[Bash] Bash scripting cheatsheet

source: https://devhints.io/bash

Example

#!/usr/bin/env bash NAME="John" echo "Hello $NAME!"

Variables

NAME="John" echo $NAME echo "$NAME" echo "${NAME}!"

String quotes

NAME="John" echo "Hi $NAME" #=> Hi John echo 'Hi $NAME' #=> Hi $NAME

Shell execution

echo "I'm in $(pwd)" echo "I'm in `pwd`" # Same

See Command substitution

Conditional execution

git commit && git push git commit || echo "Commit failed"

Functions

get_name() { echo "John" } echo "You are $(get_name)"

See: Functions

Conditionals

if [[ -z "$string" ]]; then echo "String is empty" elif [[ -n "$string" ]]; then echo "String is not empty" fi

See: Conditionals

Strict mode

set -euo pipefail IFS=$'\n\t'

See: Unofficial bash strict mode

Brace expansion

echo {A,B}.js

{A,B}	Same as A B
{A,B}.js	Same as A.js B.js
{1..5}	Same as 1 2 3 4 5

See: Brace expansion

#Parameter expansions

Basics

name="John" echo ${name} echo ${name/J/j} #=> "john" (substitution) echo ${name:0:2} #=> "Jo" (slicing) echo ${name::2} #=> "Jo" (slicing) echo ${name::-1} #=> "Joh" (slicing) echo ${name:(-1)} #=> "n" (slicing from right) echo ${name:(-2):1} #=> "h" (slicing from right) echo ${food:-Cake} #=> $food or "Cake" length=2 echo ${name:0:length} #=> "Jo"

See: Parameter expansion

STR="/path/to/foo.cpp" echo ${STR%.cpp} # /path/to/foo echo ${STR%.cpp}.o # /path/to/foo.o echo ${STR##*.} # cpp (extension) echo ${STR##*/} # foo.cpp (basepath) echo ${STR#*/} # path/to/foo.cpp echo ${STR##*/} # foo.cpp echo ${STR/foo/bar} # /path/to/bar.cpp STR="Hello world" echo ${STR:6:5} # "world" echo ${STR:-5:5} # "world" SRC="/path/to/foo.cpp" BASE=${SRC##*/} #=> "foo.cpp" (basepath) DIR=${SRC%$BASE} #=> "/path/to/" (dirpath)

Substitution

${FOO%suffix}	Remove suffix
${FOO#prefix}	Remove prefix
${FOO%%suffix}	Remove long suffix
${FOO##prefix}	Remove long prefix
${FOO/from/to}	Replace first match
${FOO//from/to}	Replace all
${FOO/%from/to}	Replace suffix
${FOO/#from/to}	Replace prefix

Comments

# Single line comment : ' This is a multi line comment '

Substrings

${FOO:0:3}	Substring (position, length)
${FOO:-3:3}	Substring from the right

Length

${#FOO}

Length of $FOO

Manipulation

STR="HELLO WORLD!" echo ${STR,} #=> "hELLO WORLD!" (lowercase 1st letter) echo ${STR,,} #=> "hello world!" (all lowercase) STR="hello world!" echo ${STR^} #=> "Hello world!" (uppercase 1st letter) echo ${STR^^} #=> "HELLO WORLD!" (all uppercase)

Default values

${FOO:-val}	$FOO, or val if not set
${FOO:=val}	Set $FOO to val if not set
${FOO:+val}	val if $FOO is set
${FOO:?message}	Show error message and exit if $FOO is not set

The : is optional (eg, ${FOO=word} works)

#Loops

Basic for loop

for i in /etc/rc.*; do echo $i done

C-like for loop

for ((i = 0 ; i < 100 ; i++)); do echo $i done

Ranges

for i in {1..5}; do echo "Welcome $i" done With step size for i in {5..50..5}; do echo "Welcome $i" done

Reading lines

< file.txt | while read line; do echo $line done

Forever

while true; do ··· done

#Functions

Defining functions

myfunc() { echo "hello $1" } # Same as above (alternate syntax) function myfunc() { echo "hello $1" } myfunc "John"

Returning values

myfunc() { local myresult='some value' echo $myresult } result="$(myfunc)"

Raising errors

myfunc() { return 1 } if myfunc; then echo "success" else echo "failure" fi

Arguments

$#	Number of arguments
$*	All arguments
$@	All arguments, starting from first
$1	First argument

See Special parameters.

