Writing and executing shell scripts

A shell script is a collection of commands written in a scripting language that is interpreted by a shell in an operating system such as Linux. The shell acts as a command-line interface (CLI), allowing users to interact with the operating system by executing commands.

Shell scripts are commonly used to:

• Automate repetitive tasks
• Execute a sequence of commands
• Simplify system administration
• Improve efficiency and consistency

Writing Your First Shell Script

Step 1: Choose a Text Editor

Before you start writing your shell script, you need a text editor. Popular choices include Nano, Vim, and Emacs. For beginners, Nano is user-friendly and easy to use.

Command:

nano myscript.sh

Step 2: Write Your Script

Let's create a simple script that prints "Hello, World!" to the terminal.

Script:

#!/bin/bash
# This is a simple shell script

echo "Hello, World!"

Explanation:

• The first line (#!/bin/bash) is called the shebang. It specifies the shell that should be used to interpret the script.
• Lines starting with # are comments. They are ignored by the shell and help improve readability.
• The echo command prints text to the terminal.

Step 3: Save and Exit

Save the script in Nano editor by pressing Ctrl + O, then Enter to confirm, and finally Ctrl + X to exit.

Step 4: Make the Script Executable

To run the script, you need to make it executable:

chmod +x myscript.sh

Executing Shell Scripts

Running the Script

Now that your script is executable, you can run it by typing.

Command:

./myscript.sh

• The ./ indicates that the script is located in the current directory.

Output:

Alternative Method (Without Changing Permissions)

A script can also be executed by explicitly specifying the interpreter:

Command:

bash myscript.sh

• In this case, execute permission is not required.

Variables and Input

Shell scripts can use variables to store and manipulate data. Here's an example:

Script:

#!/bin/bash

# Get user input
echo "Enter your name:"
read name

# Print a greeting
echo "Hello, $name!"

• read: Stores user input in the variable name
• $name retrieves the value of the variable

• Run the script, and it will prompt you to enter your name.

Output:

Control Flow

Shell scripts support basic control flow structures like if statements, case statements, and loops.

Example:

This script checks whether the entered number is even or odd.

Script:

#!/bin/bash

echo "Enter a number:"
read num

if [ $((num % 2)) -eq 0 ]; then
  echo "$num is even."
else
  echo "$num is odd."
fi

Output: