Getting started

Introduction

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 $NAME echo "$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=$'

\t'

See: Unofficial bash strict mode

Brace expansion

echo {A,B}.js

Expression Description {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%/*} # /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

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

Code Description ${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

# Single line comment

: ' This is a multi line comment '

Substrings

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

Length

Expression Description ${#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

Expression Description ${FOO:-val} $FOO , or val if unset (or null) ${FOO:=val} Set $FOO to val if unset (or null) ${FOO:+val} val if $FOO is set (and not null) ${FOO:?message} Show error message and exit if $FOO is unset (or null)

Omitting the : removes the (non)nullity checks, e.g. ${FOO-val} 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

cat 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

Expression Description $# Number of arguments $* All arguments $@ All arguments, starting from first $1 First argument $_ Last argument of the previous command

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.

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 ${Fruits[0]} # Element #0 echo ${Fruits[-1]} # Last element 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) echo ${!Fruits[@]} # Keys of all elements, space-separated

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

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 !<command> Expand most recent invocation of command <command>

Operations

Code Description !! 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

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

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

Expression Description $? 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

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