Bash

This is a quick reference cheat sheet to getting started with linux bash shell scripting.

#Getting started

#hello.sh

#!/bin/bash

VAR="world"
echo "Hello $VAR!" # => Hello world!

Execute the script

$ bash hello.sh

#Variables

NAME="John"

echo ${NAME}    # => John
echo $NAME      # => John
echo "$NAME"    # => John
echo '$NAME'    # => $NAME
echo "${NAME}!" # => John!

NAME = "John" # => Error (about space)

#Comments

# This is an inline Bash comment.
: '
This is a
very neat comment
in bash
'

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

#Arguments

Expression Description
$1$9 Parameter 1 ... 9
$0 Name of the script itself
$1 First argument
${10} Positional parameter 10
$# Number of arguments
$$ Process id of the shell
$* All arguments
[email protected] All arguments, starting from first
$- Current options
$_ Last argument of the previous command

See: Special parameters.

#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

#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

#Shell execution

echo "I'm in $(PWD)"
# Same
echo "I'm in `pwd`"

See: Command substitution

#Parameter expansions

#Syntax

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

#Substrings

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

#Length

Expression Description
${#FOO} Length of $FOO

#Default values

Expression Description
${FOO:-val} $FOO, or val if unset
${FOO:=val} Set $FOO to val if unset
${FOO:+val} val if $FOO is set
${FOO:?message} Show message and exit if $FOO is unset

#Substitution

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

#Slicing

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

#basepath & dirpath

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


DIRPATH=${SRC%$BASEPATH}
echo $DIRPATH   # => "/path/to/"

#Transform

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

#Arrays

#Defining arrays

Fruits=('Apple' 'Banana' 'Orange')

Fruits[0]="Apple"
Fruits[1]="Banana"
Fruits[2]="Orange"

ARRAY2=(foo{1..2}) # => foo1 foo2
ARRAY3=({A..D})    # => A B C D

# declare construct
declare -a Numbers=(1 2 3 4 5 6)

#Indexing

- -
${Fruits[0]} First element
${Fruits[-1]} Last element
${Fruits[*]} All elements
${Fruits[@]} All elements
${#Fruits[@]} Number of all
${#Fruits} Length of 1st
${#Fruits[3]} Length of nth
${Fruits[@]:3:2} Range
${!Fruits[@]} Keys of all

#Iteration

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

#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

#Arrays as arguments

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

#Dictionaries

#Defining

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

#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

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

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

#Conditionals

#Integer conditions

Condition Description
[[ 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
(( NUM < NUM )) Less than
(( NUM <= NUM )) Less than or equal
(( NUM > NUM )) Greater than
(( NUM >= NUM )) Greater than or equal

#String conditions

Condition Description
[[ -z STR ]] Empty string
[[ -n STR ]] Not empty string
[[ STR == STR ]] Equal
[[ STR = STR ]] Equal (Same above)
[[ STR < STR ]] Less than (ASCII)
[[ STR > STR ]] Greater than (ASCII)
[[ STR != STR ]] Not Equal
[[ STR =~ STR ]] Regexp

#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" ]]; 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

#File conditions

Condition Description
[[ -e FILE ]] Exists
[[ -d FILE ]] Directory
[[ -f FILE ]] File
[[ -h FILE ]] Symlink
[[ -s FILE ]] Size is > 0 bytes
[[ -r FILE ]] Readable
[[ -w FILE ]] Writable
[[ -x FILE ]] Executable
[[ f1 -nt f2 ]] f1 newer than f2
[[ f1 -ot f2 ]] f2 older than f1
[[ f1 -ef f2 ]] Same files

#More conditions

Condition Description
[[ -o noclobber ]] If OPTION is enabled
[[ ! EXPR ]] Not
[[ X && Y ]] And
[[ X || Y ]] Or

#logical and, or

if [ "$1" = 'y' -a $2 -gt 0 ]; then
    echo "yes"
fi

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

#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

#Auto increment

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

#Auto decrement

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

#Continue

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

#Break

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

#Until

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

#Forever

while true; do
    # here is some code.
done

#Forever (shorthand)

while :; do
    # here is some code.
done

#Reading lines

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

#Options

#Options

# 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

#Glob options

# 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   

#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

#Subshells

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

#Inspecting commands

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

#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

#Source relative

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

#Directory of script

DIR="${0%/*}"

#Case/switch

case "$1" in
    start | up)
    vagrant up
    ;;

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

#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

#printf

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"

#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

#Check for command's result

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

#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.

#Grep check

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

#Backslash escapes

  •  
  • !
  • "
  • #
  • &
  • '
  • (
  • )
  • ,
  • ;
  • <
  • >
  • [
  • |
  • \
  • ]
  • ^
  • {
  • }
  • `
  • $
  • *
  • ?

Escape these special characters with \

#Heredoc

cat <<END
hello world
END

#Go to previous directory

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

#Reading input

echo -n "Proceed? [y/n]: "
read ans
echo $ans
read -n 1 ans    # Just one character

#Conditional execution

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

#Strict mode

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

See: Unofficial bash strict mode

#Also see