Bash Scripting

Bash (Bourne Again SHell) is the default shell for most Linux distributions. Learning to write bash scripts allows you to automate repetitive tasks, create custom commands, and build powerful tools. This guide will introduce you to the basics of bash scripting.

Getting Started with Bash Scripts

A bash script is a plain text file containing a series of commands. Here's how to create and run your first script:

1. Create a Script File

Use any text editor to create a file with the .sh extension:

$ nano myscript.sh

2. Add the Shebang Line

The first line of a bash script should be the shebang, which tells the system which interpreter to use:

#!/bin/bash

3. Add Commands

Add commands to your script:

#!/bin/bash

# This is a comment
echo "Hello, World!"
echo "My first bash script"

# Create a variable
name="Linux User"
echo "Hello, $name!"

4. Make the Script Executable

$ chmod +x myscript.sh

5. Run the Script

$ ./myscript.sh

Variables in Bash

Variables allow you to store and manipulate data in your scripts:

#!/bin/bash

# Define variables
name="John"
age=30
is_student=true

# Access variables with $
echo "Name: $name"
echo "Age: $age"
echo "Student: $is_student"

# Command substitution
current_date=$(date)
echo "Current date: $current_date"

# Arithmetic operations
result=$((age + 5))
echo "$name will be $result in 5 years"

Conditional Statements

Conditionals allow your scripts to make decisions:

If-Else Statement

#!/bin/bash

# If-else statement
age=20

if [ $age -ge 18 ]; then
    echo "You are an adult"
else
    echo "You are a minor"
fi

# File test operators
if [ -f "config.txt" ]; then
    echo "config.txt exists"
else
    echo "config.txt does not exist"
fi

Common Test Operators

Operator	Description	Example
-eq	Equal to	[ $a -eq $b ]
-ne	Not equal to	[ $a -ne $b ]
-gt	Greater than	[ $a -gt $b ]
-lt	Less than	[ $a -lt $b ]
-f	File exists and is a regular file	[ -f $file ]
-d	File exists and is a directory	[ -d $directory ]

Loops

Loops allow you to repeat actions multiple times:

For Loop

#!/bin/bash

# Basic for loop
for i in 1 2 3 4 5
do
    echo "Number: $i"
done

# Loop through a range
for i in {1..5}
do
    echo "Range number: $i"
done

# Loop through files
for file in /etc/*.conf
do
    echo "Found config file: $file"
done

While Loop

#!/bin/bash

# While loop
count=1
while [ $count -le 5 ]
do
    echo "Count: $count"
    count=$((count + 1))
done

Functions

Functions allow you to organize your code into reusable blocks:

#!/bin/bash

# Define a function
greet() {
    echo "Hello, $1!"
    echo "Today is $(date)"
}

# Call the function with a parameter
greet "User"

# Function with return value
calculate_sum() {
    local num1=$1
    local num2=$2
    local sum=$((num1 + num2))
    return $sum
}

calculate_sum 5 7
result=$?
echo "Sum: $result"

Next Steps

Now that you've learned the basics of bash scripting, you might want to explore:

Advanced string manipulation
Regular expressions in bash
Error handling and debugging
Handling command line arguments
Creating interactive scripts

Practice is key to mastering bash scripting. Try to create scripts to automate tasks you perform regularly.