

Section 45.4: Splitting by separator changes
We can just do simple replacement of separators from space to new line, as following example.
echo $sentence | tr " " "\n"

It'll split the value of the variable sentence and show it line by line respectively.

Section 45.5: Splitting with IFS
To be more clear, let's create a script named showarg:
#!/usr/bin/env bash
printf "%d args:" $#
printf " <%s>" "$@"
echo

Now let's see the differences:
$ var="This is an example"
$ showarg $var
4 args: <This> <is> <an> <example>

$var is split into 4 args. IFS is white space characters and thus word splitting occurred in spaces

$ var="This/is/an/example"
$ showarg $var
1 args: <This/is/an/example>

In above word splitting didn't occur because the IFS characters weren't found.
Now let's set IFS=/
$
$
$
4

IFS=/
var="This/is/an/example"
showarg $var
args: <This> <is> <an> <example>

The $var is splitting into 4 arguments not a single argument.

Section 45.6: IFS & word splitting
See what, when and why if you don't know about the affiliation of IFS to word splitting
let's set the IFS to space character only:
set -x
var='I am

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a
multiline string'
IFS=' '
fun() {
echo "-$1-"
echo "*$2*"
echo ".$3."
}
fun $var

This time word splitting will only work on spaces. The fun function will be executed like this:
fun I 'am
a
multiline' string

$var is split into 3 args. I, am\na\nmultiline and string will be printed

Let's set the IFS to newline only:
IFS=$'\n'
...

Now the fun will be executed like:
fun 'I am' a 'multiline string'

$var is split into 3 args. I am, a, multiline string will be printed

Let's see what happens if we set IFS to nullstring:
IFS=
...

This time the fun will be executed like this:
fun 'I am
a
multiline string'

$var is not split i.e it remained a single arg.

You can prevent word splitting by setting the IFS to nullstring
A general way of preventing word splitting is to use double quote:
fun "$var"

will prevent word splitting in all the cases discussed above i.e the fun function will be executed with only one
argument.

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Chapter 46: Avoiding date using printf
In Bash 4.2, a shell built-in time conversion for printf was introduced: the format specification %(datefmt)T makes
printf output the date-time string corresponding to the format string datefmt as understood by strftime.

Section 46.1: Get the current date
$ printf '%(%F)T\n'
2016-08-17

Section 46.2: Set variable to current time
$ printf -v now '%(%T)T'
$ echo "$now"
12:42:47

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Chapter 47: Using "trap" to react to
signals and system events
Parameter
Meaning
-p
List currently installed traps
-l
List signal names and corresponding numbers

Section 47.1: Introduction: clean up temporary files
You can use the trap command to "trap" signals; this is the shell equivalent of the signal() or sigaction() call in C
and most other programming languages to catch signals.
One of the most common uses of trap is to clean up temporary files on both an expected and unexpected exit.
Unfortunately not enough shell scripts do this :-(
#!/bin/sh
# Make a cleanup function
cleanup() {
rm --force -- "${tmp}"
}
# Trap the special "EXIT" group, which is always run when the shell exits.
trap cleanup EXIT
# Create a temporary file
tmp="$(mktemp -p /tmp tmpfileXXXXXXX)"
echo "Hello, world!" >> "${tmp}"
# No rm -f "$tmp" needed. The advantage of using EXIT is that it still works
# even if there was an error or if you used exit.

Section 47.2: Catching SIGINT or Ctl+C
The trap is reset for subshells, so the sleep will still act on the SIGINT signal sent by ^C (usually by quitting), but the
parent process (i.e. the shell script) won't.
#!/bin/sh
# Run a command on signal 2 (SIGINT, which is what ^C sends)
sigint() {
echo "Killed subshell!"
}
trap sigint INT
# Or use the no-op command for no output
#trap : INT
# This will be killed on the first ^C
echo "Sleeping..."
sleep 500
echo "Sleeping..."
sleep 500

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And a variant which still allows you to quit the main program by pressing ^C twice in a second:
last=0
allow_quit() {
[ $(date +%s) -lt $(( $last + 1 )) ] && exit
echo "Press ^C twice in a row to quit"
last=$(date +%s)
}
trap allow_quit INT

Section 47.3: Accumulate a list of trap work to run at exit
Have you ever forgotten to add a trap to clean up a temporary file or do other work at exit?
Have you ever set one trap which canceled another?
This code makes it easy to add things to be done on exit one item at a time, rather than having one large trap
statement somewhere in your code, which may be easy to forget.
# on_exit and add_on_exit
# Usage:
#
add_on_exit rm -f /tmp/foo
#
add_on_exit echo "I am exiting"
#
tempfile=$(mktemp)
#
add_on_exit rm -f "$tempfile"
# Based on http://www.linuxjournal.com/content/use-bash-trap-statement-cleanup-temporary-files
function on_exit()
{
for i in "${on_exit_items[@]}"
do
eval $i
done
}
function add_on_exit()
{
local n=${#on_exit_items[*]}
on_exit_items[$n]="$*"
if [[ $n -eq 0 ]]; then
trap on_exit EXIT
fi
}

Section 47.4: Killing Child Processes on Exit
Trap expressions don't have to be individual functions or programs, they can be more complex expressions as well.
By combining jobs -p and kill, we can kill all spawned child processes of the shell on exit:
trap 'jobs -p | xargs kill' EXIT

Section 47.5: react on change of terminals window size
There is a signal WINCH ( WINdowCHange), which is fired when one resizes a terminal window.
declare -x rows cols
update_size(){

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rows=$(tput lines) # get actual lines of term
cols=$(tput cols) # get actual columns of term
echo DEBUG terminal window has no $rows lines and is $cols characters wide
}
trap update_size WINCH

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Chapter 48: Chain of commands and
operations
There are some means to chain commands together. Simple ones like just a ; or more complex ones like logical
chains which run depending on some conditions. The third one is piping commands, which effectively hands over
the output data to the next command in the chain.

Section 48.1: Counting a text pattern ocurrence
Using a pipe makes the output of a command be the input of the next one.
ls -1 | grep -c ".conf"

In this case the output of the ls command is used as the input of the grep command. The result will be the number
of files that include ".conf" in their name.
This can be used to contruct chains of subsequent commands as long as needed:
ls -1 | grep ".conf" | grep -c .

