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Bash Cheatsheet

A quick reference for Bash scripting, covering essential syntax, commands, and best practices for writing efficient and robust scripts.

Basics & Syntax

Getting Started

Shebang:

#!/usr/bin/env bash

This line specifies the interpreter for the script.

Variables:

name="John"
echo $name
echo "$name"
echo "${name} and Jane"

Variable assignment and usage. Always quote variables unless you need wildcard expansion or command substitution.

String Quotes:

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

Double quotes allow variable interpolation, single quotes do not.

Command Substitution:

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

Executes a command and inserts its output.

Conditional Execution:

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

Execute commands based on the success or failure of previous commands.

Strict Mode:

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

Enable strict mode to catch errors and improve script reliability.

Comments

Single-line comment:

# This is a single-line comment

Multi-line comment:

: '
This is a
multi line
comment
'

Brace Expansion

{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 Expansion & Manipulation

Parameter Expansion

Basics:

name="John"
echo "${name}"
echo "${name/J/j}"    #=> "john" (substitution)
echo "${name:0:2}"    #=> "Jo" (slicing)

More basics:

length=2
echo "${name:0:length}"  #=> "Jo"

Path manipulation:

str="/path/to/foo.cpp"
echo "${str%.cpp}"    # /path/to/foo
echo "${str%.cpp}.o"  # /path/to/foo.o
echo "${str%/*}"      # /path/to

More path manipulation:

str="/path/to/foo.cpp"
echo "${str##*.}"     # cpp (extension)
echo "${str##*/}"     # foo.cpp (basepath)

More string manipulation:

str="Hello world"
echo "${str:6:5}"    # "world"
echo "${str: -5:5}"  # "world"

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

String Manipulation

Case conversion:

str="HELLO WORLD!"
echo "${str,}"   #=> "hELLO WORLD!" (lowercase 1st letter)
echo "${str,,}"  #=> "hello world!" (all lowercase)

More case conversion:

str="hello world!"
echo "${str^}"   #=> "Hello world!" (uppercase 1st letter)
echo "${str^^}"  #=> "HELLO WORLD!" (all uppercase)

Default values

$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)

Loops & Functions

Loops

Basic for loop:

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

C-style for loop:

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

Ranges in for loop:

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

Ranges with step size:

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

Reading lines from a file:

while read -r line; do
  echo "$line"
done <file.txt

Infinite loop:

while true; do
  #...
done

Functions

Defining a function:

myfunc() {
    echo "hello $1"
}

Alternative syntax:

function myfunc {
    echo "hello $1"
}

Calling a function:

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

Function Arguments:

echo $#
echo $@

Arguments

$#

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

Conditionals, Arrays, Dictionaries

Conditionals

String conditions:

if [[ -z "$string" ]]; then
  echo "String is empty"
elif [[ -n "$string" ]]; then
  echo "String is not empty"
fi

Numeric conditions:

if (( $a < $b )); then
   echo "$a is smaller than $b"
fi

File conditions:

if [[ -e "file.txt" ]]; then
  echo "file exists"
fi

Conditions

[[ -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

Arrays

Defining an array:

Fruits=('Apple' 'Banana' 'Orange')

Fruits[0]="Apple"
Fruits[1]="Banana"
Fruits[2]="Orange"

Accessing array elements:

echo "${Fruits[0]}"           # Element #0
echo "${Fruits[-1]}"          # Last element
echo "${Fruits[@]}"           # All elements, space-separated

Array operations:

Fruits=("${Fruits[@]}" "Watermelon")    # Push
Fruits+=('Watermelon')                  # Also Push
unset Fruits[2]                         # Remove one item

Iterate array:

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

Dictionaries

Defining a dictionary:

declare -A sounds
sounds[dog]="bark"
sounds[cow]="moo"

Accessing dictionary elements:

echo "${sounds[dog]}" # Dog's sound
echo "${sounds[@]}"   # All values
echo "${!sounds[@]}"  # All keys

Dictionary operations:

unset sounds[dog]     # Delete dog

Iterating dictionary (values):

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

Iterating dictionary (keys):

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

Options & Miscellaneous

Options

Setting options:

set -o noclobber  # Avoid overlay files (echo "hi" > foo)
set -o errexit    # Used to exit upon error
set -o pipefail   # Unveils hidden failures
set -o nounset    # Exposes unset variables

Glob options:

shopt -s nullglob    # Non-matching globs are removed
shopt -s failglob    # Non-matching globs throw errors
shopt -s nocaseglob  # Case insensitive globs

History

Commands:

history             # Show history
shopt -s histverify # Don't execute expanded result immediately

Expansions:

!$         # Expand last parameter of most recent command
!*         # Expand all parameters of most recent command
!-n        # Expand nth most recent command

Operations:

!!                 # Execute last command again
!!:s/<FROM>/<TO>/  # Replace first occurrence in most recent command
!!:gs/<FROM>/<TO>/ # Replace all occurrences in most recent command

Miscellaneous

Numeric calculations:

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

Redirection:

python hello.py > output.txt            # stdout to (file)
python hello.py 2> error.log            # stderr to (file)
python hello.py 2>&1                    # stderr to stdout

Inspecting commands:

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

Case/switch statment:

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

Go to the previous directory:

cd -

Special Variables

$?

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
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