What is Shell Scripting?

Have you ever felt stuck doing the same simple tasks repeatedly on your computer, and it becomes too boring? Copying files, organizing folders, or checking system status can take up a lot of time. Shell scripting lets you automate these tasks easily and accurately for developers. By writing shell scripts, IT professionals can save time and reduce errors. In this article, we will explore shell scripting, one of the most powerful yet often overlooked skills, that will boost your system management abilities and help advance your career. Mastering shell scripting opens the door to greater efficiency and control over your computing environment.

Table of Contents:

• What Is a Shell Script?
• Why Learn Shell Scripting?
• Is Shell Scripting Still Relevant Today?
• How Does a Shell Script Work?
• Basic Shell Scripting Concepts
  • Commands and Syntax
  • Variables in Shell Scripts
  • Comments in Shell Scripts
  • If-Else Statements in Shell Scripts
• Examples of Simple Shell Scripts
  • Hello World
  • File Handling
  • Conditional Logic
• Types of Shells
• Online Shell Script Environment: Should You Use One?
• Advantages and Disadvantages of Shell Scripting
• Conclusion

What Is a Shell Script?

A shell script is a computer program that a developer writes and executes in the command-line interface of an operating system. Users use it to automate tasks by coding a set of commands and executing them on an operating system. If you have ever used a command line prompt, you must be familiar that we have to type each line one by one, or it behaves unpredictably. A shell script is a text file that contains a series of commands that you would normally type one by one in the terminal. Therefore, it makes the work of a developer easier and saves her some time.

For example, imagine you need to create three folders, copy files to them, and set permissions. Instead of typing each command separately, you can write a shell script that does all this work automatically.

Difference Between a Command and a Script

A command is a single line of instruction you give to the computer (or OS), whereas a shell script is a list of these commands that a developer saves in a file. When you run a script, all the commands execute one by one, automatically.

Master C & Data Structures for Free!

Enroll Now for Free and kickstart your journey as a problem-solving C programmer!

Explore Program

Why Learn Shell Scripting?

The benefit of shell scripting is that it provides an awesome advantage to anyone who works with a computer to perform tasks daily, especially if they do so in a Linux operating system or environment.

Below are some examples of how these scripts are widely used as part of the daily work in an organization:

Shell scripts help in tasks such as:

• System administration: Under this category come tasks such as backing up data, provisioning user accounts, and monitoring systems
• Software deployment: A user codes a script to install applications onto servers or even local computers.
• Data processing: A developer may also use a shell script for file conversions and giving structure to datasets.
• Work automation: Daily tasks such as scheduling tasks, weekly and monthly system maintenance, and report generation use a scripting language.  
• Development workflows: In a software development environment, such scripts automate various processes, such as building projects and executing tests.

Who Specifically Uses Shell Scripts?

There are a variety of people who are using shell scripting as part of their work. They are:

• System administrators manage servers and networks. They use scripts to automate daily server backups and monitor system health.
• DevOps engineers automate deployment processes. They leverage the power of scripts to streamline application deployments across multiple environments.
• Software developers who work on their development workflows use them to automate their build processes and execute test suites.
• Data analysts who work on data processing use scripts for faster extraction and transformation of data and various formats such as CSV, JSON, and more.
• IT personnel who maintain computer systems automate routine maintenance tasks like cleaning temporary files or managing user accounts using these scripts.

Is Shell Scripting Still Relevant Today?

Yes, even after all these advancements in technology, shell scripting remains one of the important skills a developer should have. There are several reasons why you should consider it as one of the top skills that are relevant in the industry or for your use. With computing infrastructures implementing command-line interfaces and automation adoption, there has been a phenomenal growth in this computer programming language. It allows professionals to be more efficient and responsive in operational tasks in IT and is important for working with the building blocks of modern technologies, such as containerization.

How Does a Shell Script Work?

Let us understand how a script works when executed, so that you can customize it for better efficiency and faster work. It works in a simple, straightforward way with very few steps. Let us look at them one by one.

Step 1: Initiate the Script

You perform this by entering the following command ‘./myscript.sh’ in your terminal and pressing Enter. This instructs your OS to run the requested file, but remember, the file needs the “execute” type permissions turned on before it can run. To grant it the execute permission, use the following command: chmod +x my_script.sh.

Step 2: The Shell Identifies the Interpreter

The first line of most shell scripts, called the “shebang” line (for example, #!/bin/bash or #!/usr/bin/python3), tells the OS what program to use to run the script, and that program is called an interpreter. If the file you are executing is a Bash script, the line #!/bin/bash points to the Bash shell program. If it is a Python script, the shebang line will point to the Python interpreter, and so on. Now, the operating system will deliver this script to the proper processing engine using the shebang line.

