How to Write a Basic Shell Script in Linux: A Step-by-Step Guide

Writing a script in Linux typically involves creating a file with a list of commands that you want to execute. Here’s a step-by-step guide on how to write a basic shell script:

1. Choose a Text Editor

To write a shell script, you need a text editor. Linux offers several options:

• nano

• vi

• vim

• gedit

2. Create a New Script File

Create a new file with a .sh extension. For example, you can create a file named myscript.sh: nano myscript.sh

3. Add the Shebang Line

The first line of the script should be the shebang (#!) followed by the path to the shell you want to use. For a Bash script, it would look like this: #!/bin/bash

4. Write Your Script

Add the commands you want to execute. Here’s an example script that prints the username and the current date and time:

#!/bin/bash

# Print the current user

echo "The current user is: $(whoami)"

# Print the current date and time

echo "The current date and time is: $(date)"

# List files in the current directory

echo "Listing files in the current directory:" ls -la

5. Save and Exit

Save the file and exit the text editor. In nano, you can do this by pressing Ctrl+O to write out the file, then Enter to confirm, and Ctrl+X to exit.

6. Make the Script Executable

To run the script, you need to make it executable. Use the chmod command to change the file permissions: chmod +x myscript.sh

7. Run the Script

You can now run your script by typing ./ followed by the script name: ./myscript.sh

Example of a Complete Script

Here’s another example that checks if a directory exists and creates it if it doesn’t, then writes the username to a file within that directory:

#!/bin/bash

# Define the directory and file name

DIR="/tmp/mytestdir"

FILE="$DIR/username.txt"

# Check if the directory exists

if [ ! -d "$DIR" ]; then

# If not, create the directory

mkdir -p "$DIR"

echo "Directory $DIR created."

else

echo "Directory $DIR already exists."

fi

# Write the current user to the file

echo "The current user is: $(whoami)" > "$FILE"

echo "Username written to $FILE."

# Output the contents of the file

cat "$FILE"

Running the Script

Follow the same steps to create, save, and run this script. This script will:

1. Check if /tmp/mytestdir exists.

2. Create the directory if it does not exist.

3. Write the current username to a file named username.txt inside that directory.

4. Display the contents of username.txt.

Tips for Writing Shell Scripts

• Comments: Use # to add comments to your script for better readability.

• Variables: Store reusable values in variables.

• Conditionals: Use if, elif, and else to make decisions.

• Loops: Use for, while, and until to repeat tasks.

• Functions: Define functions for reusable blocks of code.