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

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

Bash Scripting Cheatsheet

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