Section 48.2: transfer root cmd output to user file
Often one want to show the result of a command executed by root to other users. The tee command allows easily
to write a file with user perms from a command running as root:
su -c ifconfig | tee ~/results-of-ifconfig.txt

Only ifconfig runs as root.

Section 48.3: logical chaining of commands with && and ||
&& chains two commands. The second one runs only if the first one exits with success. || chains two commands.
But second one runs only if first one exits with failure.
[ a = b

] && echo "yes" || echo "no"

#
#
#
#
[

if you want to run more commands within a logical chain, use curly braces
which designate a block of commands
They do need a ; before closing bracket so bash can diffentiate from other uses
of curly braces
a = b ] && { echo "let me see."
echo "hmmm, yes, i think it is true" ; } \
|| { echo "as i am in the negation i think "
echo "this is false. a is a not b." ; }
# mind the use of line continuation sign \
# only needed to chain yes block with || ....

Section 48.4: serial chaining of commands with semicolon
A semicolon separates just two commands.
echo "i am first" ; echo "i am second" ; echo " i am third"

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Section 48.5: chaining commands with |
The | takes the output of the left command and pipes it as input the right command. Mind, that this is done in a
subshell. Hence you cannot set values of vars of the calling process within a pipe.
find . -type f -a -iname '*.mp3' | \
while read filename; do
mute --noise "$filename"
done

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Chapter 49: Type of Shells
Section 49.1: Start an interactive shell
bash

Section 49.2: Detect type of shell
shopt -q login_shell && echo 'login' || echo 'not-login'

Section 49.3: Introduction to dot files
In Unix, files and directories beginning with a period usually contain settings for a specific program/a series of
programs. Dot files are usually hidden from the user, so you would need to run ls -a to see them.
An example of a dot file is .bash_history, which contains the latest executed commands, assuming the user is
using Bash.
There are various files that are sourced when you are dropped into the Bash shell. The image below, taken from
this site, shows the decision process behind choosing which files to source at startup.

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Bash Notes for Professionals

157

Chapter 50: Color script output (crossplatform)
Section 50.1: color-output.sh
In the opening section of a bash script, it's possible to define some variables that function as helpers to color or
otherwise format the terminal output during the run of the script.
Different platforms use different character sequences to express color. However, there's a utility called tput which
works on all *nix systems and returns platform-specific terminal coloring strings via a consistent cross-platform API.
For example, to store the character sequence which turns the terminal text red or green:
red=$(tput setaf 1) green=$(tput setaf 2)
Or, to store the character sequence which resets the text to default appearance:
reset=$(tput sgr0)
Then, if the BASH script needed to show different colored outputs, this can be achieved with:
echo "${green}Success!${reset}"
echo "${red}Failure.${reset}"

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Chapter 51: co-processes
Section 51.1: Hello World
# create the co-process
coproc bash
# send a command to it (echo a)
echo 'echo Hello World' >&"${COPROC[1]}"
# read a line from its output
read line <&"${COPROC[0]}"
# show the line
echo "$line"

The output is "Hello World".

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Chapter 52: Typing variables
Section 52.1: declare weakly typed variables
declare is an internal command of bash. (internal command use help for displaying "manpage"). It is used to show
and define variables or show function bodies.
Syntax: declare [options] [name[=value]]...
# options are used to define
# an integer
declare -i myInteger
declare -i anotherInt=10
# an array with values
declare -a anArray=( one two three)
# an assoc Array
declare -A assocArray=( [element1]="something" [second]=anotherthing )
# note that bash recognizes the string context within []
# some modifiers exist
# uppercase content
declare -u big='this will be uppercase'
# same for lower case
declare -l small='THIS WILL BE LOWERCASE'
# readonly array
declare -ra constarray=( eternal true and unchangeable )
# export integer to environment
declare -xi importantInt=42

You can use also the + which takes away the given attribute. Mostly useless, just for completness.
To display variables and/or functions there are some options too
# printing definded vars and functions
declare -f
# restrict output to functions only
declare -F # if debugging prints line number and filename defined in too

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Chapter 53: Jobs at specific times
Section 53.1: Execute job once at specific time
Note: at is not installed by default on most of modern distributions.
To execute a job once at some other time than now, in this example 5pm, you can use
echo "somecommand &" | at 5pm

If you want to catch the output, you can do that in the usual way:
echo "somecommand > out.txt 2>err.txt &" | at 5pm
at understands many time formats, so you can also say
echo
echo
echo
echo

"somecommand
"somecommand
"somecommand
"somecommand

&"
&"
&"
&"

|
|
|
|

at
at
at
at

now + 2 minutes
17:00
17:00 Jul 7
4pm 12.03.17

If no year or date are given, it assumes the next time the time you specified occurs. So if you give a hour that
already passed today, it will assume tomorrow, and if you give a month that already passed this year, it will assume
next year.
This also works together with nohup like you would expect.
echo "nohup somecommand > out.txt 2>err.txt &" | at 5pm

There are some more commands to control timed jobs:
atq lists all timed jobs (atqueue)
atrm removes a timed job (atremove )
batch does basically the same like at, but runs jobs only when system load is lower than 0.8
All commands apply to jobs of the user logged in. If logged in as root, system wide jobs are handled of course.

Section 53.2: Doing jobs at specified times repeatedly using
systemd.timer
systemd provides a modern implementation of cron. To execute a script periodical a service and a timer file ist
needed. The service and timer files should be placed in /etc/systemd/{system,user}. The service file:
[Unit]
Description=my script or programm does the very best and this is the description
[Service]
# type is important!
Type=simple
# program|script to call. Always use absolute pathes
# and redirect STDIN and STDERR as there is no terminal while being executed
ExecStart=/absolute/path/to/someCommand >>/path/to/output 2>/path/to/STDERRoutput
#NO install section!!!! Is handled by the timer facitlities itself.
#[Install]

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#WantedBy=multi-user.target

Next the timer file:
[Unit]
Description=my very first systemd timer
[Timer]
# Syntax for date/time specifications is Y-m-d H:M:S
# a * means "each", and a comma separated list of items can be given too
# *-*-* *,15,30,45:00 says every year, every month, every day, each hour,
# at minute 15,30,45 and zero seconds
OnCalendar=*-*-* *:01:00
# this one runs each hour at one minute zero second e.g. 13:01:00

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Chapter 54: Handling the system prompt
Escape

