Bash Scripting Introduction

We usually start by creating a file with the .sh extension, for example script.sh, and then add Bash commands to it.

Basic Script

#!/bin/bash

echo "Hello World"

You can run this script with bash script.sh or ./script.sh. The second form only works if the file is executable. To make it executable, run chmod +x script.sh.

Shebang

The first line of a Bash script is called the shebang. It tells the system which interpreter should run the file. For Bash, a common shebang is #!/bin/bash.

Variables

A variable is a named value. In Bash, we use $ to read it, for example $NAME. You can also write ${NAME} when you want to make the variable boundary explicit.

#!/bin/bash

# Variable Declaration
NAME="John"

# Variable Usage

echo "My name is $NAME"
echo "My name is ${NAME}"

NOTE: If you set NAME="John" in the shell, it is only a shell variable. To make it available to child processes, export it with export NAME="John".

If you close the terminal, that exported variable is gone. To make it persistent for future shell sessions, add it to ~/.bashrc or your shell startup file.

User Input

We can take input from the user using the read command.

#!/bin/bash

echo "Enter your name: "
read NAME
echo "Hello $NAME, nice to meet you!"

Arguments

We can pass arguments to the script. The arguments are stored in the $1, $2, $3 and so on. $0 is the name of the script.

Example: bash script.sh arg1 arg2

#!/bin/bash

echo "First Argument: $1"

Arithmetic Operations

We can do arithmetic operations in Bash with (( )).

#!/bin/bash

echo $(( 5 + 5 ))

Arithmetic Operators

• + - Addition
• - - Subtraction
• * - Multiplication
• / - Division
• % - Modulus

Conditional Statements

We can use the if statement to check for a condition. The syntax is:

#!/bin/bash

if [ "$1" == "John" ]
then
    echo "Hello John"
    exit 1
elif [ "$1" == "Doe" ]
then
    echo "Hello Doe"
else
    echo "I don't know you"
fi

The compact one-line form exists, but the block form is easier to read and debug.

Comparison Operators

• == - Equal to
• > - Greater than
• < - Less than
• >= - Greater than or equal to
• <= - Less than or equal to
• != - Not equal to
• -ge - Greater than or equal to
• -le - Less than or equal to

Boolean Operators

• -a - And
• -o - Or
• ! - Not

Loops

We can use loops to repeat a set of commands. There are two types of loops in bash. for and while. The body is enclosed in do and done.

For Loop

#!/bin/bash

for i in 1 2 3 4 5
do
    echo $i
done

While Loop

#!/bin/bash

i=1
while [ $i -le 5 ]
do
    echo $i
    (( i++ ))
done

Break and Continue

We can use break and continue in loops. break will break the loop and continue will skip the current iteration.

#!/bin/bash

for i in 1 2 3 4 5
do
    if [ $i -eq 3 ]
    then
        continue
    fi
    echo $i
done

Functions

We can create functions in Bash like this:

#!/bin/bash

function sayHello() {
    echo "Hello World"
}

sayHello

• exit 1 exits the script with a non-zero status code.
• $RANDOM gives a random number between 0 and 32767.
• $SHELL gives the path of the shell.
• $USER gives the username of the current user.
• $HOSTNAME gives the hostname of the machine.

jq

jq is a command-line JSON processor. It is useful for reading, filtering, transforming, and generating JSON from shell scripts.

Installation

sudo apt install jq

Usage

The format in JSON.

echo '{"name": "John", "age": 30}' | jq

It will print out the specified key.

echo '{"name": "John", "age": 30}' | jq '.name'

What's next?

• Learning Resources - Learn more about Bash Scripting with these resources.
• Other Resources - Explore more about Bash Scripting with these resources.
• Tools - Explore the tools used in Bash Scripting.