[OS][S4] Shell programming

Implement Shell programming
- Command syntax
- Write simple functions with basic tests, loops, patterns

You can make a simple calculator

Language: `Bash`

[Working Directory](https://github.com/dscmbcet/hacktoberfest-2021/tree/main/DS%20and%20Algorithms/S4)

Start by inspecting the DS and Algorithms/S4 working directory and any existing entries or tests to understand the repository’s conventions. The issue’s stated completion criteria are Bash coverage for command syntax, simple functions, tests, loops, patterns, and optionally a calculator, but it does not identify specific files or a narrower scope.

Written by the indexing model from the issue text.