Details
A bell character.
The date, in "Weekday Month Date" format (e.g., "Tue May 26").
The FORMAT is passed to `strftime'(3) and the result is inserted into the prompt string; an empty
\D{FORMAT}
FORMAT results in a locale-specific time representation. The braces are required.
\e
An escape character. \033 works of course too.
\h
The hostname, up to the first `.'. (i.e. no domain part)
\H
The hostname eventually with domain part
\j
The number of jobs currently managed by the shell.
\l
The basename of the shell's terminal device name.
\n
A newline.
\r
A carriage return.
\s
The name of the shell, the basename of `$0' (the portion following the final slash).
\t
The time, in 24-hour HH:MM:SS format.
\T
The time, in 12-hour HH:MM:SS format.
@
The time, in 12-hour am/pm format.
\A
The time, in 24-hour HH:MM format.
\u
The username of the current user.
\v
The version of Bash (e.g., 2.00)
\V
The release of Bash, version + patchlevel (e.g., 2.00.0)
The current working directory, with $HOME abbreviated with a tilde (uses the $PROMPT_DIRTRIM
\w
variable).
\W
The basename of $PWD, with $HOME abbreviated with a tilde.
!
The history number of this command.
#
The command number of this command.
If the effective uid is 0, #, otherwise $.
$
\NNN
The character whose ASCII code is the octal value NNN.
\
A backslash.
Begin a sequence of non-printing characters. This could be used to embed a terminal control
\[
sequence into the prompt.
\]
End a sequence of non-printing characters.
\a
\d

Section 54.1: Using the PROMPT_COMMAND envrionment
variable
When the last command in an interactive bash instance is done, the evaluated PS1 variable is displayes. Before
actually displaying PS1 bash looks whether the PROMPT_COMMAND is set. This value of this var must be a callable
program or script. If this var is set this program/script is called BEFORE the PS1 prompt is displayed.
# just a stupid function, we will use to demonstrate
# we check the date if Hour is 12 and Minute is lower than 59
lunchbreak(){
if (( $(date +%H) == 12 && $(date +%M) < 59 )); then
# and print colored \033[ starts the escape sequence
# 5; is blinking attribute
# 2; means bold
# 31 says red
printf "\033[5;1;31mmind the lunch break\033[0m\n";
else
printf "\033[33mstill working...\033[0m\n";
fi;
}

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# activating it
export PROMPT_COMMAND=lunchbreak

Section 54.2: Using PS2
PS2 is displayed when a command extends to more than one line and bash awaits more keystrokes. It is displayed
too when a compound command like while...do..done and alike is entered.
export PS2="would you please complete this command?\n"
# now enter a command extending to at least two lines to see PS2

Section 54.3: Using PS3
When the select statement is executed, it displays the given items prefixed with a number and then displays the
PS3 prompt:
export PS3=" To choose your language type the preceding number : "
select lang in EN CA FR DE; do
# check input here until valid.
break
done

Section 54.4: Using PS4
PS4 is displayes when bash is in debugging mode.
#!/usr/bin/env bash
# switch on debugging
set -x
# define a stupid_func
stupid_func(){
echo I am line 1 of stupid_func
echo I am line 2 of stupid_func
}
# setting the PS4 "DEBUG" prompt
export PS4='\nDEBUG level:$SHLVL subshell-level: $BASH_SUBSHELL \nsource-file:${BASH_SOURCE}
line#:${LINENO} function:${FUNCNAME[0]:+${FUNCNAME[0]}(): }\nstatement: '
# a normal statement
echo something
# function call
stupid_func
# a pipeline of commands running in a subshell
( ls -l | grep 'x' )

Section 54.5: Using PS1
PS1 is the normal system prompt indicating that bash waits for commands being typed in. It understands some
escape sequences and can execute functions or progams. As bash has to position the cursor after the displayes
prompt, it needs to know how to calculate the effective length of the prompt string. To indicate non printing
sequences of chars within the PS1 variable escaped braces are used: \[ a non printing sequence of chars \]. All being
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said holds true for all PS* vars.
(The black caret indicates cursor)
#everything not being an escape sequence will be literally printed
export PS1="literal sequence " # Prompt is now:
literal sequence ?
# \u == user \h == host \w == actual working directory
# mind the single quotes avoiding interpretation by shell
export PS1='\u@\h:\w > ' # \u == user, \h == host, \w actual working dir
looser@host:/some/path > ?
# executing some commands within PS1
# following line will set foreground color to red, if user==root,
# else it resets attributes to default
# $( (($EUID == 0)) && tput setaf 1)
# later we do reset attributes to default with
# $( tput sgr0 )
# assuming being root:
PS1="\[$( (($EUID == 0)) && tput setaf 1 \]\u\[$(tput sgr0)\]@\w:\w \$ "
looser@host:/some/path > ? # if not root else <red>root<default>@host....

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Chapter 55: The cut command
Parameter
Details
-f, --fields
Field-based selection
-d, --delimiter
Delimiter for field-based selection
-c, --characters
Character-based selection, delimiter ignored or error
-s, --only-delimited Suppress lines with no delimiter characters (printed as-is otherwise)
--complement
Inverted selection (extract all except specified fields/characters
--output-delimiter Specify when it has to be different from the input delimiter
The cut command is a fast way to extract parts of lines of text files. It belongs to the oldest Unix commands. Its
most popular implementations are the GNU version found on Linux and the FreeBSD version found on MacOS, but
each flavor of Unix has its own. See below for differences. The input lines are read either from stdin or from files
listed as arguments on the command line.

Section 55.1: Only one delimiter character
You cannot have more than one delimiter: if you specify something like -d ",;:", some implementations will use
only the first character as a delimiter (in this case, the comma.) Other implementations (e.g. GNU cut) will give you
an error message.
$ cut -d ",;:" -f2 <<<"J.Smith,1 Main Road,cell:1234567890;land:4081234567"
cut: the delimiter must be a single character
Try `cut --help' for more information.

Section 55.2: Repeated delimiters are interpreted as empty
fields
$ cut -d, -f1,3 <<<"a,,b,c,d,e"
a,b

is rather obvious, but with space-delimited strings it might be less obvious to some
$ cut -d ' ' -f1,3 <<<"a
a b

b c d e"

cut cannot be used to parse arguments as the shell and other programs do.