Step 3: The Interpreter Reads the Script Line by Line

When an interpreter, like Bash, begins to execute a script, it reads the contents of the script file sequentially, from top to bottom, and line by line. As it does this, the interpreter parses each line of the script, which means it reads each line to detect and understand the commands, variables, or flow control structures like loops or conditionals.

Step 4: Commands are Executed Sequentially

As the interpreter parses the script line by line, it executes the commands in the order it comes along. For example, if it finds a line that says ls -l, the shell executes the command ls, passing to ls the -l option. If it finds a line that defines a variable, it stores that value in memory. If it finds a line that has a flow control structure, like an if statement, it evaluates the condition and follows the appropriate path based on the condition, resulting in true or false. The order of execution follows the sequential order, which means they execute line by line based on when they are encountered. This flow of execution does not occur only when the interpreter is directed somewhere else by a flow control statement.

Step 5: Output is Displayed, and Execution Concludes

To alert the users of the success or failure of the command, it is always best practice to display a message on the screen for the readers to read. Use the echo command to directly display the message in your terminal that you are using as the environment. The script continues to execute line by line until it reaches the end of the file, encounters an explicit exit command, or finds an unrecoverable error. When the processing is over, the terminal shell returns control to your main terminal in the way it found it.

What Happens in Shell Memory?

When executing a script, the shell invokes a distinct process in memory, which stores:

• Variables: Temporary storage for data that is or was being used by the script.
• Environment: Any settings or information inherited from the parent shell.
• Command syntax: A record of all the commands that are executed for ease of debugging.
• File descriptors: Connections to the input, output, and error streams.

Basic Shell Scripting Concepts

When it comes to learning shell scripting, there are just a few basic ideas that you need to learn to get you started.  These ideas are the building blocks for every script you will write, and they will help you tell your computer what to do and understand how scripts interpret data.

1. Commands and Syntax

A shell script is nothing more than a series of commands executed by a shell interpreter.

• When you create such a script, it will generally start with a “shebang” line, like #!/bin/bash. The shebang tells your operating system which exact program, the interpreter such as Bash, should be used to execute the rest of the script.
• Commands are typically entered one per line, but if you have more complicated tasks, you can use operators like &&, also known as “and,” meaning to execute the next command only if the previous command was successful.
• In contrast, you can also use ||, which translates to “or,” meaning to execute the next command only if the previous command was unsuccessful. These two operators help to string commands together in one line and decide which to execute.

Example

When using commands, type the command name, followed by options, and then arguments. Use quotes if the text has spaces or special characters. You can also use a semicolon (;) to run multiple commands on the same line.

1. This is a basic command.

ls

An ls command is used to list the content of the current working directory.

2. This is a basic command with an option.

ls -l

An ls -l command is used to list the content of a directory in a long format, providing detailed information such as permissions, ownership, size, and modification date for each item.

3. This is a command with an argument.

ls /home/user/documents

An ls /home/user/documents command is used to list the content of a specific directory (in this case, /home/user/documents), rather than the directory the controller is currently in.

4. This is a command with both an option and an argument.

ls -l /var/log

An ls -l /var/log command is used to list the content of a specific directory (/var/log) in a long format, showing detailed information for each item within that particular directory.

2. Variables in Shell Scripts

Variables in shell scripts temporarily store data like strings, numbers, or file paths for reuse throughout the script. This offers a significant amount of flexibility and reusability in scripts.

• You create a variable by assigning a value to a name, such as: USER_NAME=”Mohan” or FILE_COUNT=10.
• To make it a reference for the value stored in a variable, use a dollar sign ($) in front of the name, such as echo $USER_NAME. This enables you to define information once and subsequently be able to refer to it throughout your script. You can easily update the value without having to change it everywhere it is used.

3. Comments in Shell Scripts

Comments are important, non-executable lines in your shell script. It usually begins with a hash symbol (#) and is just ignored by the shell interpreter during the script execution. The only goal for comments is to explain your code so that someone can make sense of it.

# This script greets the user
echo "Hello, $USER_NAME!"  # Displays a personalized greeting

Good comments can be extremely valuable. They can serve as documentation for you to remember the intent of specific code sections when you revisit your script years later. They are invaluable for sharing scripts with colleagues, collaborating, and ongoing maintenance.

4. If Else Statements in Shell Scripts

The if-else statement is probably the most important control structure because it lets your script make choices based on specific Boolean (either true or false) conditions. The script can behave intelligently by changing what it does based on different conditions it encounters while running.

if [ condition ]
then
      # Code 
else
      # Code
fi

One example of using an if-else statement would be to check if a file exists before creating a new one, or to check if a user has administrative rights before allowing an important command to run. All these forms of deductive logic help complete a functional and useful shell script.

