Bash

Getting started

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"

Function

get_name() {
  echo "John"
}

echo "You are $(get_name)"

Conditional

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'

See: Unofficial bash strict mode

Brace expansion

echo {A,B}.js
Code 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####*.}     # 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

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

Length

Code 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

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

The : is optional (eg, ${FOO=word} works)

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

< 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

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
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

Examples

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

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

# String
if [[ -z "$string" ]]; then
  echo "String is empty"
elif [[ -n "$string" ]]; then
  echo "String is not empty"
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[@]}           # 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)

Operation

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

Iterations

Iterate over values

for val in "${sounds[@]}"; do
  echo $val
done

Iterate over keys

for key in "${!sounds[@]}"; do
  echo $key
done

Option

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

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

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

Expansions

Code 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..200

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

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

Code Description
$? Exit status of last task
$! PID of last background task
$$ PID of shell

See Special parameters.

Go to previous directory

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

Also see