Bash Scripting Cheat Sheet

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 longtime users may forget something every once in a while and that is why we have created this Bash scripting cheat sheet.

For times like these, it’s very handy to have a compiled list of Bash scripting components that have been sorted by category. That way, it only takes a few moments to reference the list whenever you forget the exact syntax of an operator or conditional statement, etc.

In this tutorial, we’ll present you with a curated list of the most handy things to know for Bash scripting. These are some of the most useful components, but they aren’t 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.

In this tutorial you will learn:

• Bash Scripting Cheat Sheet

Software Requirements and Linux Command Line Conventions

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

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Here are some of the most basic things to know about Bash scripting. If you are not sure where to start, this would be a good choice.

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