Bash cheat sheet

A very usefull bash cheat sheet.

Michael KAMM
Michael KAMM   Follow

# bash cheat sheet

A highly opinionated bash cheat sheet.

# Conditions

Basics:

    #!/usr/bin/env bash
    
    if [ expr ] && [ expr ]; then
        echo 'and operator'
    elif [ expr ] || [ expr ]; then
        echo 'or operator'
    elif [ ! expr ]; then
        echo 'not operator'
    else
        echo 'third'
    fi

# String comparison

    #!/usr/bin/env bash
    
    if [[ $a == z* ]]; then
        echo 'starts with "z"'
    elif [ $a == "z*" ]; then
        echo 'is equal to "z*"'
    elif [ $a == z* ]; then
        echo 'file globbing and word splitting take place'
    elif [ "$a" != "$b" ]; then
        echo 'is not equal'
    elif [ -z '' ]; then
        echo 'is null, or empty'
    elif [ -n "$string1" ]; then
        echo 'is not null, or is initialized'
    fi

# Integer comparison

    #!/usr/bin/env bash
    
    if [ "$a" -eq "$b" ]; then
        echo 'is equal to'
    elif [ "$a" -ne "$b" ]; then
        echo 'is not equal to'
    elif [ "$a" -gt "$b" ]; then
        echo 'is greater than'
    elif [ "$a" -ge "$b" ]; then
        echo 'is greater than or equal to'
    elif [ "$a" -lt "$b" ]; then
        echo 'is less than'
    elif [ "$a" -le "$b" ]; then
        echo 'is less than or equal to'
    fi

# File checking

    #!/usr/bin/env bash
    
    if [ -e $file]; then
        echo 'File exists'
    elif [ -f $file ]; then
        echo 'is a regular file'
    elif [ -d $directory]; then
        echo 'is a directory'
    elif [ -L $symbolic_link ]; then
        echo 'is a symbolic link'
    elif [ -r $readable_file ]; then
        echo 'has read permission'
    elif [ -w $writable_file ]; then
        echo 'has write permission'
    elif [ -x $executable_file ]; then
        echo 'has execution permission'
    elif [ -O $your_file ]; then
        echo 'is owned by you'
    elif [ -N $edited_file ]; then
        echo 'modified since last read'
    elif [ $newer -nt $older ]; then
        echo 'is newer than'
    elif [ $older -ot $newer ]; then
        echo 'is older than'
    fi

# getopts

Synopis:

while getopts OPTSTRING opt; do
    case $opt in
        # Conditions
    esac
done

With OPTSTRING describing the short options:

  • starting OPTSTRING with a : will set Error Reporting to "silent mode"
  • each character describes a flag (eg for script.sh -h -e -y, OPTSTRING would be hey)
  • a character followed by a : describes a flag with an argument (eg for script -f /tmp/filename, OPTSTRING would be f:)

In case of error, opt will be set with the following value:

  • invalid option:
    • ?
      • in "silent mode", OPTARG is set with the invalid option character
  • missing required argument:
    • in "verbose mode": ?, and an error message is printed
    • in "silent mode": :, and OPTARG is set with the option character

Example:

#!/bin/bash
 
while getopts ":ab:" opt; do
    case $opt in
        a)
            echo "-a was triggered!" >&2
            ;;
        b)
            echo "-b was triggered, Parameter: $OPTARG" >&2
            ;;
        \?)
            echo "Invalid option: -$OPTARG" >&2
            exit 1
            ;;
        :)
            echo "Option -$OPTARG requires an argument." >&2
            exit 1
            ;;
    esac
done

# References

Linux Cheat
Conditions
String comparison
Integer comparison
File checking
getopts
References