Section 55.3: No quoting
There is no way to protect the delimiter. Spreadsheets and similar CSV-handling software usually can recognize a
text-quoting character which makes it possible to define strings containing a delimiter. With cut you cannot.
$ cut -d, -f3 <<<'John,Smith,"1, Main Street"'
"1

Section 55.4: Extracting, not manipulating
You can only extract portions of lines, not reorder or repeat fields.
$ cut -d, -f2,1 <<<'John,Smith,USA' ## Just like -f1,2
John,Smith
$ cut -d, -f2,2 <<<'John,Smith,USA' ## Just like -f2

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Smith

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Chapter 56: Bash on Windows 10
Section 56.1: Readme
The simpler way to use Bash in Windows is to install Git for Windows. It's shipped with Git Bash which is a real Bash.
You can access it with shortcut in :
Start > All Programs > Git > Git Bash

Commands like grep, ls, find, sed, vi etc is working.

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Chapter 57: Cut Command
Option

Description
Print the bytes listed in the LIST parameter
-c LIST, --characters=LIST Print characters in positions specified in LIST parameter
-f LIST, --fields=LIST
Print fields or columns
-d DELIMITER
Used to separate columns or fields
-b LIST, --bytes=LIST

In Bash, the cut command is useful for dividing a file into several smaller parts.

Section 57.1: Show the first column of a file
Suppose you have a file that looks like this
John Smith 31
Robert Jones 27
...

This file has 3 columns separated by spaces. To select only the first column, do the following.
cut -d ' ' -f1 filename

Here the -d flag, specifies the delimiter, or what separates the records. The -f flag specifies the field or column
number. This will display the following output
John
Robert
...

Section 57.2: Show columns x to y of a file
Sometimes, it's useful to display a range of columns in a file. Suppose you have this file
Apple California 2017 1.00 47
Mango Oregon 2015 2.30 33

To select the first 3 columns do
cut -d ' ' -f1-3 filename

This will display the following output
Apple California 2017
Mango Oregon 2015

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Chapter 58: global and local variables
By default, every variable in bash is global to every function, script and even the outside shell if you are declaring
your variables inside a script.
If you want your variable to be local to a function, you can use local to have that variable a new variable that is
independent to the global scope and whose value will only be accessible inside that function.

Section 58.1: Global variables
var="hello"
function foo(){
echo $var
}
foo

Will obviously output "hello", but this works the other way around too:
function foo() {
var="hello"
}
foo
echo $var

Will also output "hello"

Section 58.2: Local variables
function foo() {
local var
var="hello"
}
foo
echo $var

Will output nothing, as var is a variable local to the function foo, and its value is not visible from outside of it.

Section 58.3: Mixing the two together
var="hello"
function foo(){
local var="sup?"
echo "inside function, var=$var"
}
foo
echo "outside function, var=$var"

Will output

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inside function, var=sup?
outside function, var=hello

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Chapter 59: CGI Scripts
Section 59.1: Request Method: GET
It is quite easy to call a CGI-Script via GET.
First you will need the encoded url of the script.
Then you add a question mark ? followed by variables.
Every variable should have two sections separated by =.
First section should be always a unique name for each variable,
while the second part has values in it only
Variables are separated by &
Total length of the string should not rise above 255 characters
Names and values needs to be html-encoded (replace: </ , / ? : @ & = + $ )
Hint:
When using html-forms the request method can be generated by it self.
With Ajax you can encode all via encodeURI and encodeURIComponent
Example:
http://www.example.com/cgi-bin/script.sh?var1=Hello%20World!&var2=This%20is%20a%20Test.&

The server should communicate via Cross-Origin Resource Sharing (CORS) only, to make request more secure. In
this showcase we use CORS to determine the Data-Type we want to use.
There are many Data-Types we can choose from, the most common are...
text/html
text/plain
application/json
When sending a request, the server will also create many environment variables. For now the most important
environment variables are $REQUEST_METHOD and $QUERY_STRING.
The Request Method has to be GET nothing else!
The Query String includes all the html-endoded data.
The Script
#!/bin/bash
# CORS is the way to communicate, so lets response to the server first
echo "Content-type: text/html"
# set the data-type we want to use
echo ""
# we don't need more rules, the empty line initiate this.
# CORS are set in stone and any communication from now on will be like reading a html-document.
# Therefor we need to create any stdout in html format!
# create html scructure and send it to stdout
echo "<!DOCTYPE html>"
echo "<html><head>"
# The content will be created depending on the Request Method
if [ "$REQUEST_METHOD" = "GET" ]; then

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# Note that the environment variables $REQUEST_METHOD and $QUERY_STRING can be processed by the
shell directly.
# One must filter the input to avoid cross site scripting.
Var1=$(echo "$QUERY_STRING" | sed -n 's/^.*var1=\([^&]*\).*$/\1/p')
# read value of "var1"
Var1_Dec=$(echo -e $(echo "$Var1" | sed 's/+/ /g;s/%\(..\)/\\x\1/g;'))
# html decode
Var2=$(echo "$QUERY_STRING" | sed -n 's/^.*var2=\([^&]*\).*$/\1/p')
Var2_Dec=$(echo -e $(echo "$Var2" | sed 's/+/ /g;s/%\(..\)/\\x\1/g;'))
# create content for stdout
echo "<title>Bash-CGI Example 1</title>"
echo "</head><body>"
echo "<h1>Bash-CGI Example 1</h1>"
echo "<p>QUERY_STRING: ${QUERY_STRING}<br>var1=${Var1_Dec}<br>var2=${Var2_Dec}</p>"
the values to stdout

# print

else
echo
echo
echo
echo

"<title>456 Wrong Request Method</title>"
"</head><body>"
"<h1>456</h1>"
"<p>Requesting data went wrong.<br>The Request method has to be \"GET\" only!</p>"

fi
echo "<hr>"
echo "$SERVER_SIGNATURE"
# an other environment variable
echo "</body></html>"
# close html
exit 0

The html-document will look like this ...
<html><head>
<title>Bash-CGI Example 1</title>
</head><body>
<h1>Bash-CGI Example 1</h1>
<p>QUERY_STRING: var1=Hello%20World!&amp;var2=This%20is%20a%20Test.&amp;<br>var1=Hello
World!<br>var2=This is a Test.</p>
<hr>
<address>Apache/2.4.10 (Debian) Server at example.com Port 80</address>

</body></html>

The output of the variables will look like this ...
var1=Hello%20World!&var2=This%20is%20a%20Test.&
Hello World!
This is a Test.
Apache/2.4.10 (Debian) Server at example.com Port 80