#Conditionals

Conditions

Note that [[ is actually a command/program that returns either 0 (true) or 1 (false). Any program that obeys the same logic (like all base utils, such as grep(1) or ping(1)) can be used as condition, see examples.

[[ -z STRING ]]	Empty string
[[ -n STRING ]]	Not empty string
[[ STRING == STRING ]]	Equal
[[ STRING != STRING ]]	Not Equal
[[ NUM -eq NUM ]]	Equal
[[ NUM -ne NUM ]]	Not equal
[[ NUM -lt NUM ]]	Less than
[[ NUM -le NUM ]]	Less than or equal
[[ NUM -gt NUM ]]	Greater than
[[ NUM -ge NUM ]]	Greater than or equal
[[ STRING =~ STRING ]]	Regexp
(( NUM < NUM ))	Numeric conditions

[[ -o noclobber ]]	If OPTIONNAME is enabled
[[ ! EXPR ]]	Not
[[ X ]] && [[ Y ]]	And
[[ X ]] || [[ Y ]]	Or

File conditions

[[ -e FILE ]]	Exists
[[ -r FILE ]]	Readable
[[ -h FILE ]]	Symlink
[[ -d FILE ]]	Directory
[[ -w FILE ]]	Writable
[[ -s FILE ]]	Size is > 0 bytes
[[ -f FILE ]]	File
[[ -x FILE ]]	Executable
[[ FILE1 -nt FILE2 ]]	1 is more recent than 2
[[ FILE1 -ot FILE2 ]]	2 is more recent than 1
[[ FILE1 -ef FILE2 ]]	Same files

Example

if ping -c 1 google.com; then echo "It appears you have a working internet connection" fi if grep -q 'foo' ~/.bash_history; then echo "You appear to have typed 'foo' in the past" fi # String if [[ -z "$string" ]]; then echo "String is empty" elif [[ -n "$string" ]]; then echo "String is not empty" fi # Combinations if [[ X ]] && [[ Y ]]; then ... fi # Equal if [[ "$A" == "$B" ]] # Regex if [[ "A" =~ "." ]] if (( $a < $b )); then echo "$a is smaller than $b" fi if [[ -e "file.txt" ]]; then echo "file exists" fi

#Arrays

Defining arrays

Fruits=('Apple' 'Banana' 'Orange') Fruits[0]="Apple" Fruits[1]="Banana" Fruits[2]="Orange"

Working with arrays

echo ${Fruits[0]} # Element #0 echo ${Fruits[@]} # All elements, space-separated echo ${#Fruits[@]} # Number of elements echo ${#Fruits} # String length of the 1st element echo ${#Fruits[3]} # String length of the Nth element echo ${Fruits[@]:3:2} # Range (from position 3, length 2)

Operations

Fruits=("${Fruits[@]}" "Watermelon") # Push Fruits+=('Watermelon') # Also Push Fruits=( ${Fruits[@]/Ap*/} ) # Remove by regex match unset Fruits[2] # Remove one item Fruits=("${Fruits[@]}") # Duplicate Fruits=("${Fruits[@]}" "${Veggies[@]}") # Concatenate lines=(`cat "logfile"`) # Read from file

Iteration

for i in "${arrayName[@]}"; do echo $i done

#Dictionaries

Defining

declare -A sounds sounds[dog]="bark" sounds[cow]="moo" sounds[bird]="tweet" sounds[wolf]="howl"

Declares sound as a Dictionary object (aka associative array).

