title

Getting started

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Introduction

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

Learn bash in y minutes (learnxinyminutes.com)
Bash Guide (mywiki.wooledge.org)

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

{: id='functions-example'}

get_name() {
  echo "John"
}

echo "You are $(get_name)"

See: Functions

Conditionals

{: id='conditionals-example'}

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`

See: Brace expansion

Parameter expansions

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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

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

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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

{: .-three-column}

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 postional arguments (as a single word)
`$@`	All postitional 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

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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

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

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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

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

{: .-one-column}

Bash-hackers wiki (bash-hackers.org)
Shell vars (bash-hackers.org)
Learn bash in y minutes (learnxinyminutes.com)
Bash Guide (mywiki.wooledge.org)
ShellCheck (shellcheck.net)

Files

bash.md

Latest commit

History

bash.md

File metadata and controls

Getting started

Introduction

Example

Variables

String quotes

Shell execution

Conditional execution

Functions

Conditionals

Strict mode

Brace expansion

Parameter expansions

Basics

Substitution

Comments

Substrings

Length

Manipulation

Default values

Loops

Basic for loop

C-like for loop

Ranges

With step size

Reading lines

Forever

Functions

Defining functions

Returning values

Raising errors

Arguments

Conditionals

Conditions

More conditions

File conditions

Example

Arrays

Defining arrays

Working with arrays

Operations

Iteration

Dictionaries

Defining

Working with dictionaries

Iteration

Iterate over values

Iterate over keys

Options

Options

Glob options

History

Commands

Expansions

Operations

Slices

Miscellaneous

Numeric calculations

Subshells

Redirection

Inspecting commands

Trap errors

Case/switch

Source relative

printf

Directory of script

Getting options

Heredoc

Reading input

Special variables

Go to previous directory

Check for command's result

Grep check

Also see