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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

Watch log file in reverse order and paginated​

tac /var/log/cron-20240211 | less