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Bash Scripting Essentials Cheatsheet

A concise reference for Bash scripting, covering essential syntax, commands, and best practices. Includes variables, loops, conditionals, functions, and more, to help you write effective and maintainable scripts.

Basics & Variables

Script Structure

Every bash script starts with a shebang line, specifying the interpreter.

#!/usr/bin/env bash

Use comments to explain your code.

# This is a comment

Best practice: use strict mode to catch errors early.

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

Variables

Defining a variable:

 
name="John"

Accessing a variable:

 
echo $name
echo "$name"
echo "${name} has a value"

String quotes:

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

Variable scope (local):

 
local myvar="local value"  # Function scope

Readonly variables:

 
declare -r readonly_var="cannot be changed"

Brace Expansion

Generate combinations of strings.

echo {A,B}.js   # A.js B.js
echo {1..5}     # 1 2 3 4 5
echo {{1..3},{7..9}} # 1 2 3 7 8 9

Loops & Conditionals

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

Looping through a range:

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

Looping 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

Conditionals

Basic if statement:

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

Using test command (alternative):

 
if test -e "file.txt"; then
  echo "File exists"
fi

Numeric conditions:

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

File existence check:

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

Case Statements 

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

  *)
    echo "Usage: $0 {start|stop|ssh}"
    ;;
esac

Functions & Arrays

Functions

Defining a function:

 
myfunc() {
    echo "hello $1"
}

# Alternate 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

Passing arguments:

 
myfunc() {
  echo "Argument 1: $1, Argument 2: $2"
}

myfunc "arg1" "arg2"

Arrays

Defining arrays:

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

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

Accessing elements:

echo "${Fruits[0]}"   # Element #0
echo "${Fruits[-1]}"  # Last element

Iterating through elements:

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

Getting array length:

echo "${#Fruits[@]}"  # Number of elements

Adding elements:

Fruits=("${Fruits[@]}" "Watermelon")  # Push
Fruits+=('Watermelon')                # Also Push

Parameter Expansion & Redirection

Parameter Expansion

Basics:

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

String manipulation:

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

Default values:

echo "${food:-Cake}"  #=> $food or "Cake"

Redirection

Stdout to file:

 
python hello.py > output.txt

Stdout to file (append):

 
python hello.py >> output.txt

Stderr to file:

 
python hello.py 2> error.log

Stderr to stdout:

 
python hello.py 2>&1

Both stdout and stderr to file:

 
python hello.py > output.txt 2>&1
# or
python hello.py &> output.txt

Feed file to stdin:

 
python hello.py < foo.txt

Advanced Features

History Expansion

Show history:

history

Expand last parameter of most recent command:

!$

Expand all parameters of most recent command:

!*

Expand nth most recent command:

!-n

Execute last command again:

!!

Replace in last command:

!!:s/<FROM>/<TO>/

Options and Globbing

Disable file overwriting:

 
set -o noclobber

Exit on error:

 
set -e

Fail on pipe errors:

 
set -o pipefail

Unset variables are errors:

 
set -u

Wildcards match dotfiles:

 
shopt -s dotglob

Recursive globbing

 
shopt -s globstar

String Manipulation

Uppercase and Lowercase Transformations:

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

Transforming strings using tr:

echo "Welcome To Devhints" | tr '[:lower:]' '[:upper:]'
#=> WELCOME TO DEVHINTS

Getting the directory of the script:

dir=${0%/*}
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