Bash Cheat Sheet

Sommaire

Introduction #

This is a quick reference to getting started with Bash

Bash Shortcuts #

Moving #

command	description
`ctrl` + `a`	Goto BEGINNING of command line
`ctrl` + `e`	Goto END of command line
`ctrl` + `b`	move back one character
`ctrl` + `f`	move forward one character
`alt` + `f`	move cursor FORWARD one word
`alt` + `b`	move cursor BACK one word
`ctrl` + `xx`	Toggle between the start of line and current cursor position
`ctrl` + `]` + `x`	Where x is any character, moves the cursor forward to the next occurance of x
`alt` + `ctrl` + `]` + `x`	Where x is any character, moves the cursor backwards to the previous occurance of x

Edit / Other #

command	description
`ctrl` + `d`	Delete the character under the cursor
`ctrl` + `h`	Delete the previous character before cursor
`ctrl` + `u`	Clear all / cut BEFORE cursor
`ctrl` + `k`	Clear all / cut AFTER cursor
`ctrl` + `w`	delete the word BEFORE the cursor
`alt` + `d`	delete the word FROM the cursor
`ctrl` + `y`	paste (if you used a previous command to delete)
`ctrl` + `i`	command completion like Tab
`ctrl` + `l`	Clear the screen (same as clear command)
`ctrl` + `c`	kill whatever is running
`ctrl` + `d`	Exit shell (same as exit command when cursor line is empty)
`ctrl` + `z`	Place current process in background
`ctrl` + `_`	Undo
`ctrl` + `x` `ctrl` + `u`	Undo the last changes. `ctrl`+ _ does the same
`ctrl` + `t`	Swap the last two characters before the cursor
`esc` + `t`	Swap last two words before the cursor
`alt` + `t`	swap current word with previous
`esc` + `.`
`esc` + `_`
`alt` + `[Backspace]`	delete PREVIOUS word
`alt` + `<`	Move to the first line in the history
`alt` + `>`	Move to the end of the input history, i.e., the line currently being entered
`alt` + `?`	display the file/folder names in the current path as help
`alt` + `*`	print all the file/folder names in the current path as parameter
`alt` + `.`	print the LAST ARGUMENT (ie “vim file1.txt file2.txt” will yield “file2.txt”)
`alt` + `c`	capitalize the first character to end of word starting at cursor (whole word if cursor is at the beginning of word)
`alt` + `u`	make uppercase from cursor to end of word
`alt` + `l`	make lowercase from cursor to end of word
`alt` + `n`
`alt` + `p`	Non-incremental reverse search of history.
`alt` + `r`	Undo all changes to the line
`alt` + `ctl` + `e`	Expand command line.
~[TAB][TAB]	List all users
$[TAB][TAB]	List all system variables
@[TAB][TAB]	List all entries in your /etc/hosts file
[TAB]	Auto complete
cd -	change to PREVIOUS working directory

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)"
# Same

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

Strict mode #

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

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

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 #

{: .-three-column}

Defining functions #

myfunc() {
    echo "hello $1"
}

# Same as above (<kbd>alt</kbd>ernate 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).

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

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 #

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 nth most recent command
!n	Expand nth command in history
!	Expand most recent invocation of command

Operations #

Code	Description
!!	Execute last command again
!!:s///	Replace first occurrence of to in most recent command
!!:gs///	Replace all occurrences of 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

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

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)

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