Bash scripting cheatsheet

The ability to automate tasks with Bash scripts in Linux is one of the operating system’s most powerful components.

However, due to the sheer amount of scripting components, it can be intimidating for newcomers. Even long-time users can sometimes forget something and that’s why we’ve made this bash scripting cheat sheet. For such times, it is very useful to have a compiled list of bash scripting components sorted by category. That way, whenever you forget the exact syntax of an operator, conditional statement, etc., it only takes a few moments to refer to the list. In this tutorial, we will present you a curated list of the most useful things to know for bash scripting. These are some of the most useful elements, but they are not easy to remember for everyone.Next time your mind is blanking when writing a Bash script, take a look at the Bash scripting cheat sheet below for some quick help.

Category	Requirements, Conventions or Software Version Used
System	Any Linux distro
Software	Bash shell (installed by default)
Other	Privileged access to your Linux system as root or via the `sudo` command.
Conventions	# – requires given linux commands to be executed with root privileges either directly as a root user or by use of `sudo` command $ – requires given linux commands to be executed as a regular non-privileged user

Bash Scripting Basics:

Here are some of the most basic things to know about Bash scripting. This would be a good option if you’re not sure where to start.

Syntax	Description
`#!/bin/bash`	Shebang that goes on the first line of every Bash script
`#!/usr/bin/env bash`	Alternative (and better) shebang – using environment variable
`#`	Used to make comments, text that comes after it will not be executed
`chmod +x script.sh && ./script.sh`	Give script executable permissions and execute it
`$#`	Stores the number of arguments passed to the Bash script
`$1, $2, $3`	Variables that store the values passed as arguments to the Bash script
`exit`	Exit from the Bash script, optionally add an error code
`Ctrl + C`	Keyboard combination to stop Bash script in the middle of execution
`$( )`	Execute a command inside of a subshell
`sleep`	Pause for a specified number of seconds, minutes, hours, or days

Conditional statements

Conditional statements with if or case allow for us to check if a certain condition is true or not. Depending on the answer, the script can proceed different ways.

Syntax	Description
`if then fi`	Test a condition and execute the `then` clause if it is true
`if then else fi`	Execute the `then` clause if the condition is true, otherwise execute the `else` clause
`if then elif else fi`	Test multiple conditions and execute whichever clause is true

For case statements it is best to just see a basic example:

#!/bin/bash

day=$(date +"%a")

case $day in 

  Mon | Tue | Wed | Thu | Fri)
    echo "today is a weekday"
    ;;

  Sat | Sun) 
    echo "today is the weekend"
    ;;

  *)
    echo "date not recognized"
    ;; 
esac

Basic if example script:

#!/bin/bash

if [ $1 -eq $2 ]; then
    echo "they are equal"
else
    echo "they are NOT equal"
fi

Bash Loops

Bash loops allow the script to continue executing a set of instructions as long as a condition continues to evaluate to true.

Syntax	Description
`for do done`	Continue to loop for a predetermined number of lines, files, etc
`until do done`	Continue to loop until a certain condition is met
`while do done`	Continue to loop as long as a certain condition is true
`break`	Exit the loop and continue to the next part of the Bash script
`continue`	Exit the current iteration of the loop but continue to run the loop

Read User Input

Prompt the user for information to enter by using read command:

#!/bin/bash

read -p "What is your name? " name

echo "Enjoy this tutorial, $name"

Parse input given as arguments to the Bash script:

#!/bin/bash

if [ $# -ne 2 ]; then
	echo "wrong number of arguments entered. please enter two."
	exit 1
fi

echo You have entered $1 and $2.

Arithmetic Operators

Arithmetic operators in Bash give us the ability to do things like addition, subtraction, multiplication, division, and other basic arithmetic inside of a Bash script.

Syntax	Description
`+`	Addition
`-`	Subtraction
`*`	Multiplication
`/`	Division
`%`	Modulus
`**`	Raise to a power
`((i++))`	Increment a variable
`((i--))`	Decrement a variable

Arithmetic Conditional Operators

Arithmetic conditional operators are usually used on two numbers to determine if a certain condition is true or false.

-lt	<
-gt	>
-le	<=
-ge	>=
-eq	==
-ne	!=

Note that the operators in the left column will work with single brackets [ ] or double brackets [[ ]], whereas the operators in the right column will work only with double brackets.

String Comparison Operators

We can use string comparison operators to determine if a string is empty or not, and to check if a string is equal, less, or greater in length to another string.

=	equal
!=	not equal
<	less then
>	greater then
-n s1	string s1 is not empty
-z s1	string s1 is empty

Bash File Testing Operators

In Bash, we can test to see different characteristics about a file or directory.

-b filename	Block special file
-c filename	Special character file
-d directoryname	Check for directory existence
-e filename	Check for file existence
-f filename	Check for regular file existence not a directory
-G filename	Check if file exists and is owned by effective group ID.
-g filename	true if file exists and is set-group-id.
-k filename	Sticky bit
-L filename	Symbolic link
-O filename	True if file exists and is owned by the effective user id.
-r filename	Check if file is a readable
-S filename	Check if file is socket
-s filename	Check if file is nonzero size
-u filename	Check if file set-ser-id bit is set
-w filename	Check if file is writable
-x filename	Check if file is executable

Boolean Operators

Boolean operators include and &&, or || and not equal to !. These operators allow us to test if two or more conditions are true or not.

syntax	Description
`&&`	Logical AND operator
`\|\|`	Logical OR operator
`!`	NOT equal to operator

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Mahesh Wabale

Admin at DevOpsLover.com

Mahesh Wabale is the admin & author of this blog, Technology enthusiast with interest in DevOps, CICD, Spring Boot, Microservices Architecture He has more than 7 years of experience in IT industry, programming language and development stuff. He loves to share his experience with his blog.