Negative side effects...
All the encoding and decoding does not look nice, but is needed
The Request will be public readable and leave a tray behind
The size of a request is limited

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Needs protection against Cross-Side-Scripting (XSS)

Section 59.2: Request Method: POST /w JSON
Using Request Method POST in combination with SSL makes datatransfer more secure.
In addition...
Most of the encoding and decoding is not needed any more
The URL will be visible to any one and needs to be url encoded.
The data will be send separately and therefor should be secured via SSL
The size of the data is almost unlitmited
Still needs protection against Cross-Side-Scripting (XSS)

To keep this showcase simple we want to receive JSON Data
and communication should be over Cross-Origin Resource Sharing (CORS).
The following script will also demonstrate two different Content-Types.
#!/bin/bash
exec 2>/dev/null
# We don't want any error messages be printed to stdout
trap "response_with_html && exit 0" ERR
# response with an html message when an error occurred
and close the script
function
echo
echo
echo
echo
echo
echo
echo
echo
echo
echo
echo
}

response_with_html(){
"Content-type: text/html"
""
"<!DOCTYPE html>"
"<html><head>"
"<title>456</title>"
"</head><body>"
"<h1>456</h1>"
"<p>Attempt to communicate with the server went wrong.</p>"
"<hr>"
"$SERVER_SIGNATURE"
"</body></html>"

function
echo
echo
echo
}

response_with_json(){
"Content-type: application/json"
""
"{\"message\": \"Hello World!\"}"

if [ "$REQUEST_METHOD" = "POST" ]; then
# The environment variabe $CONTENT_TYPE describes the data-type received
case "$CONTENT_TYPE" in
application/json)
# The environment variabe $CONTENT_LENGTH describes the size of the data
read -n "$CONTENT_LENGTH" QUERY_STRING_POST
# read datastream
# The following lines will prevent XSS and check for valide JSON-Data.
# But these Symbols need to be encoded somehow before sending to this script
QUERY_STRING_POST=$(echo "$QUERY_STRING_POST" | sed "s/'//g" | sed
's/\$//g;s/`//g;s/\*//g;s/\\//g' )
# removes some symbols (like \ * ` $ ') to prevent XSS
with Bash and SQL.

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QUERY_STRING_POST=$(echo "$QUERY_STRING_POST" | sed -e :a -e 's/<[^>]*>//g;/</N;//ba')
removes most html declarations to prevent XSS within documents
JSON=$(echo "$QUERY_STRING_POST" | jq .)
# json encode - This is a pretty save way
to check for valide json code
;;
*)
response_with_html
exit 0
;;
esac

#

else
response_with_html
exit 0
fi
# Some Commands ...
response_with_json
exit 0

You will get {"message":"Hello World!"} as an answer when sending JSON-Data via POST to this Script. Every
thing else will receive the html document.
Important is also the varialbe $JSON. This variable is free of XSS, but still could have wrong values in it and needs to
be verify first. Please keep that in mind.
This code works similar without JSON.
You could get any data this way.
You just need to change the Content-Type for your needs.
Example:
if [ "$REQUEST_METHOD" = "POST" ]; then
case "$CONTENT_TYPE" in
application/x-www-form-urlencoded)
read -n "$CONTENT_LENGTH" QUERY_STRING_POST
text/plain)
read -n "$CONTENT_LENGTH" QUERY_STRING_POST
;;
esac
fi

Last but not least, don't forget to response to all requests, otherwise third party programms won't know if they
succeeded

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Chapter 60: Select keyword
Select keyword can be used for getting input argument in a menu format.

Section 60.1: Select keyword can be used for getting input
argument in a menu format
Suppose you want the user to SELECT keywords from a menu, we can create a script similar to
#!/usr/bin/env bash
select os in "linux" "windows" "mac"
do
echo "${os}"
break
done

Explanation: Here SELECT keyword is used to loop through a list of items that will be presented at the command
prompt for a user to pick from. Notice the break keyword for breaking out of the loop once the user makes a
choice. Otherwise, the loop will be endless!
Results: Upon running this script, a menu of these items will be displayed and the user will be prompted for a
selection. Upon selection, the value will be displayed, returning back to command prompt.
>bash select_menu.sh
1) linux
2) windows
3) mac
#? 3
mac
>

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Chapter 61: When to use eval
First and foremost: know what you're doing! Secondly, while you should avoid using eval, if its use makes for
cleaner code, go ahead.

Section 61.1: Using Eval
For example, consider the following that sets the contents of $@ to the contents of a given variable:
a=(1 2 3)
eval set -- "${a[@]}"

This code is often accompanied by getopt or getopts to set $@ to the output of the aforementioned option parsers,
however, you can also use it to create a simple pop function that can operate on variables silently and directly
without having to store the result to the original variable:
isnum()
{
# is argument an integer?
local re='^[0-9]+$'
if [[ -n $1 ]]; then
[[ $1 =~ $re ]] && return 0
return 1
else
return 2
fi
}
isvar()
{
if isnum "$1"; then
return 1
fi
local arr="$(eval eval -- echo -n "\$$1")"
if [[ -n ${arr[@]} ]]; then
return 0
fi
return 1
}
pop()
{
if [[ -z $@ ]]; then
return 1
fi
local var=
local isvar=0
local arr=()
if isvar "$1"; then # let's check to see if this is a variable or just a bare array
var="$1"
isvar=1
arr=($(eval eval -- echo -n "\${$1[@]}")) # if it is a var, get its contents
else
arr=($@)
fi

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# we need to reverse the contents of $@ so that we can shift
# the last element into nothingness
arr=($(awk <<<"${arr[@]}" '{ for (i=NF; i>1; --i) printf("%s ",$i); print $1; }'
# set $@ to ${arr[@]} so that we can run shift against it.
eval set -- "${arr[@]}"
shift # remove the last element
# put the array back to its original order
arr=($(awk <<<"$@" '{ for (i=NF; i>1; --i) printf("%s ",$i); print $1; }'
# echo the contents for the benefit of users and for bare arrays
echo "${arr[@]}"
if ((isvar)); then
# set the contents of the original var to the new modified array
eval -- "$var=(${arr[@]})"
fi
}

Section 61.2: Using Eval with Getopt
While eval may not be needed for a pop like function, it is however required whenever you use getopt:
Consider the following function that accepts -h as an option:
f()
{
local __me__="${FUNCNAME[0]}"
local argv="$(getopt -o 'h' -n $__me__ -- "$@")"
eval set -- "$argv"
while :; do
case "$1" in
-h)
echo "LOLOLOLOL"
return 0
;;
--)
shift
break
;;
done
echo "$@"
}

Without eval set -- "$argv" generates
-h -instead of the desired (-h --) and subsequently enters an infinite loop because
-h -doesn't match -- or -h.

