Bash Cheat Sheet

args=("$@") args+=(foo) args+=(bar) echo "${args[@]}" 

Put the arguments into an array and then append

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

See: Unofficial bash strict mode

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

echo -n "Proceed? [y/n]: " read ans echo $ans 

read -n 1 ans # Just one character 

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

cat <<END hello world END 

• Â
• !
• "
• #
• &
• '
• (
• )
• ,
• ;
• <
• >
• [
• |
• \
• ]
• ^
• {
• }
• `
• $
• *
• ? Escape these special characters with \

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

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

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 

printf "Hello %s, I'm %s" Sven Olga #=> "Hello Sven, I'm Olga printf "1 + 1 = %d" 2 #=> "1 + 1 = 2" printf "Print a float: %f" 2 #=> "Print a float: 2.000000" 

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

DIR="${0%/\*}" 

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

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 

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

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

$((a + 200)) # Add 200 to $a 

$(($RANDOM%200)) # Random number 0..199 

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

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

myfunc() { return 1 } 

if myfunc; then echo "success" else echo "failure" fi 

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

result="$(myfunc)" 

myfunc() { echo "hello $1" } 

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

myfunc "John" 

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

while :; do # here is some code. done 

while true; do # here is some code. done 

count=0 until [ $count -gt 10 ]; do echo "$count" ((count++)) done 

for number in $(seq 1 3); do if [[ $number == 2 ]]; then # Skip entire rest of loop. break; fi # This will only print 1 echo "$number" done 

for number in $(seq 1 3); do if [[ $number == 2 ]]; then continue; fi echo "$number" done 

i=3 while [[ $i -gt 0 ]]; do echo "Number: $i" ((i--)) done 

i=1 while [[ $i -lt 4 ]]; do echo "Number: $i" ((i++)) done 

for i in {1..5}; do echo "Welcome $i" done 

#With step size

for i in {5..50..5}; do echo "Welcome $i" done 

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

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

if [ "$1" = 'y' -a $2 -gt 0 ]; then echo "yes" fi if [ "$1" = 'n' -o $2 -lt 0 ]; then echo "no" fi 

#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" ]]; then ... fi 

#Regex

if [[ '1. abc' =~ ([a-z]+) ]]; then echo ${BASH\_REMATCH[1]} fi 

#Smaller

if (( $a < $b )); then echo "$a is smaller than $b" fi 

#Exists

if [[ -e "file.txt" ]]; then echo "file exists" fi 

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

declare -A sounds 

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

function extract() { local -n myarray=$1 local idx=$2 echo "${myarray[$idx]}" } Fruits=('Apple' 'Banana' 'Orange') extract Fruits 2 # => Orangle 

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 

Fruits=('Apple' 'Banana' 'Orange') for e in "${Fruits[@]}"; do echo $e done 

#With index

for i in "${!Fruits[@]}"; do printf "%s\t%s\n" "$i" "${Fruits[$i]}" done 

Fruits=('Apple' 'Banana' 'Orange') Fruits[0]="Apple" Fruits[1]="Banana" Fruits[2]="Orange" ARRAY1=(foo{1..2}) # => foo1 foo2 ARRAY2=({A..D}) # => A B C D # Merge => foo1 foo2 A B C D ARRAY3=(${ARRAY1[@]} ${ARRAY2[@]}) # declare construct declare -a Numbers=(1 2 3) Numbers+=(4 5) # Append => 1 2 3 4 5 

STR="HELLO WORLD!" echo ${STR,} # => hELLO WORLD! echo ${STR,,} # => hello world! STR="hello world!" echo ${STR^} # => Hello world! echo ${STR^^} # => HELLO WORLD! ARR=(hello World) echo "${ARR[@],}" # => hello world echo "${ARR[@]^}" # => Hello World 

SRC="/path/to/foo.cpp" 

BASEPATH=${SRC##\*/} echo $BASEPATH # => "foo.cpp" DIRPATH=${SRC%$BASEPATH} echo $DIRPATH # => "/path/to/" 

name="John" echo ${name} # => John echo ${name:0:2} # => Jo echo ${name::2} # => Jo echo ${name::-1} # => Joh echo ${name:(-1)} # => n echo ${name:(-2)} # => hn echo ${name:(-2):2} # => hn length=2 echo ${name:0:length} # => Jo 

See: Parameter expansion

echo ${food:-Cake} #=> $food or "Cake" 

STR="/path/to/foo.cpp" echo ${STR%.cpp} # /path/to/foo echo ${STR%.cpp}.o # /path/to/foo.o echo ${STR%/\*} # /path/to 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 

#Substrings

#Length

#Default values

# => I'm in /path/of/current echo "I'm in $(PWD)" # Same as: echo "I'm in `pwd`" 

See: Command substitution

echo {A,B}.js 

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

See: Conditionals

get\_name() { echo "John" } echo "You are $(get\_name)" 

See: Functions

# This is an inline Bash comment. 

: ' This is a very neat comment in bash ' 

Multi-line comments use :' to open and ' to close

NAME="John" echo ${NAME} # => John (Variables) echo $NAME # => John (Variables) echo "$NAME" # => John (Variables) echo '$NAME' # => $NAME (Exact string) echo "${NAME}!" # => John! (Variables) NAME = "John" # => Error (about space) 

#!/bin/bash VAR="world" echo "Hello $VAR!" # => Hello world! 

Execute the script

$ bash hello.sh