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What is Shell Script, Sha-Bang, and First Script?

What is Shell Script?

A shell script is a sequence of commands saved to a file. This file can be executed as a script.
In a shell script, we can use programming features like:

  • Conditional statements
  • Loops
  • Functions

Shell scripts are best suited for automating tasks and can handle complex requirements with ease.

How to Write and Run a Shell Script?

Step 1: Write the Script

Create a file, for example, demo.sh, and include the following commands:

echo "Welcome to shell script"
date
cal

Step 2: Provide Execute Permissions to the Script

Use the following command to grant execute permissions:

$ chmod a+x demo.sh

Step 3: Run the Script

You can run the script in multiple ways:

$ /bin/bash ./demo.sh
$ bash ./demo.sh
$ /bin/bash /home/durgasoft/scripts/demo.sh
$ ./demo.sh  # If the default shell is bash

Note:

  • The default shell is bash, which executes the script by default.
  • If you want to use a different shell (e.g., Bourne shell), run the script as follows:
$ /bin/sh ./demo.sh
$ sh ./demo.sh

Importance of Sha-Bang

The sha-bang (#!) is used to specify the interpreter responsible for executing a script.
It provides clear instructions about which interpreter to use for the script.

  • # → Sharp
  • ! → Bang
  • #! → Sharp Bang, Sha-Bang, or Shebang

Examples:

  • #! /bin/bash → Specifies that the script should be executed by the Bash shell.
  • #! /bin/sh → Specifies that the script should be executed by the Bourne Shell.
  • #! /usr/bin/python3 → Specifies that the script should be executed by the Python 3 interpreter.

Why is Sha-Bang Important?

When a sha-bang is included in the script, you can execute the script directly without explicitly providing the interpreter command.

Examples of Scripts with and Without Sha-Bang

Q1 Write a Python Script and Execute Without Sha-Bang and With Sha-Bang

Script: test.py

#! /usr/bin/python3

import random
print("Good Morning")
name = input("Enter your name: ")
l = ["sejal", "shrushti", "archana", "arti", "gauri", "preeti"]
print("congratulation ", name)
print("Your best frined is ", random.choice(l))

Without Sha-Bang:

$ python3 ./test.py
$ python3 /home/durgasoft/scripts/test.py

With Sha-Bang:

$ ./test.py
$ /home/durgasoft/scripts/test.py

Q2 Consider the Following Script:

Script: demo.sh

#! /bin/rm
echo "It is a beautiful script"

What Happens When We Execute This Script?

$ ./demo.sh
  • This is equivalent to running: 
    $ rm ./demo.sh
  • The demo.sh file will be removed as the script is executed by the rm command.

Write and Run a Shell Script That Prints the Current System Date and Time

Script: date.sh

#! /bin/bash
echo "The current System Date and Time:"
date

Steps to Execute:

  1. Provide execute permissions:

    $ chmod a+x date.sh

  2. Run the script:

    $ ./date.sh

    Output:

    The current System Date and Time:
Wed Dec 4 21:01:49 IST 2019