Bash Scripting Cheatsheet
Just about every geek and nerd that wants to be a hacker always wants to skip straight to the good stuff, and it’s no wonder why. There’s a certain amount of mystery, intrigue, and awe in being able to used advanced tools (such as those found on Kali Linux like Metasploit) to hack into another computing system. Hackers have been around for decades now, and are frequently present in action and thriller films, such as The Matrix.
But there’s one big problem. These tools work almost exclusively at the command line, or in Linux, the BASH shell.
Here is the Cheatsheet for Bash that will help you to build your skills:
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
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'
Brace expansion
echo {A,B}.js
{A,B}Same as A B{A,B}.jsSame as A.js B.js{1..5}Same as 1 2 3 4 5
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"
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 ${FOO%suffix}Remove suffix${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${FOO:0:3}Substring (position, length)${FOO:-3:3}Substring from the right
Length${#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 ${STR^^} #=> "HELLO WORLD!" (all uppercase)
Default values${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
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
cat file.txt | while read line; do
echo $line
done
Forever
while true; do
···
done
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 $#Number of arguments$*All arguments$@All arguments, starting from first$1First argument$_Last argument of the previous command
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. [[ -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 [[ -o noclobber ]]If OPTIONNAME is enabled[[ ! EXPR ]]Not[[ X ]] && [[ Y ]]And[[ X ]] || [[ Y ]]Or
File conditions [[ -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"
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
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 ${Fruits[@]:3:2} # Range (from position 3, length 2)
Operations
Fruits=("${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
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 ${#sounds[@]} # Number of elements
unset sounds[dog] # Delete dog
Iteration
Iterate over values
for val in "${sounds[@]}"; do
echo $val
done
Iterate over keys
for key in "${!sounds[@]}"; do
echo $key
done
Options
set -o noclobber # Avoid overlay files (echo "hi" > foo)
set -o errexit # Used to exit upon error, avoiding cascading errors
set -o pipefail # Unveils hidden failures
set -o nounset # Exposes unset variables
Glob options
shopt -s nullglob # Non-matching globs are removed ('*.foo' => '')
shopt -s failglob # Non-matching globs throw errors
shopt -s nocaseglob # Case insensitive globs
shopt -s dotglob # Wildcards match dotfiles ("*.sh" => ".foo.sh")
shopt -s globstar # Allow ** for recursive matches ('lib/**/*.rb' => 'lib/a/b/c.rb')
CommandshistoryShow historyshopt -s histverifyDon’t execute expanded result immediately
Expansions!$Expand last parameter of most recent command!*Expand all parameters of most recent command!-nExpand nth most recent command!nExpand nth command in history!<command>Expand most recent invocation of command <command>
Operations!!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!$:tExpand only basename from last parameter of most recent command!$:hExpand only directory from last parameter of most recent command
!! and !$ can be replaced with any valid expansion.
Slices!!:nExpand only nth token from most recent command (command is 0; first argument is 1)!^Expand first argument from most recent command!$Expand last token from most recent command!!:n-mExpand range of tokens from most recent command!!:n-$Expand nth token to last from most recent command
!! can be replaced with any valid expansion i.e. !cat, !-2, !42, etc.
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
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
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 $ans
read -n 1 ans # Just one character
Special variables$?Exit status of last task$!PID of last background task$$PID of shell$0Filename of the shell script
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