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Chapter 62: Networking With Bash
Bash is often commonly used in the management and maintenance of servers and clusters. Information pertaining
to typical commands used by network operations, when to use which command for which purpose, and
examples/samples of unique and/or interesting applications of it should be included

Section 62.1: Networking commands
ifconfig

The above command will show all active interface of the machine and also give the information of
1. IP address assign to interface
2. MAC address of the interface
3. Broadcast address
4. Transmit and Receive bytes
Some example
ifconfig -a

The above command also show the disable interface
ifconfig eth0

The above command will only show the eth0 interface
ifconfig eth0 192.168.1.100 netmask 255.255.255.0

The above command will assign the static IP to eth0 interface
ifup eth0

The above command will enable the eth0 interface
ifdown eth0

The below command will disable the eth0 interface
ping

The above command (Packet Internet Grouper) is to test the connectivity between the two nodes
ping -c2 8.8.8.8

The above command will ping or test the connectivity with google server for 2 seconds.
traceroute

The above command is to use in troubleshooting to find out the number of hops taken to reach the destination.
netstat

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The above command (Network statistics) give the connection info and their state
dig www.google.com

The above command (domain information grouper) query the DNS related information
nslookup www.google.com

The above command query the DNS and find out the IP address of corresponding the website name.
route

The above command is used to check the Netwrok route information. It basically show you the routing table
router add default gw 192.168.1.1 eth0

The above command will add the default route of network of eth0 Interface to 192.168.1.1 in routing table.
route del default

The above command will delete the default route from the routing table

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Chapter 63: Parallel
Option

Description
Run n jobs in parallel
-k
Keep same order
-X
Multiple arguments with context replace
--colsep regexp
Split input on regexp for positional replacements
{} {.} {/} {/.} {#} Replacement strings
{3} {3.} {3/} {3/.} Positional replacement strings
-j n

-S sshlogin
--trc {}.bar
--onall
--nonall
--pipe
--recend str
--recstart str

Example: foo@server.example.com

Shorthand for --transfer --return {}.bar --cleanup
Run the given command with argument on all sshlogins
Run the given command with no arguments on all sshlogins
Split stdin (standard input) to multiple jobs.
Record end separator for --pipe.
Record start separator for --pipe.

Jobs in GNU Linux can be parallelized using GNU parallel. A job can be a single command or a small script that has
to be run for each of the lines in the input. The typical input is a list of files, a list of hosts, a list of users, a list of
URLs, or a list of tables. A job can also be a command that reads from a pipe.

Section 63.1: Parallelize repetitive tasks on list of files
Many repetitive jobs can be performed more efficiently if you utilize more of your computer's resources (i.e. CPU's
and RAM). Below is an example of running multiple jobs in parallel.
Suppose you have a < list of files >, say output from ls. Also, let these files are bz2 compressed and the
following order of tasks need to be operated on them.
1. Decompress the bz2 files using bzcat to stdout
2. Grep (e.g. filter) lines with specific keyword(s) using grep <some key word>
3. Pipe the output to be concatenated into one single gzipped file using gzip
Running this using a while-loop may look like this
filenames="file_list.txt"
while read -r line
do
name="$line"
## grab lines with puppies in them
bzcat $line | grep puppies | gzip >> output.gz
done < "$filenames"

Using GNU Parallel, we can run 3 parallel jobs at once by simply doing
parallel -j 3 "bzcat {} | grep puppies" ::: $( cat filelist.txt ) | gzip > output.gz

This command is simple, concise and more efficient when number of files and file size is large. The jobs gets
initiated by parallel, option -j 3 launches 3 parallel jobs and input to the parallel jobs is taken in by :::. The
output is eventually piped to gzip > output.gz

Section 63.2: Parallelize STDIN
Now, let's imagine we have 1 large file (e.g. 30 GB) that needs to be converted, line by line. Say we have a script,
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convert.sh, that does this <task>. We can pipe contents of this file to stdin for parallel to take in and work with in

chunks such as
<stdin> | parallel --pipe --block <block size> -k <task> > output.txt

where <stdin> can originate from anything such as cat <file>.
As a reproducible example, our task will be nl -n rz. Take any file, mine will be data.bz2, and pass it to <stdin>
bzcat data.bz2 | nl | parallel --pipe --block 10M -k nl -n rz | gzip > ouptput.gz

The above example takes <stdin> from bzcat data.bz2 | nl, where I included nl just as a proof of concept that
the final output output.gz will be saved in the order it was received. Then, parallel divides the <stdin> into
chunks of size 10 MB, and for each chunk it passes it through nl -n rz where it just appends a numbers rightly
justified (see nl --help for further details). The options --pipe tells parallel to split <stdin> into multiple jobs
and -- block specifies the size of the blocks. The option -k specifies that ordering must be maintained.
Your final output should look something like
000001
000002
000003
000004
000005
...
000587
000588
000589
000590
000591

1
2
3
4
5

<data>
<data>
<data>
<data>
<data>

552409
552410
552411
552412
552413

<data>
<data>
<data>
<data>
<data>

My original file had 552,413 lines. The first column represents the parallel jobs, and the second column represents
the original line numbering that was passed to parallel in chunks. You should notice that the order in the second
column (and rest of the file) is maintained.

