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Introduction to Shell Scripting

Why Write Scripts?

Think about a task you do often on your computer. Maybe you back up important files to another folder every day. You open the terminal, type a copy command, specify the source, specify the destination, and hit enter. It’s not hard, but doing it repeatedly is tedious. A shell script is simply a file containing a list of those commands. Instead of typing them all out each time, you just run the script.

Shell scripting is a way to automate repetitive tasks. It saves time, reduces the chance of human error, and makes complex processes easy to repeat.

Think of it like a recipe. You write down the steps once, and then you (or your computer) can follow them perfectly every time. This is the core benefit: you capture a process in a file, and you can execute that process with a single command.

Anatomy of a Shell Script

A shell script is a plain text file, but it has a few key parts. Let's look at a simple example.

#!/bin/bash

# This is a comment. The computer ignores it.
# It's here to explain what the script does.

echo "Hello, world!"
pwd

Let's break that down:

  1. The Shebang: The very first line, #!/bin/bash, is special. It's called a shebang. It tells the operating system which interpreter to use to run the commands in the file. In this case, we're telling it to use Bash, a common command-line shell.

  2. Comments: Any line that starts with a # is a comment. The computer completely ignores these lines. They are notes for you and other people who might read your script later. Good comments explain why the script does something, not just what it does.

  3. Commands: The rest of the file is a sequence of commands, just as you would type them in your terminal. Here, we have echo "Hello, world!", which prints that message to the screen, and pwd, which prints the current working directory.

Creating and Running Your First Script

Let's create and run a script from scratch. It's a three-step process: write, permit, and execute.

First, open a text editor and create a new file. A common convention is to give shell script files a .sh extension. We'll call ours greeting.sh.

nano greeting.sh

Now, type the following code into the file. This script will print a greeting and then list the files in the current directory.

#!/bin/bash

# A simple script to greet the user and list files.

echo "Welcome to your first script!"
echo "Here are the files in your current directory:"
ls -l

Save and close the file. (In nano, you press Ctrl+X, then Y, then Enter).

Next, you need to give the file permission to be executed. By default, new text files are not runnable for security reasons. We use the chmod (change mode) command to add execute permission (+x).

chmod +x greeting.sh

Finally, you can run your script. To run a program that's in your current directory, you need to tell the shell to look right here by starting the command with ./.

./greeting.sh

You should see the output of your commands printed directly to the terminal. You've just automated a two-step process!

Now that you understand the basics, let's test your knowledge.

Quiz Questions 1/5

What is the primary purpose of a shell script?

Quiz Questions 2/5

What is the function of the #!/bin/bash line at the very beginning of a script?