Bash Scripting: From Command to Automation

Up until now, we have been using the Linux terminal "Live"—typing one command at a time. This is like playing a piano note by note. Bash Scripting is about writing the "Sheet Music." It allows you to save a sequence of commands into a file so you can run thousands of operations with a single keystroke.

In this lesson, we will move from a user of the shell to a Programmer of the shell.

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1. Anatomy of a Shell Script ADAD

A shell script is simply a text file containing commands. However, it needs two special things to work properly as a script.

I. The Shebang (#!)

The very first line of every script must start with #!. This tells Linux which "Interpreter" should read this file.

• #!/bin/bash: Use the standard Bash shell.
• #!/usr/bin/python3: Use Python to read this file.

II. The Execution Bit

Just because a file contains code doesn't mean Linux will "Run" it. You must give the file Execute permission.

chmod +x my_script.sh

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2. Your First Script: hello.sh

Create a file named hello.sh and add the following:

#!/bin/bash

# This is a comment - it is ignored by the shell
echo "Starting my first automation..."
echo "My name is: $(whoami)"
echo "Today is: $(date)"
echo "My system has been up for: $(uptime -p)"

Running the script:

./hello.sh

Note: We use ./ to tell Bash to look in the "Current Directory" for the script.

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3. Positional Parameters: Passing Info to Scripts

Scripts become useful when you can send them "Inputs." These are called Arguments.

• $0: The name of the script itself.
• $1: The first argument.
• $2: The second argument.
• $#: The total number of arguments passed.

Example: greet.sh

#!/bin/bash
echo "Hello $1! You are currently in the $2 directory."

Running it: ./greet.sh Sudeep /var/log

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4. The Exit Status: Success or Failure?

Every command in Linux returns a "hidden" number when it finishes.

• 0: Success.
• Non-zero (1-255): Failure.

You can see the status of the last command by checking the special variable $?.

ls /folder_that_doesnt_exist
echo $?
# Output: 2 (Generic error code)

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5. Practical: The "Health Check" Script

Here is a script you might use as a sysadmin to quickly check if a server is ready for work.

#!/bin/bash

# server_check.sh
echo "--- PRE-FLIGHT CHECK ---"

# 1. Check if we are connected to the network
ping -c 1 8.8.8.8 > /dev/null 2>&1
if [ $? -eq 0 ]; then
    echo "[OK] Internet is reachable."
else
    echo "[FAIL] No internet connection!"
fi

# 2. Check Disk Space
FREE_DISK=$(df -h / | tail -1 | awk '{print $4}')
echo "[INFO] Disk space available: $FREE_DISK"

# 3. List top 3 memory users
echo "[INFO] Top processes:"
ps aux --sort=-%mem | head -4

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6. Example: A Script Template Generator (Python)

If you find yourself creating scripts often, you don't want to manually type the shebang and run chmod every time. Here is a Python script that creates a new Bash script for you and sets the permissions.

import sys
import os

def create_shell_script(name):
    """
    Bootstraps a new bash script with a shebang and sets execute bit.
    """
    if not name.endswith(".sh"):
        name += ".sh"
        
    content = "#!/bin/bash\n\n# Created automatically by ShShell Architect\necho 'Script running...'\n"
    
    with open(name, "w") as f:
        f.write(content)
        
    # Set permissions to 755 (rwxr-xr-x)
    os.chmod(name, 0o755)
    print(f"Successfully created executable script: {name}")

if __name__ == "__main__":
    if len(sys.argv) < 2:
        print("Usage: python3 create_script.py [script_name]")
    else:
        create_shell_script(sys.argv[1])

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7. Professional Tip: Path Environment

If you want to run your script from anywhere (just by typing my_script without the ./), move your script to the /usr/local/bin directory or add your home "bin" folder to your $PATH.

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8. Summary

Scripts are the "Automated Memory" of a systems engineer.

• #!bin/bash is the mandatory first line.
• chmod +x makes the file "Alive."
• $1, $2, ... are the inputs from the user.
• $? tells you if the previous command worked.
• echo is for output; # is for comments.

In the next lesson, we will learn how to make our scripts smarter by using Variables, Data Types, and User Input.

Quiz Questions

1. What does the #! (shebang) actually do when you run a script?
2. How do you find out the exit status of the command you just ran?
3. Why do we usually run scripts with ./script.sh instead of just script.sh?

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Continue to Lesson 2: Variables and User Input—Dynamic Shell Scripting.