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Chapter 64: Decoding URL
Section 64.1: Simple example
Encoded URL
http%3A%2F%2Fwww.foo.com%2Findex.php%3Fid%3Dqwerty
Use this command to decode the URL
echo "http%3A%2F%2Fwww.foo.com%2Findex.php%3Fid%3Dqwerty" | sed -e "s/%\([0-9A-F][0-9AF]\)/\\\\\x\1/g" | xargs -0 echo -e

Decoded URL (result of command)
http://www.foo.com/index.php?id=qwerty

Section 64.2: Using printf to decode a string
#!bin/bash
$ string='Question%20%20%22how%20do%20I%20decode%20a%20percent%20encoded%20string%3F%22%0AAnswer%20%20%20%20Use%20printf%20%3A)'
$ printf '%b\n' "${string//%/\\x}"
# the result
Question - "how do I decode a percent encoded string?"
Answer
- Use printf :)

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Chapter 65: Design Patterns
Accomplish some common design patterns in Bash

Section 65.1: The Publish/Subscribe (Pub/Sub) Pattern
When a Bash project turns into a library, it can become difficult to add new functionality. Function names, variables
and parameters usually need to be changed in the scripts that utilize them. In scenarios like this, it is helpful to
decouple the code and use an event driven design pattern. In said pattern, an external script can subscribe to an
event. When that event is triggered (published) the script can execute the code that it registered with the event.
pubsub.sh:
#!/usr/bin/env bash
#
# Save the path to this script's directory in a global env variable
#
DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
#
# Array that will contain all registered events
#
EVENTS=()
function action1() {
echo "Action #1 was performed ${2}"
}
function action2() {
echo "Action #2 was performed"
}
#
# @desc
:: Registers an event
# @param :: string $1 - The name of the event. Basically an alias for a function name
# @param :: string $2 - The name of the function to be called
# @param :: string $3 - Full path to script that includes the function being called
#
function subscribe() {
EVENTS+=("${1};${2};${3}")
}
#
# @desc
:: Public an event
# @param :: string $1 - The name of the event being published
#
function publish() {
for event in ${EVENTS[@]}; do
local IFS=";"
read -r -a event <<< "$event"
if [[ "${event[0]}" == "${1}" ]]; then
${event[1]} "$@"
fi
done
}
#
# Register our events and the functions that handle them

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#
subscribe "/do/work"
"action1" "${DIR}"
subscribe "/do/more/work"
"action2" "${DIR}"
subscribe "/do/even/more/work" "action1" "${DIR}"
#
# Execute our events
#
publish "/do/work"
publish "/do/more/work"
publish "/do/even/more/work" "again"

Run:
chmod +x pubsub.sh
./pubsub.sh

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Chapter 66: Pitfalls
Section 66.1: Whitespace When Assigning Variables
Whitespace matters when assigning variables.
foo = 'bar' # incorrect foo= 'bar' # incorrect foo='bar' # correct
The first two will result in syntax errors (or worse, executing an incorrect command). The last example will correctly
set the variable $foo to the text "bar".

Section 66.2: Failed commands do not stop script execution
In most scripting languages, if a function call fails, it may throw an exception and stop execution of the program.
Bash commands do not have exceptions, but they do have exit codes. A non-zero exit code signals failure, however,
a non-zero exit code will not stop execution of the program.
This can lead to dangerous (although admittedly contrived) situations like so:
#!/bin/bash
cd ~/non/existent/directory
rm -rf *

If cd-ing to this directory fails, Bash will ignore the failure and move onto the next command, wiping clean the
directory from where you ran the script.
The best way to deal with this problem is to make use of the set command:
#!/bin/bash
set -e
cd ~/non/existent/directory
rm -rf *
set -e tells Bash to exit the script immediately if any command returns a non-zero status.

Section 66.3: Missing The Last Line in a File
The C standard says that files should end with a new line, so if EOF comes at the end of a line, that line may not be
missed by some commands. As an example:
$ echo 'one\ntwo\nthree\c' > file.txt $ cat file.txt one two three $ while read line ; do echo "line $line" ; done <
file.txt one two
To make sure this works correctly for in the above example, add a test so that it will continue the loop if the last line
is not empty.
$ while read line || [ -n "$line" ] ; do echo "line $line" ; done < file.txt one two three

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Appendix A: Keyboard shortcuts
Section A.1: Editing Shortcuts
Shortcut
Description
+ a
move to the beginning of the line
+ e
move to the end of the line
+ k
Kill the text from the current cursor position to the end of the line.
+ u
Kill the text from the current cursor position to the beginning of the line
+ w
Kill the word behind the current cursor position
+ b
move backward one word
+ f
move forward one word
+ Alt + e shell expand line
+ y
Yank the most recently killed text back into the buffer at the cursor.
Rotate through killed text. You can only do this if the prior command is Ctrl + y or
Alt + y
Alt + y .
Ctrl
Ctrl
Ctrl
Ctrl
Ctrl
Alt
Alt
Ctrl
Ctrl

Killing text will delete text, but save it so that the user can reinsert it by yanking. Similar to cut and paste except that
the text is placed on a kill ring which allows for storing more than one set of text to be yanked back on to the
command line.
You can find out more in the emacs manual.

Section A.2: Recall Shortcuts
Description
search the history backwards
previous command in history
next command in history
quit history searching mode
use the last word of the previous command
repeat to get the last word of the previous + 1 command
Alt + n
Alt + . use the nth word of the previous command
!! + Return
execute the last command again (useful when you forgot sudo: sudo !!)
Ctrl
Ctrl
Ctrl
Ctrl
Alt

Shortcut
+ r
+ p
+ n
+ g
+ .

Section A.3: Macros
Shortcut
Description
Ctrl + x , ( start recording a macro
Ctrl + x , ) stop recording a macro
Ctrl + x , e execute the last recorded macro

Section A.4: Custome Key Bindings
With the bind command it is possible to define custom key bindings.
The next example bind an Alt + w to >/dev/null 2>&1:
bind '"\ew"':"\" >/dev/null 2>&1\""

If you want to execute the line immediately add \C-m ( Enter ) to it:
bind '"\ew"':"\" >/dev/null 2>&1\C-m\""

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Section A.5: Job Control
Shortcut
Description
Ctrl + c Stop the current job
Ctrl + z Suspend the current job (send a SIGTSTP signal)

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Credits
Thank you greatly to all the people from Stack Overflow Documentation who helped provide this content,
more changes can be sent to web@petercv.com for new content to be published or updated
Ajay Sangale
Ajinkya
Alessandro Mascolo
Alexej Magura
Amir Rachum
Anil
anishsane
Antoine Bolvy
Archemar
Arronical
Ashari
Ashkan
Batsu
Benjamin W.
binki
Blachshma
Bob Bagwill
Bostjan
BrunoLM
Brydon Gibson
Bubblepop
Burkhard
BurnsBA
Carpetsmoker
cb0
Chandrahas Aroori
chaos
charneykaye
chepner
Chris Rasys
Christopher Bottoms
codeforester
Cody
Colin Yang
Cows quack
CraftedCart
Crazy
criw
Daniel Käfer
Danny
Dario
David Grayson
Deepak K M
deepmax
depperm
dhimanta
dimo414
dingalapadum
divyum

