Shell scripting is one of the most commonly used skills in DevOps. It is mainly used on Linux systems and helps automate tasks, which reduces manual work and human involvement.
Even today, there are many automation tools and programming languages like Python. Still, shell scripting remains widely used in the industry because it is simple, fast, lightweight, and efficient.
This is the first blog in the shell scripting series. In this blog, we will cover an introduction to the shell along with some basic concepts.
You will learn about the shell, variables, user input, command-line arguments, and basic arithmetic operations. By the end of this blog, you will have a basic understanding of shell scripting.
What is Shell and Shell Scripting?
A shell is a program that allows users to interact with the operating system. When you type commands in the Linux terminal, the shell reads those commands and instructs the operating system to perform the required actions.
Linux supports different types of shells, such as:
- Bash (Bourne Again Shell)
- sh (Bourne Shell)
- zsh
Among these, Bash is the most commonly used shell and is the default shell in most Linux distributions.
A shell script is a collection of shell commands written inside a single file.
Writing Your First Shell Script
Step 1: Create a Shell Script File
touch hello.sh
Step 2: Add the Shebang Line
#!/bin/bash
Step 3: Write a Simple Command
echo "Hello, Shell Scripting!"
Step 4: Make the Script Executable
chmod +x hello.sh
Step 5: Run the Script
./hello.sh
Output:
Hello, Shell Scripting!
echo Command
echo "Hello World"
Output:
Hello World
To print in New Line
echo -e "Hello\nWorld"
Output:
Hello
World
Print Tab Space
echo -e "Hello\tWorld"
Output:
Hello World
No New Line at the End
echo -n "Hello World"; echo " Hi"
Output:
Hello World Hi
Variables in Shell Scripting
name="Linux" echo "Welcome to $name"
Output:
Welcome to Linux
User-Defined Variables
city="Hyderabad" echo "City is $city"
Output:
City is Hyderabad
Environment Variables
env
Output:
HOME=/home/user
USER=user
PATH=/usr/local/bin:/usr/bin:/bin
Access an Environment Variable
echo $HOME
Output:
/home/user
Taking User Input
echo "Enter your name:" read username echo "Welcome $username"
Input: Venkat
Output:
Welcome Venkat
Input with Prompt
read -p "Enter your name: " name echo "Hello $name"
Input: Bye
Hello Bye
Password Input
read -sp "Enter password: " password echo echo "Password saved"
Output:
Password saved
Default REPLY Variable
read echo "You entered: $REPLY"
Input: Linux
You entered: Linux
Command-Line Arguments
./script.sh AWS DevOps
echo "Script name: $0" echo "First argument: $1" echo "Second argument: $2" echo "Total arguments: $#" echo "All arguments: $@"
Output:
Script name: ./script.sh
First argument: AWS
Second argument: DevOps
Total arguments: 2
All arguments: AWS DevOps
Arithmetic Operations in Shell Scripting
a=10 b=5 echo "Addition: $((a + b))" echo "Subtraction: $((a - b))" echo "Multiplication: $((a * b))" echo "Division: $((a / b))"
Output:
Addition: 15
Subtraction: 5
Multiplication: 50
Division: 2
Execution Flow of a Shell Script
A shell script runs commands in a linear manner. The shell reads the script from top to bottom and executes each command one by one in the order they appear.
Example
#!/bin/bash echo "Script Started" echo "Script Stopped"
Output:
Script Started
Script Stopped
This example shows that the first command executes before the second command. The execution always follows the written order unless conditions or loops are used.
Comments in Shell Scripts
Comments are used to explain what a script does or to describe specific lines of code. They improve readability and make scripts easier to understand and maintain.
In shell scripting, comments start with the # symbol. The shell ignores comment lines and does not execute them.
Single-Line Comment Example
# This is a comment echo "Hello Linux"
Output:
Hello Linux
Multiple-Line Comments
Shell scripting does not support true multi-line comments. To write comments on multiple lines, use the # symbol at the beginning of each line.
# This script prints a message # It is written for beginners # Used to understand comments echo "Shell scripting is simple"
Output:
Shell scripting is simple
Exit Status in Shell Scripting
Every command executed in Linux returns an exit status. This value indicates whether the command ran successfully or failed.
An exit status of 0 means the command was successful. Any non-zero value indicates an error or failure.
Example
ls / echo $?
Output:
0
Here, $? stores the exit status of the previously executed command. Exit status is important when writing scripts that require basic error checking.
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