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Bash

Functions​

Access arguments from within a function​

  • All: "@"
  • All starting after x: "$\{@:x}"
  • Positional: $1, $2, $x
  • Make argument list start after the first one: call shift or shift n to begin after the nth.

Define function​

function hello () {
    echo "Hello world"
}

Get file count in target directory​

Including dotfiles​

ls -1A | wc -l

Excluding dotfiles​

ls -1 | wc -l
tip

For directories other than the current one the path can be passed to the ls command.

Get full path of target file or dir​

readlink -f filename

Iterate over a range of numbers including both limits​

for i in "$(seq 1 3)"; do echo "$i"; done

Jobs​

To pause the current job press Ctrl+z and to restart the last job run fg.

Merge files in dir with cat and glob​

Merges all files in the current dir into a single file.

cat * >> bigfile.txt
tip

Use *.extension instead of * to merge only files with the given extension.

Return to previous position in history after searching with Ctrl+r​

Ctrl+c - End of history.

Ctrl+g - Last position before starting search.

Run command during logout​

Add command to ~/.bash_logout.

Run executable file​

Mark the file as executable.

chmod +x filename

Execute the file in the terminal with one of the following commands.

./filename
bash filename
tip

The second option works even if the file doesn't have execution permission.

Send string or command output to stdin of another command​

# Using pipes.
echo 'testOutput' | cut -c 5-
# With here-docs (strings only).
# Single-line string.
cat <<< 'test str'
# Multi-line string and command options.
cat <<EOF -n -s
"$var"
line 2
EOF
# Using process substitution.
# < is for redirection to stdin, <() is the process substitution syntax.
cat < <(echo 'result sent to stdin')

Set environment variables​

For all users​

Add a line to /etc/profile.

export VARNAME=value

Or create a file with .sh extension containing it in /etc/profile.d.

For a given user​

Add line to ~/.bash_profile or ~/.bashrc.

note

These statements are executed after user log in.

Show pipe operation status with pv​

The basic usage is similar to the cat command. For instance to show progress while creating a tarball:

tar cf - /target | pv > target.tar

And when extracting files from one:

pv target.tar | tar xf -

Use parts of last command in current one​

# Positional arguments
$ echo a
a
$ echo !:1
a

$ echo a b
a b
$ echo !:2
b

$ echo c d
c d
$ echo !:2 q !:1
d q c

# Name of the command
$ rm somefile
$ echo !:0
rm

Useful Environment Variables​

DISPLAY

Display identifier from the display server (e.g. x11).

EDITOR

Text editor used by different applications (e.g. crontab, visudo).

HOME

Current user's home directory.

HOSTNAME

System's name.

PATH

List of dirs where executables are looked for by the shell.

PWD

Current working directory.

OLDPWD

Previous working directory.

SHELL

Path to the current user's shell.

UID

Current user's numeric ID.

USER

Name of the current user.

XDG_SESSION_TYPE

Graphical session type (i.e. wayland, x11)

Watch log file in reverse order and paginated​

tac /var/log/cron-20240211 | less