The cat command dumps the contents of a file to the terminal:
$ cat haiku
Yesterday it worked
Today it is not working
Software is like that.
This is an easy way of looking at a file--simpler than opening it with a text editor.
The haiku file is short.
For longer files, a pager is more appropriate to use.
Let's change to the data-shell/data directory
and view the contents of the file planets.txt
with the pager more:
$ cd data-shell/data/
$ ls
amino-acids.txt animals.txt morse.txt planets.txt sunspot.txt
animal-counts elements pdb salmon.txt
$ more planets.txt
"Planet Name","Pl. Mass","Pl. Radius","Pl. Period",
"55 Cnc e","0.027","0.194","0.7365449",
"CoRoT-1 b","1.03","1.49","1.5089557",
"CoRoT-10 b","2.75","0.97","13.2406",
"CoRoT-11 b","2.33","1.43","2.99433",
"CoRoT-12 b","0.917","1.44","2.828042",
...
This text file is quite long,
so more pages the output.
You can hit Enter or Spc to advance through the file,
and q to quit back to the command prompt.
The more command also has a more advanced sibling, less:
$ less planets.txt
Related to pagers,
the head and tail commands
dump the first/last (respectively) 10 lines of text from a file.
$ head planets.txt
"Planet Name","Pl. Mass","Pl. Radius","Pl. Period",
"55 Cnc e","0.027","0.194","0.7365449",
"CoRoT-1 b","1.03","1.49","1.5089557",
"CoRoT-10 b","2.75","0.97","13.2406",
"CoRoT-11 b","2.33","1.43","2.99433",
"CoRoT-12 b","0.917","1.44","2.828042",
"CoRoT-13 b","1.308","0.885","4.03519",
"CoRoT-14 b","7.6","1.09","1.51214",
"CoRoT-16 b","0.535","1.17","5.35227",
"CoRoT-17 b","2.45","1.02","3.768125",
$ tail planets.txt
"WASP-68 b","0.8","0.9","5.1",
"WASP-69 b","0.3","1","3.9",
"WASP-7 b","0.96","1.33","4.9546416",
"WASP-70 b","0.6","0.8","3.7",
"WASP-8 b","2.244","1.038","8.158715",
"XO-1 b","0.9","1.184","3.9415128",
"XO-2 b","0.62","0.973","2.615838",
"XO-3 b","11.79","1.217","3.1915239",
"XO-4 b","1.72","1.34","4.1250823",
"XO-5 b","1.077","1.03","4.1877537",
The wc command counts the number of lines, words, and characters in a file.
For example, for our haiku:
$ wc haiku
3 12 67 haiku
The file haiku has
- 3 lines,
- 12 words, and
- 67 characters.
The vertical bar | performs the pipe action:
it takes the output from one command
and feeds them as input to another command.
For example,
get the first five lines of the long directory listing
for the data-shell/data/pdb directory:
$ ls -l data-shell/data/pdb | head -5
total 208
-rw-r--r--. 1 mpiper csdms 1516 Aug 7 2019 aldrin.pdb
-rw-r--r--. 1 mpiper csdms 306 Aug 7 2019 ammonia.pdb
-rw-r--r--. 1 mpiper csdms 1444 Aug 7 2019 ascorbic-acid.pdb
-rw-r--r--. 1 mpiper csdms 1030 Aug 7 2019 benzaldehyde.pdb
Actually, how many files are in this directory?
$ ls -1 data-shell/data/pdb/ | wc -l
48
In this example,
the 1 option forces ls to print one file per line,
and the l option to wc returns only the number of lines.
The greater than symbol > performs the redirect action:
it takes the output from a command
and redirects it from the terminal to a file.
For example,
there are a set of data files in the data-shell/molecules directory:
$ ls data-shell/molecules/
cubane.pdb ethane.pdb methane.pdb octane.pdb pentane.pdb propane.pdb
Let's find the lengths of these files and store them in a new file:
$ cd data-shell/molecules/
$ wc -l *.pdb > ~/Desktop/molecule_pdb_lengths
$ cd -
/home/mpiper/Desktop
$ cat molecule_pdb_lengths
20 cubane.pdb
12 ethane.pdb
9 methane.pdb
30 octane.pdb
21 pentane.pdb
15 propane.pdb
107 total
Note that I used the asterisk * as a wildcard in this example.
The asterisk matches any set of characters;
here, it's used to match any file that ends with .pdb.
Related to the asterisk is the question mark ?,
which is used to match a single character in a filename.
For example,
change to the data-shell directory and view its contents:
$ cd data-shell/
$ ls
creatures molecules notes.txt solar.pdf
data north-pacific-gyre pizza.cfg writing
Now find all files in this directory with five characters in their name and any extension:
$ ls ?????.*
notes.txt pizza.cfg solar.pdf
Switch back to the previous directory:
$ cd -
/home/mpiper/Desktop
Finally,
the sort command can be used to sort the contents of a file.
Sort the contents of the molecule_pdb_lengths file
we created earlier:
$ sort -n molecule_pdb_lengths
9 methane.pdb
12 ethane.pdb
15 propane.pdb
20 cubane.pdb
21 pentane.pdb
30 octane.pdb
107 total
As a bonus,
the sed command is a stream editor.
It can programmatically edit files
instead of interactively, like with a text editor.
Use sed to programmatically edit the molecule_pdb_lengths file,
replacing "pdb" with "txt":
$ sed -i.bak s@pdb@txt@g molecule_pdb_lengths
The editing is done in-place on the file.
The i.bak creates a backup file with the contents of the original file.
View the results:
$ cat molecule_pdb_lengths
20 cubane.txt
12 ethane.txt
9 methane.txt
30 octane.txt
21 pentane.txt
15 propane.txt
107 total
$ cat molecule_pdb_lengths.bak
20 cubane.pdb
12 ethane.pdb
9 methane.pdb
30 octane.pdb
21 pentane.pdb
15 propane.pdb
107 total
Rather than visually inspecting the differences between these files,
use the diff command:
$ diff molecule_pdb_lengths*
1,6c1,6
< 20 cubane.txt
< 12 ethane.txt
< 9 methane.txt
< 30 octane.txt
< 21 pentane.txt
< 15 propane.txt
---
> 20 cubane.pdb
> 12 ethane.pdb
> 9 methane.pdb
> 30 octane.pdb
> 21 pentane.pdb
> 15 propane.pdb
The table below summarizes the commands and special characters described in this section.
| Command/Character | Description |
|---|---|
| cat | dump text to terminal |
| more | screen pager |
| less | better screen pager |
| head | print first lines of file |
| tail | print last lines of file |
| wc | count characters, words, lines |
| > | redirect output of command |
| * | match all characters |
| ? | match single characters |
| sort | sorts input |
| sed | stream editor |
| diff | show differences between files |
Introduction to the shell | Previous: Creating things | Next: Finding things