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 $name # see below echo "$name" echo "${name}!"

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

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
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
{{1..3},{7..9}} Same as 1 2 3 7 8 9

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)

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
${foo%suffix} Remove suffix
${foo#prefix} Remove prefix
${foo%%suffix} Remove long suffix
${foo/%suffix} Remove long suffix
${foo##prefix} Remove long prefix
${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

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

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).

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[@]}" "${Veggies[@]}") # Concatenate words=($(< datafile)) # From file (split by IFS)

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 nth most recent command
!n Expand nth 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 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..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

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" 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

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

Heredoc allows a section of your source code to be treated as a file. See Bash Reference Manual.

Herestring

tr '[:lower:]' '[:upper:]' <<< "Will be uppercased, even $variable"

Herestring allows a string to be treated as a standard input (stdin). See Bash Reference Manual.

Process substitution

# loop on myfunc output lines while read -r line; do echo "$line" done < <(myfunc) # compare content of two folders diff <(ls "$dir1") <(ls "$dir2")

Process substitution allows the input (or output) of a command to be treated as a file. See Bash Reference Manual.

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
${PIPESTATUS[n]} return value of piped commands (array)

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

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