files-and-directories.md

Files and directories

Information on a computer is stored in files and directories. The part of an operating system that handles this information is called the filesystem. Think of the filesystem as a tree with branches (directories) and leaves (files).

Let's find out where we are in the filesystem when we start a terminal.

$ pwd
/home/mpiper

The pwd command prints the current directory to the terminal. You can see that I'm in the directory mpiper under the directory home. The forward slash / is used as a delimiter between directory names.

This directory, /home/mpiper, is special--it's the home directory for the user mpiper. Every user has a home directory. It's the default location for you to create and store information.

Note that I executed this command on a Linux machine; the home directory on a macOS or Windows machine will be slightly different.

Directory listings

Next, let's look at the contents of this directory.

$ ls
anaconda3  build  Desktop  local  packages  scratch  tmp
bin        data   dist     nb     projects  testing

The ls command provides a directory listing. This home directory holds a set of subdirectories.

We can drill down through directories by providing arguments to ls. For example, to see the contents of the Desktop directory, type:

$ ls Desktop
data-shell  data-shell.zip

You can see the sample files we downloaded earlier. We can drill further; for example, to see into the directory containing the sample files:

$ ls Desktop/data-shell
creatures  molecules           notes.txt  solar.pdf
data       north-pacific-gyre  pizza.cfg  writing

Changing directories

Although we've peered into the directory of sample files, we remain in the home directory. To change directories, use the cd command:

$ cd Desktop
$ pwd
/home/mpiper/Desktop

The pwd command shows that we've switched to the Desktop directory.

The cd command can take a directory name as an argument. It can also take no arguments:

$ cd
$ pwd
/home/mpiper

With no arguments, cd always returns to your home directory.

The cd command can also take a set of special characters. To switch to the previous directory, use a dash -:

$ cd -
$ pwd
/home/mpiper/Desktop

When working with directories, a dot . is a shortcut for the current directory, while dot dot .. is a shortcut for the parent directory:

$ cd .
$ pwd
/home/mpiper/Desktop
$ cd ..
$ pwd
/home/mpiper

Likewise, the tilde ~ is a shortcut for the user's home directory:

$ cd ~
$ pwd
/home/mpiper

Directory queues

Directories can be stacked in a queue. This can be really helpful when you're frequently moving between a set of directories.

The pushd command pushes a directory onto the stack:

$ pushd Desktop/
~/Desktop ~

There are now two directories on the stack: the home directory ~, where we started, and ~/Desktop. A nice way to view the stack is with the dirs command; in particular, with the v option (for verbose):

$ dirs -v
 0  ~/Desktop
 1  ~

To switch to another directory on the stack, use the pushd command again, but this time with an argument, the index in the stack. For example, to switch to the home directory:

$ pushd +1
~ ~/Desktop
$ dirs -v
 0  ~
 1  ~/Desktop

The current directory is now the home directory.

To remove a directory from the stack, use the popd command. For example, pop the home directory off the stack with:

$ popd
~/Desktop

The current directory is now Desktop.

Handy things

I'm not the best typist. The shell saves me time, though, with tab completion. Anytime you're in the process of entering a command in a terminal, hit the Tab key, and the shell will attempt to complete the command. For example, from the Desktop directory, try listing the contents of data-shell by typing the first few characters, then hitting the Tab key:

$ ls da

Tab completion is wonderful.

The tree command is nonstandard, but it can be installed into a shell. It acts like an ASCII-art Finder, printing to the terminal the branch-leaf structure from a point in the filesystem. For example, from the Desktop directory, view the contents of the data-shell/writing directory with tree:

$ tree data-shell/writing
data-shell/writing/
|-- data
|   |-- LittleWomen.txt
|   |-- one.txt
|   `-- two.txt
|-- haiku.txt
|-- thesis
|   `-- empty-draft.md
`-- tools
    |-- format
    |-- old
    |   `-- oldtool
    `-- stats

4 directories, 8 files

Summary

The table below summarizes the commands and special characters described in this section.

Command/Character	Description
pwd	print working directory
ls	directory listing
cd	change directory
/	path delimiter
~	home directory
.	current directory
..	one directory up
-	previous directory
pushd	push directory on stack
popd	pop directory off stack
dirs	print directory stack
tree	view directory tree (nonstandard)
tab	complete command

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