Chapter 1
Chapter 20
Chapters 26 and 11
Chapters 9, 12, 36 and 61
Chapter 8
Chapter 1
Chapter 5
Chapter 9
Chapter 9
Chapter 12
Chapter 36
Chapters 36 and 43
Chapter 17
Chapters 1, 47, 9, 12, 15, 24, 31, 36 and 46
Chapter 21
Chapter 1
Chapter 1
Chapter 7
Chapter 14
Chapter 9
Chapters 1, 8, 5, 12 and 24
Chapter 1
Chapter 22
Chapter 47
Chapter 28
Chapter 6
Chapter 9
Chapter 50
Chapters 15, 27 and 46
Chapter 34
Chapters 1, 5 and 3
Chapter 12
Chapter 66
Chapter 1
Chapter 30
Chapter 1
Chapter 64
Chapter 36
Chapter 67
Chapter 1
Chapters 28, 36 and 55
Chapter 9
Chapter 20
Chapter 25
Chapters 4 and 35
Chapter 62
Chapter 14
Chapters 7 and 16
Chapters 1 and 14

Bash Notes for Professionals

189

DocSalvager
Doctor J
DonyorM
Dr Beco
Dunatotatos
Echoes_86
Edgar Rokyan
edi9999
Eric Renouf
fedorqui
fifaltra
Flows
Gavyn
George Vasiliou
Gilles
glenn jackman
Grexis
Grisha Levit
gzh
hedgar2017
Holt Johnson
I0_ol
Iain
IamaTacos
Inanc Gumus
Inian
intboolstring
Jahid
James Taylor
Jamie Metzger
jandob
janos
Jeffrey Lin
JepZ
jerblack
Jesse Chen
JHS
jimsug
John Kugelman
Jon
Jon Ericson
Jonny Henly
jordi
Judd Rogers
Kamal Soni
Kelum Senanayake
leftaroundabout
Leo Ufimtsev
liborm
lynxlynxlynx
m02ph3u5
markjwill
Markus V.
Mateusz Piotrowski
Matt Clark

Chapter 10
Chapter 28
Chapter 10
Chapter 36
Chapter 51
Chapter 17
Chapter 10
Chapter 14
Chapter 9
Chapters 12, 15, 17, 20, 28 and 34
Chapters 8 and 53
Chapter 18
Chapters 9, 26, 33 and 36
Chapters 9, 15 and 58
Chapters 21 and 22
Chapters 1, 4, 7 and 5
Chapter 15
Chapter 36
Chapter 10
Chapters 9, 15 and 22
Chapter 4
Chapter 64
Chapters 4 and 20
Chapter 35
Chapter 1
Chapters 17 and 28
Chapters 4, 7 and 5
Chapters 1, 9, 10, 5, 12, 14, 15, 17, 20, 21, 22, 23, 30, 34, 45, 39, 43 and 44
Chapter 23
Chapter 31
Chapter 29
Chapters 7, 10, 12, 14, 20 and 24
Chapter 49
Chapter 3
Chapter 12
Chapters 15, 26 and 45
Chapters 7, 19 and 67
Chapter 24
Chapter 12
Chapter 63
Chapter 9
Chapter 4
Chapter 48
Chapters 9 and 67
Chapter 30
Chapter 23
Chapter 17
Chapter 33
Chapter 9
Chapter 43
Chapter 67
Chapter 12
Chapter 4
Chapter 12
Chapters 1, 9, 14, 19, 17 and 23

Bash Notes for Professionals

190

mattmc
Michael Le Barbier
Grünewald
Mike Metzger
miken32
Misa Lazovic
Mohima Chaudhuri
nautical
NeilWang
Neui
Ocab19
ormaaj
Osaka
P.P.
Pavel Kazhevets
Peter
phs
Pooyan Khosravi
Rafa Moyano
Reboot
Riccardo Petraglia
Richard Hamilton
Riker
Roman Piták
Root
Sameer Srivastava
Samik
Samuel
Saqib Rokadia
satyanarayan rao
Scroff
Sergey
sjsam
Sk606
Skynet
SLePort
Stephane Chazelas
Stobor
suleiman
Sundeep
Sylvain Bugat
Thomas Champion
Tim Rijavec
TomOnTime
Trevor Clarke
tripleee
tversteeg
uhelp
UNagaswamy
user1336087
vielmetti
vmaroli
Warren Harper
Wenzhong
Will

Chapters 36 and 65
Chapter 14
Chapter 8
Chapters 9 and 10
Chapters 4 and 30
Chapters 18 and 41
Chapter 34
Chapter 12
Chapter 8
Chapter 58
Chapter 12
Chapter 4
Chapter 38
Chapter 25
Chapter 31
Chapter 47
Chapter 9
Chapter 42
Chapter 42
Chapter 8
Chapters 4, 16, 57 and 41
Chapters 1 and 40
Chapter 47
Chapters 8, 9 and 5
Chapter 8
Chapters 4, 10, 5, 12, 14 and 37
Chapter 5
Chapter 67
Chapter 1
Chapter 66
Chapter 14
Chapters 1 and 32
Chapters 8, 12 and 33
Chapter 45
Chapters 10 and 5
Chapters 15 and 36
Chapter 20
Chapter 59
Chapter 1
Chapters 4, 9, 14, 15 and 2
Chapter 56
Chapter 25
Chapter 47
Chapter 1
Chapters 1, 5, 14, 17 and 36
Chapter 30
Chapters 47, 7, 20, 31, 2, 36, 48, 52, 53, 13 and 54
Chapters 12, 13 and 60
Chapters 1 and 26
Chapter 5
Chapter 39
Chapter 9
Chapter 30
Chapters 12, 15 and 21

Bash Notes for Professionals

191

Will Barnwell
William Pursell
Wojciech Kazior
Wolfgang
xhienne
ymbirtt
zarak
Zaz
Мона_Сах
南山竹

Chapter 24
Chapters 1, 36 and 49
Chapter 36
Chapter 9
Chapter 5
Chapter 15
Chapters 8, 24 and 31
Chapter 1
Chapter 28
Chapters 1, 9, 5, 12 and 17

Bash Notes for Professionals

192

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