How to Create a Shell Script in Linux?

Introduction to Shell and Shell Scripting

In the world of Linux, the shell is a command-line interface that allows users to interact with the operating system. It mediates between the user and the system, interpreting and executing commands. One of the powerful features of the shell is the ability to create shell scripts, which are files containing a series of commands that can be executed as a single unit.

Shell scripting is a way to automate tasks and perform complex operations by combining multiple commands and control structures. It provides a convenient and efficient method to accomplish various tasks, from simple file operations to complex system administration tasks.

Types of Linux Shell

Several types of shells are available in Linux, each with its features and syntax.

Some commonly used shells include:

1. Bash (Bourne Again Shell): This is the most popular shell used in Linux distributions. It is a superset of the original Bourne shell (sh) and provides a rich set of features, including command history, tab completion, and advanced scripting capabilities.
2. Csh (C Shell): Csh has a syntax similar to the C programming language and offers interactive features like command-line editing and job control. It is often preferred by users familiar with the C programming language.
3. Ksh (Korn Shell): Ksh is an enhanced version of the Bourne shell, offering improved scripting capabilities and command-line editing features. It is widely used in commercial environments.
4. Zsh (Z Shell): Zsh is a highly customizable shell with advanced features, such as extensive completion capabilities and plugin support. It provides a user-friendly interface and is gaining popularity among power users.

Creating a Linux Shell Script

To create a Linux shell script, follow these steps:

1. Choose a shell: Decide which shell you want to use for your script. For this article, we'll focus on bash, which is available on most Linux distributions.
2. Create a new file: Open a text editor and create a new file with a .sh extension. For example, you can use the command nano myscript.sh to create a new script file named myscript.sh.
3. Specify the shell: On the first line of the script, add the shebang (#!) followed by the path to the shell interpreter. In our case, we'll use #!/bin/bash to specify bash as the interpreter.
4. Write the script: Below the shebang line, you can start writing your script by adding commands and comments. Commands are executed in sequential order.
5. Save the script: Once you've finished writing the script, save the file and exit the text editor.
6. Make the script executable: To make the script executable, you need to change its file permissions. Run the command chmod +x myscript.sh to give the script execute permissions.
7. Execute the script: Finally, you can execute the script by typing its name preceded by ./ (current directory). For example, run ./myscript.sh to execute the script.

Here's an example of a simple Linux shell script that displays a greeting message:

By following these steps, you can create and execute your shell scripts to automate tasks and perform various operations in Linux.

Comments in the Shell Script

Comments in a shell script are lines that are not executed but provide information and explanations about the script. They are useful for documenting the script's purpose and adding notes for future reference.

In Bash, comments start with the # symbol and continue until the end of the line. Anything written after the # is ignored by the shell.

Here's an example of a Linux shell script with comments:

Output:

In the above example, the comments provide context and explanation for each step of the script, making it easier for others (including yourself) to understand and modify the script in the future.

What are Shell Variables?

Shell variables are placeholders used to store values that can be accessed and manipulated by the shell and shell scripts. They are useful for storing temporary data, user input, command outputs, and other types of information.

In Bash, you can define a variable by assigning a value to it using the = symbol. The variable name should follow the rules of variable naming conventions, such as starting with a letter or an underscore and consisting of letters, digits, or underscores.

Here's an example of using variables in a Linux shell script:

Output:

In the above script, the variable name is assigned the value "John", and it is accessed using the $ symbol ($name). Similarly, the variables a and b are used to perform arithmetic operations and store the result in the sum variable.

Shell variables are case-sensitive, and you can use them throughout your script to store and manipulate data as per your requirements.

Extended Shell Scripts

Shell scripting provides a wide range of features and capabilities that allow you to create powerful and efficient scripts to automate tasks and perform complex operations. Let's explore some of the extended features of shell scripts:

1. Control Structures

Control structures in shell scripting allow you to control the flow of execution based on certain conditions or criteria. They enable you to make decisions, repeat commands, and perform actions based on the outcome of certain conditions.

1. If-else Statements: If-else statements are used to execute a block of code based on a specific condition. They allow you to perform different actions depending on whether a condition is true or false. Here's an example:

   Output:

2. Loops: Loops are used to repeat a block of code until a certain condition is met. Shell scripting provides different types of loops, including the for loop, the while loop, and the until loop. Here's an example of a for loop:

   Output:

3. Case statements: Case statements allow you to perform different actions based on the value of a variable. They provide a convenient way to handle multiple conditions. Here's an example:

   Output:

2. Input/Output Operations

Shell scripting provides various mechanisms to interact with users and handle input/output operations.

1. Command-Line Arguments: Command-line arguments allow you to pass values to a shell script when executing it. The script can access these values using special variables such as $1, $2, etc. Here's an example:

2. User Input: Shell scripts can prompt users to enter values using the read command. The entered values can be stored in variables and used within the script. Here's an example:

3. File Input/Output: Shell scripts can read data from files and write data to files using various commands such as cat, echo, read, and redirection operators (>, >>, <). This allows you to process and manipulate files in your scripts.

3. Functions

Functions in shell scripting allow you to encapsulate a block of code that can be called and executed multiple times. They promote code reusability and modular programming.

Here's an example of defining and calling a function in a Linux shell script:

Functions can also accept arguments and return values, allowing you to create more versatile and flexible scripts.