Bash Cheatsheet
Quick Examples
Variables
name="John"
echo "$name"
echo "${name}!"
String quotes
name="John"
echo "Hi $name" #=> Hi John
echo 'Hi $name' #=> Hi $name
Shell execution
See Command substitution
Conditional execution
git commit && git push
git commit || echo "Commit failed"
Functions
get_name() {
echo "John"
}
echo "You are $(get_name)"
Conditionals
if [[ -z "$string" ]]; then
echo "String is empty"
elif [[ -n "$string" ]]; then
echo "String is not empty"
fi
Strict mode
set -euo pipefail
IFS=$'\n\t'
See: Unofficial bash strict mode
Brace expansion
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
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 |
: '
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
ยทยทยท
done
Functions
Defining functions
myfunc() {
echo "hello $1"
}
# Same as above (alternate syntax)
function myfunc() {
echo "hello $1"
}
Returning values
myfunc() {
local myresult='some value'
echo "$myresult"
}
Raising errors
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 $*
must be quoted in order to perform as described.
Otherwise, they do exactly the same thing (arguments as separate strings).
See Special parameters.
Conditionals
Conditions
Note that [[
is actually a command/program that returns either 0
(true) or 1
(false). Any program that obeys the same logic (like all base utils, such as grep(1)
or ping(1)
) can be used as condition, see examples.
Condition |
Description |
[[ -z STRING ]] |
Empty string |
[[ -n STRING ]] |
Not empty string |
[[ STRING == STRING ]] |
Equal |
[[ STRING != STRING ]] |
Not Equal |
--- |
--- |
[[ NUM -eq NUM ]] |
Equal |
[[ NUM -ne NUM ]] |
Not equal |
[[ NUM -lt NUM ]] |
Less than |
[[ NUM -le NUM ]] |
Less than or equal |
[[ NUM -gt NUM ]] |
Greater than |
[[ NUM -ge NUM ]] |
Greater than or equal |
--- |
--- |
[[ STRING =~ STRING ]] |
Regexp |
--- |
--- |
(( NUM < NUM )) |
Numeric conditions |
More conditions
Condition |
Description |
[[ -o noclobber ]] |
If OPTIONNAME is enabled |
--- |
--- |
[[ ! EXPR ]] |
Not |
[[ X && Y ]] |
And |
[[ X || Y ]] |
Or |
File conditions
Condition |
Description |
[[ -e FILE ]] |
Exists |
[[ -r FILE ]] |
Readable |
[[ -h FILE ]] |
Symlink |
[[ -d FILE ]] |
Directory |
[[ -w FILE ]] |
Writable |
[[ -s FILE ]] |
Size is > 0 bytes |
[[ -f FILE ]] |
File |
[[ -x FILE ]] |
Executable |
--- |
--- |
[[ FILE1 -nt FILE2 ]] |
1 is more recent than 2 |
[[ FILE1 -ot FILE2 ]] |
2 is more recent than 1 |
[[ FILE1 -ef FILE2 ]] |
Same files |
Example
# String
if [[ -z "$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
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
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
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"
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
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
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 [[ "$1" == '--' ]]; then shift; fi
Heredoc
cat <<END
hello world
END
echo -n "Proceed? [y/n]: "
read -r ans
echo "$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) |
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