Working with dictionaries

echo ${sounds[dog]} # Dog's sound echo ${sounds[@]} # All values echo ${!sounds[@]} # All keys echo ${#sounds[@]} # Number of elements unset sounds[dog] # Delete dog

Iteration

Iterate over values for val in "${sounds[@]}"; do echo $val done Iterate over keys for key in "${!sounds[@]}"; do echo $key done

#Options

Options

set -o noclobber # Avoid overlay files (echo "hi" > foo) set -o errexit # Used to exit upon error, avoiding cascading errors set -o pipefail # Unveils hidden failures set -o nounset # Exposes unset variables

Glob options

set -o nullglob # Non-matching globs are removed ('*.foo' => '') set -o failglob # Non-matching globs throw errors set -o nocaseglob # Case insensitive globs set -o globdots # Wildcards match dotfiles ("*.sh" => ".foo.sh") set -o globstar # Allow ** for recursive matches ('lib/**/*.rb' => 'lib/a/b/c.rb')

Set GLOBIGNORE as a colon-separated list of patterns to be removed from glob matches.

#History

Commands

history	Show history
shopt -s histverify	Don’t execute expanded result immediately

Expansions

!$	Expand last parameter of most recent command
!*	Expand all parameters of most recent command
!-n	Expand nth most recent command
!n	Expand nth command in history
!<command>	Expand most recent invocation of command <command>

Operations

!!	Execute last command again
!!:s/<FROM>/<TO>/	Replace first occurrence of <FROM> to <TO> in most recent command
!!:gs/<FROM>/<TO>/	Replace all occurrences of <FROM> to <TO> in most recent command
!$:t	Expand only basename from last parameter of most recent command
!$:h	Expand only directory from last parameter of most recent command

!! and !$ can be replaced with any valid expansion.

Slices

!!:n	Expand only nth token from most recent command (command is 0; first argument is 1)
!^	Expand first argument from most recent command
!$	Expand last token from most recent command
!!:n-m	Expand range of tokens from most recent command
!!:n-$	Expand nth token to last from most recent command

!! can be replaced with any valid expansion i.e. !cat, !-2, !42, etc.

#Miscellaneous

Numeric calculations

$((a + 200)) # Add 200 to $a $((RANDOM%=200)) # Random number 0..200

Subshells

(cd somedir; echo "I'm now in $PWD") pwd # still in first directory

Redirection

python hello.py > output.txt # stdout to (file) python hello.py >> output.txt # stdout to (file), append python hello.py 2> error.log # stderr to (file) python hello.py 2>&1 # stderr to stdout python hello.py 2>/dev/null # stderr to (null) python hello.py &>/dev/null # stdout and stderr to (null) python hello.py < foo.txt # feed foo.txt to stdin for python

Inspecting commands

command -V cd #=> "cd is a function/alias/whatever"

Trap errors

trap 'echo Error at about $LINENO' ERR

or

traperr() { echo "ERROR: ${BASH_SOURCE[1]} at about ${BASH_LINENO[0]}" } set -o errtrace trap traperr ERR

Case/switch

case "$1" in start | up) vagrant up ;; *) echo "Usage: $0 {start|stop|ssh}" ;; esac

Source relative

source "${0%/*}/../share/foo.sh"

printf

printf "Hello %s, I'm %s" Sven Olga #=> "Hello Sven, I'm Olga printf "1 + 1 = %d" 2 #=> "1 + 1 = 2" printf "This is how you print a float: %f" 2 #=> "This is how you print a float: 2.000000"

Directory of script

DIR="${0%/*}"

Getting options

while [[ "$1" =~ ^- && ! "$1" == "--" ]]; do case $1 in -V | --version ) echo $version exit ;; -s | --string ) shift; string=$1 ;; -f | --flag ) flag=1 ;; esac; shift; done if [[ "$1" == '--' ]]; then shift; fi

Heredoc

cat <<END hello world END

Reading input

echo -n "Proceed? [y/n]: " read ans echo $ans read -n 1 ans # Just one character

Special variables

$?	Exit status of last task
$!	PID of last background task
$$	PID of shell
$0	Filename of the shell script

See Special parameters.

Go to previous directory

pwd # /home/user/foo cd bar/ pwd # /home/user/foo/bar cd - pwd # /home/user/foo