Examples of Simple Shell Scripts

Examples are a great way to help you understand how shell scripting concepts work in practice; you can see and visualize the command usage and see what goes into making them do what they are meant to do.

1. Hello World

The “Hello, World!” script is the most basic and one of the first, if not the first, programs for almost any beginner to learn. It just displays a message on the screen.

#!/bin/bash
echo "Hello, World!"

Explanation:

• #!/bin/bash: This is the shebang line, which is always the very first line of a shell script. It tells your operating system that this script should be executed using the bash interpreter, a common command-line shell program.
• echo “Hello, World!”: The echo command is the most basic, used frequently to print (or display) text to your terminal window. Whatever text you put inside the double quotes after echo will appear on your screen.

2. File Handling

Shell scripts are heavily used for automating tasks involving files and directories. This example shows how to create a directory and copy files into it.

#!/bin/bash
# create a backup directory
mkdir -p backups
# copy important files
cp *.txt backups/
echo "Files backed up successfully"

Explanation:

1. #!/bin/bash: The meaning is the same as explained above.
2. # Create a backup directory: This line starts with a #, making it a comment. Comments are ignored by the script, but are crucial for humans to understand what each part of the script does. It is also helpful when another developer reads your script.
3. mkdir -p backups: Let us understand what each part of this command does.
   • mkdir: This command stands for “make directory” and is used to create new folders (directories) in your file system.
   • -p: This is an option for mkdir. It’s short for “parents” or “path.” If the backups directory already exists, -p ensures mkdir doesn’t throw an error. If any parent directories in the path don’t exist, it will create them too.
   • backups: This is the argument to mkdir, specifying the name of the directory to be created.
4. # Copy important files: Another comment, explaining the next action.
5. The next command is cp *.txt backups/:
   • cp: This command stands for “copy” and is used to copy files or directories from one location to another.
   • *.txt: This command simply refers to all the files with the .txt extension. The * symbol acts as a placeholder for “any characters.”
   • backups/: This tells the OS where to copy the matched files. The trailing confirms it is a directory, which is important.
6. echo “Files backed up successfully”: This command simply prints a confirmation message to the terminal.

3. Conditional Logic

Conditional Logic is one of the most commonly used features of scripting since it has the ability to make decisions. They allow your script to perform different actions based on whether a certain condition is true or false.

#!/bin/bash
if [ -f "data.txt" ]
then
    echo "Data file exists"
    wc -l data.txt
else
    echo "Data file not found"
    touch data.txt
fi

Explanation:

1. if [ -f “data.txt” ]: The if keyword is used to start a conditional statement in a shell script. The condition is written inside square brackets [ ]. For example, -f "data.txt" Checks if the file exists and is a regular file. If the condition is true, all commands after the then keyword will run, until the script reaches else or fi. This helps you control what parts of the script should run based on certain conditions, making your scripts more flexible and smarter.
2. Then and else keyword: If the condition evaluated in the if statement is true, the script executes all commands under the then and if it is false, then, else block is executed.
3. echo “Data file exists”: This prints a message to show that the file was found. It runs only when the condition in the if statement is true.
4. wc -l data.txt: The wc command stands for “word count” and is used to count words, tokens, as well as lines. The -l option tells it to display only the number of lines in the file named data.txt.
5. echo “Data file not found”: This line informs the user that the file doesn’t exist. It runs only when the condition is false.
6. touch data.txt: This command creates a new file by the name ‘data.txt’, if the file doesn’t exist. If the file by that name exists, this command will simply update the timestamp.
7. fi: This keyword marks the end of the if statement. Every if block must end with fi to show where the condition concludes.

Types of Shells

There are different types of shells used that offer various features and capabilities for shell scripting. Some of the most common ones are:

1. Bash: It stands for Bourne Again Shell and is specially used in the Linux operating system.
2. Zsh: It is short for Z shell and is known for its user-friendly features and powerful customization options.
3. Fish (Friendly Interactive Shell): The most attractive feature of this shell is that it provides syntax highlighting and is made for ease of use, with helpful suggestions.
4. Ksh (Korn Shell): It combines the features of Bash and C shells and is quite popular in the enterprise environment.
5. Dash (Debian Almquist Shell): It is often included in the Ubuntu operating system and is lightweight and fast.

Unix vs Linux Shell Scripting

As you start learning shell scripting, you will probably come across the two most commonly used terms, Unix and Linux. They are very closely related and have many similarities, but they are not exactly the same. The shell scripting concepts you will be learning will translate mostly between the two, but knowing the nuances in how they are different can help you think of why certain commands or tools may act slightly differently in different environments.

