Chapter 3: Files and directories

Work on many files at once

Before Bash runs a command, it rewrites the command line: patterns become lists of matching file names, braces become sequences, ~ becomes your home directory, and $(…) becomes a command's output. These expansions let one short command act on hundreds of files. Quoting lets you switch them off when you don't want them.

• Match file names with *, ? and […], and predict which names a pattern selects.
• Generate lists of names with brace expansion, and use ~, variables and command substitution.
• Use double quotes, single quotes and backslashes to control which expansions happen.

The shell expands, the command never knows

When you type ls *.html, ls never sees the *. Bash finds the matching names first and runs ls about.html index.html. Every command therefore gets the same pattern features for free, and you can always check what a pattern will do by putting echo in front of it:

[student@servera site]$ echo rm *.html
rm about.html index.html

Match file names with patterns

Two rules surprise people:

• A pattern never matches a name that starts with a dot unless the pattern starts with a dot too. * skips hidden files; .* finds them.
• If nothing matches, Bash passes the pattern on unchanged. Then the command looks for a file literally named *.zip, and usually says it doesn't exist.

Try patterns against a directory of a website's files. The directory list highlights what matches, and the last line shows the command Bash actually runs:

Generate names with braces

Braces don't look at files at all. They generate one word for each item in a list or a range, whether or not such files exist, which makes them perfect for creating things:

[student@servera ~]$ echo {Sunday,Monday,Tuesday,Wednesday}.log
Sunday.log Monday.log Tuesday.log Wednesday.log
[student@servera ~]$ echo file{1..3}.txt
file1.txt file2.txt file3.txt
[student@servera ~]$ echo file{a..c}.txt
filea.txt fileb.txt filec.txt
[student@servera ~]$ echo file{a,b}{1,2}.txt
filea1.txt filea2.txt fileb1.txt fileb2.txt
[student@servera ~]$ echo ep{01..03}
ep01 ep02 ep03

So mkdir -p project/{src,docs,tests} creates three directories in one go, and touch report-{jan,feb,mar}.txt three files. Brace expansion happens first, so its results can still contain patterns: ls *.{html,css} lists every HTML and CSS file.

The tilde, variables and command substitution

[student@servera ~]$ echo ~
/home/student
[student@servera ~]$ echo ~root
/root
[student@servera ~]$ echo $HOME
/home/student
[student@servera ~]$ echo "Today is $(date +%A)."
Today is Saturday.

• ~ is your home directory; ~user is another user's.
• $NAME (or ${NAME}) is replaced with the value of the variable NAME. You set one with NAME=value, no spaces around the =. Chapter 5 covers variables properly.
• $(command) runs the command and puts its output in its place. That is command substitution: cd $(dirname /etc/ssh/sshd_config) changes to /etc/ssh.

[student@servera ~]$ host=$(hostname)
[student@servera ~]$ echo "***** hostname is ${host} *****"
***** hostname is servera.lab.example.com *****

Quoting turns expansion off

Leave out the quotes in the hostname example above and the asterisks become patterns. In a directory full of files, the line fills with file names:

[student@servera w]$ echo ***** hostname is ${host} *****
about.html app.js index.html logo.png ... hostname is servera.lab.example.com about.html app.js ...

Try it

This practice terminal knows a set of expansion examples. Run each and compare the output with your prediction.

Check your understanding

• Which pattern matches report1.txt and reportA.txt but not report10.txt?
• A directory holds only a.txt and b.txt. What does ls *.log run?
• What does mkdir -p app/{logs,cache,config} create?
• Which prints $HOME literally, without expanding it?
• Why does ls * not list .bashrc?
• What is the safe way to check which files rm *.tmp would remove?