Bash scripting cheatsheet

This is a quick reference to getting started with Bash scripting.

Example

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

Variables

name="John" echo $name # see below echo "$name" echo "$ !"

Generally quote your variables unless they contain wildcards to expand or command fragments.

wildcard="*.txt" options="iv" cp -$options $wildcard /tmp

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`" # obsolescent # Same

Conditional execution

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

Functions

get_name()  echo "John" > echo "You are $(get_name)"

Conditionals

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

Strict mode

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

Brace expansion

echo A,B>.js
Expression Description
Same as A B
.js Same as A.js B.js
Same as 1 2 3 4 5
,> Same as 1 2 3 7 8 9

Parameter expansions

Basics

name="John" echo "$ " echo "$/J/j>" #=> "john" (substitution) echo "$:0:2>" #=> "Jo" (slicing) echo "$::2>" #=> "Jo" (slicing) echo "$::-1>" #=> "Joh" (slicing) echo "$:(-1)>" #=> "n" (slicing from right) echo "$:(-2):1>" #=> "h" (slicing from right) echo "$:-Cake>" #=> $food or "Cake" 
length=2 echo "$:0:length>" #=> "Jo" 
str="/path/to/foo.cpp" echo "$%.cpp>" # /path/to/foo echo "$%.cpp>.o" # /path/to/foo.o echo "$%/*>" # /path/to echo "$##*.>" # cpp (extension) echo "$##*/>" # foo.cpp (basepath) echo "$#*/>" # path/to/foo.cpp echo "$##*/>" # foo.cpp echo "$/foo/bar>" # /path/to/bar.cpp 
str="Hello world" echo "$:6:5>" # "world" echo "$: -5:5>" # "world" 
src="/path/to/foo.cpp" base=$##*/> #=> "foo.cpp" (basepath) dir=$%$base> #=> "/path/to/" (dirpath)

Prefix name expansion

prefix_a=one prefix_b=two echo $!prefix_*> # all variables names starting with `prefix_` prefix_a prefix_b

Indirection

name=joe pointer=name echo $!pointer> joe

Substitution

Code Description
$ Remove suffix
$ Remove prefix
$ Remove long suffix
$ Remove long suffix
$ Remove long prefix
$ Remove long prefix
$ Replace first match
$ Replace all
$ Replace suffix
$ Replace prefix

Comments

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

Substrings

Expression Description
$ Substring (position, length)
$ Substring from the right

Length

Expression Description
$ Length of $foo

Manipulation

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

Default values

Expression Description
$ $foo , or val if unset (or null)
$ Set $foo to val if unset (or null)
$ val if $foo is set (and not null)
$ Show error message and exit if $foo is unset (or null)

Omitting the : removes the (non)nullity checks, e.g. $ expands to val if unset otherwise $foo .

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

while read -r line; do echo "$line" done file.txt

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

Expression Description
$# Number of arguments
$* All positional arguments (as a single word)
$@ All positional arguments (as separate strings)
$1 First argument
$_ Last argument of the previous command

Note: $@ and $* must be quoted in order to perform as described. Otherwise, they do exactly the same thing (arguments as separate strings).

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.

Condition Description
[[ -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

More conditions

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

File conditions

Condition Description
[[ -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

# String if [[ -z "$string" ]]; then echo "String is empty" elif [[ -n "$string" ]]; then echo "String is not empty" else echo "This never happens" 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 "$[0]>" # Element #0 echo "$[-1]>" # Last element echo "$[@]>" # All elements, space-separated echo "$#Fruits[@]>" # Number of elements echo "$#Fruits>" # String length of the 1st element echo "$#Fruits[3]>" # String length of the Nth element echo "$[@]:3:2>" # Range (from position 3, length 2) echo "$!Fruits[@]>" # Keys of all elements, space-separated

Operations

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

Iteration

for i in "$[@]>"; do echo "$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 "$[dog]>" # Dog's sound echo "$[@]>" # All values echo "$!sounds[@]>" # All keys echo "$#sounds[@]>" # Number of elements unset sounds[dog] # Delete dog

Iteration

Iterate over values

for val in "$[@]>"; do echo "$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

shopt -s nullglob # Non-matching globs are removed ('*.foo' => '') shopt -s failglob # Non-matching globs throw errors shopt -s nocaseglob # Case insensitive globs shopt -s dotglob # Wildcards match dotfiles ("*.sh" => ".foo.sh") shopt -s 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

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

Expansions

Expression Description
!$ Expand last parameter of most recent command
!* Expand all parameters of most recent command
!-n Expand n th most recent command
!n Expand n th command in history
! Expand most recent invocation of command

Operations

Code Description
!! Execute last command again
. s/// Replace first occurrence of to in most recent command
. gs/// Replace all occurrences of 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

Code Description
. n Expand only n th 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 n th 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..199 
declare -i count # Declare as type integer count+=1 # Increment

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 >output.txt 2>&1 # stdout and stderr to (file), equivalent to &> python hello.py &>/dev/null # stdout and stderr to (null) echo "$0: warning: too many users" >&2 # print diagnostic message to stderr 
python hello.py  foo.txt # feed foo.txt to stdin for python diff (ls -r) (ls) # Compare two stdout without files

Inspecting commands

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

Trap errors

trap 'echo Error at about $LINENO' ERR 
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 " ;; esac

Source relative

source "$%/*>/../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" printf '%s\n' '#!/bin/bash' 'echo hello' >file # format string is applied to each group of arguments printf '%i+%i=%i\n' 1 2 3 4 5 9

Transform strings

Command option Description
-c Operations apply to characters not in the given set
-d Delete characters
-s Replaces repeated characters with single occurrence
-t Truncates
[:upper:] All upper case letters
[:lower:] All lower case letters
[:digit:] All digits
[:space:] All whitespace
[:alpha:] All letters
[:alnum:] All letters and digits

Example

echo "Welcome To Devhints" | tr '[:lower:]' '[:upper:]' WELCOME TO DEVHINTS

Directory of script

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 -r ans echo "$ans"

The -r option disables a peculiar legacy behavior with backslashes.

read -n 1 ans # Just one character

Special variables

Expression Description
$? Exit status of last task
$! PID of last background task
$$ PID of shell
$0 Filename of the shell script
$_ Last argument of the previous command
$ return value of piped commands (array)

Go to previous directory

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

Check for command’s result

if ping -c 1 google.com; then echo "It appears you have a working internet connection" fi

Grep check

if grep -q 'foo' ~/.bash_history; then echo "You appear to have typed 'foo' in the past" fi

Also see