Online Shell Scripting Environment: Should You Use One?

An online shell scripting environment can be a valuable learning tool if you want to learn scripting basics and experiment with code before having to install any applications on your computer. An online environment provides a simple browser-based window to write and execute shell commands similar to those of a real Linux terminal.

When it helps learn and test

In general, an online shell scripting environment is useful for many reasons and particularly helpful for new learners in the following situations:

• Quick Practice & Testing: You will be able to quickly try out a single command or section of a script, to test how it works without having to be concerned about any setup or installation on your local computer. These environments are perfect for immediate testing and experimentation.
• No Local Linux Environment: If you do not want to or have not installed a Linux environment on your computer, or you do not want to deal with a complicated local development environment yet, these online environments allow you to quickly and easily start coding.
• Easy Code Sharing: Online environments usually allow for easier sharing of example scripts with classmates, instructors, or a friend when you can simply send a link to the code and permit them to run it.
• Learning Basic Concepts: An online coding environment provides a relatively distraction-free environment and allows you to concentrate on the code structure and syntax when learning about a new command or simple scripts.

Commonly Used Online Applications for Beginners

Several convenient, web-based applications can help you write and run your shell scripts:

• Replit: This is a very comprehensive application that gives you a complete, interactive Linux-like environment in your web browser. It is a great application for running all different types of shell scripts – not just basic commands.

• OnlineGDB: This is a simpler interface to run command-line scripts. OnlineGDB is a good place for testing quickly and simply.

Note for Complex Scripts: As wonderful as online environments are for learning and trying out basic scripts, they do have certain limitations. For complex scripts that require engaging with your actual computer’s file system, network configuration, or system administration tasks, you will need a real Linux system, like one installed on your actual computer, in a virtual machine, or on a cloud server.

Advantages and Disadvantages of Shell Scripting

Knowing the advantages and the disadvantages of shell scripting is important so that you can understand when to use it for automation or solution development. It is best for you, as a developer, to understand the abilities of shell scripting so that you know when and how to use shell scripting to your advantage and when to avoid it and choose a better tool for your requirements.

Advantages

Let us summarize the benefits of shell scripting for system administration and automating common transactions.

• Automation: Shell scripting makes it easy to completely ignore common repetitive tasks by doing them, saving the time and effort of a developer.
• Efficiency: Shell scripts can run multiple operations together with one command, which helps in quickly executing different processes.
• Integration: They work easily with software programs and tools available in system commands and utilities.
• Portability: Even if a developer codes the script to work on one Unix-like system, it will usually work on the newest Unix-like system as well with little or no modifications, hence making it a portable application.
• Quick Development: They are generally faster to write and implement for many tasks compared to compiled programming languages.

Disadvantages

Like most languages, shell scripting also comes with limitations. You will likely see these disadvantages when working through more complex and demanding tasks.

• Performance: Shell scripts are interpreted in a line-by-line fashion, which is slower than programs that are run after compilation.
• Error Handling: Identifying and debugging errors can be challenging due to limited built-in capabilities and less explicit error messages.
• Complex Logic: The logical flow involved in developing complex algorithms or deep logical flows can become messy, and it can be a challenge to go back and see how the logic worked.
• Security: Shell scripts can be prone to security issues, which traditionally arise if you don’t write your scripts carefully. This relates to no input validation, as well as having direct access to the system.
• Maintenance: The bigger or more complex shell scripts become, difficult it becomes to understand, modify, and troubleshoot them over time.

When Not To Use a Shell Script

With these advantages and disadvantages in mind, here are a few scenarios where you must not use these scripts.

• Do not use them for applications that need high performance, considerable computation, or advanced algorithms.
• Do not use them when developing large enterprise applications with complex logic or graphical user interfaces.
• Do not use them in scenarios where robust error handling and deep debugging capabilities are critical for application stability.

Learn Java - Zero Cost, Full Certification!

Join the Free Course and earn your Java certification while you learn.

Explore Program

Conclusion

Shell scripting continues to be an important skill to have if you want to automate processes, work with Linux systems, or perform system administration. It makes tasks like automating basic file operations to automating complex system management, and helps you streamline your workflow. Whether you are a beginner with some basic knowledge of commands or an expert with those skills, shell scripting is a great way to increase your productivity and technical skills.

Start with simple scripting, practice regularly, and over time, you will become experienced enough to build more complex automation techniques. These skills will benefit you throughout your career regardless of your chosen specialization.

What is Shell Scripting – FAQs