Bash Cheat Sheet

From WikiOD

Here is the cheatsheat of Bash and basic guide on how to use Bash

Getting started[edit | edit source]

{: .-three-column}

Introduction[edit | edit source]

{: .-intro}

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

Example[edit | edit source]

#!/usr/bin/env bash

NAME="John"
echo "Hello $NAME!"

Variables[edit | edit source]

NAME="John"
echo $NAME
echo "$NAME"
echo "${NAME}!"

String quotes[edit | edit source]

NAME="John"
echo "Hi $NAME"  #=> Hi John
echo 'Hi $NAME'  #=> Hi $NAME

Shell execution[edit | edit source]

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

See Command substitution

Conditional execution[edit | edit source]

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

Functions[edit | edit source]

{: id=‘functions-example’}

get_name() {
  echo "John"
}

echo "You are $(get_name)"

See: Functions

Conditionals[edit | edit source]

{: 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[edit | edit source]

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

See: Unofficial bash strict mode

Brace expansion[edit | edit source]

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[edit | edit source]

{: .-three-column}

Basics[edit | edit source]

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[edit | edit source]

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[edit | edit source]

# Single line comment
: '
This is a
multi line
comment
'

Substrings[edit | edit source]

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

Length[edit | edit source]

Expression Description
${#FOO} Length of $FOO

Manipulation[edit | edit source]

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[edit | edit source]

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[edit | edit source]

{: .-three-column}

Basic for loop[edit | edit source]

for i in /etc/rc.*; do
  echo $i
done

C-like for loop[edit | edit source]

for ((i = 0 ; i < 100 ; i++)); do
  echo $i
done

Ranges[edit | edit source]

for i in {1..5}; do
    echo "Welcome $i"
done

With step size[edit | edit source]

for i in {5..50..5}; do
    echo "Welcome $i"
done

Reading lines[edit | edit source]

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

Forever[edit | edit source]

while true; do
  ···
done

Functions[edit | edit source]

{: .-three-column}

Defining functions[edit | edit source]

myfunc() {
    echo "hello $1"
}
# Same as above (alternate syntax)
function myfunc() {
    echo "hello $1"
}
myfunc "John"

Returning values[edit | edit source]

myfunc() {
    local myresult='some value'
    echo $myresult
}
result="$(myfunc)"

Raising errors[edit | edit source]

myfunc() {
  return 1
}
if myfunc; then
  echo "success"
else
  echo "failure"
fi

Arguments[edit | edit source]

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[edit | edit source]

{: .-three-column}

Conditions[edit | edit source]

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[edit | edit source]

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

File conditions[edit | edit source]

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[edit | edit source]

# 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[edit | edit source]

Defining arrays[edit | edit source]

Fruits=('Apple' 'Banana' 'Orange')
Fruits[0]="Apple"
Fruits[1]="Banana"
Fruits[2]="Orange"

Working with arrays[edit | edit source]

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[edit | edit source]

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[edit | edit source]

for i in "${arrayName[@]}"; do
  echo $i
done

Dictionaries[edit | edit source]

{: .-three-column}

Defining[edit | edit source]

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[edit | edit source]

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[edit | edit source]

Iterate over values[edit | edit source]

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

Iterate over keys[edit | edit source]

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

Options[edit | edit source]

Options[edit | edit source]

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[edit | edit source]

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[edit | edit source]

Commands[edit | edit source]

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

Expansions[edit | edit source]

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[edit | edit source]

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[edit | edit source]

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[edit | edit source]

Numeric calculations[edit | edit source]

$((a + 200))      # Add 200 to $a
$(($RANDOM%200))  # Random number 0..199

Subshells[edit | edit source]

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

Redirection[edit | edit source]

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[edit | edit source]

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

Trap errors[edit | edit source]

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[edit | edit source]

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

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

Source relative[edit | edit source]

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

printf[edit | edit source]

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[edit | edit source]

DIR="${0%/*}"

Getting options[edit | edit source]

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[edit | edit source]

cat <<END
hello world
END

Reading input[edit | edit source]

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

Special variables[edit | edit source]

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[edit | edit source]

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

Check for command’s result[edit | edit source]

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

Grep check[edit | edit source]

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

Also see[edit | edit source]

{: .-one-column}

Credit:rstacruz