Simple Shell

Building a Simple Unix Shell Program

Isa Isa
Lawal Tajudeen

A Unix shell is a tool that allows users to interact with the OS through a command-line interface. It provides a convenient way to execute programs/ commands, run programs, and manage files and processes.

This READ ME write-up explores the process of building a simple Unix shell program using the C programming language. It covers the basics of a Unix shell termed Simple Shell, including the following:

1. Understanding the Basics:

• Basics:

  • A shell reads commands from the user,
  • Interprets them, and
  • Executes the corresponding programs.

• It also handles features like:

  • input/output redirection,
  • piping, and
  • command history.

The main goal is to create a simplified version of a shell that performs the essential functions of a shell.

2. Command Line Input:

• The first step:
  • Read commands entered by the user. We can use the getline() function to read a line of input from the user, this depends on the allowed functions from the project requirements.
  • Next we can parse the command line to separate the command and its arguments.

3. Executing Commands:

• Once we have the command and its arguments, we can create a child process using fork(). In the child process, we can use execve() to replace the child process with the desired command.

NOTE: Please man all the functions listed in this Read me file to have a better understanding of what they do and how they connect to create a simple shell process.

The execve() function loads and executes the specified program file, and it requires the full path to the program along with the arguments.

4. Handling Built-in Commands:

• In addition to external commands, shells often support built-in commands, such as cd or exit. (make tresearch as to what these commands do)
• We can add support for these built-in commands by checking if the command matches any of the built-in commands and executing them accordingly.

5. Managing Processes:

• A shell should handle multiple processes efficiently.

• We can use system calls like fork(), execve(), wait(), and waitpid() to manage child processes and ensure proper execution and termination. By using these functions, we can:

• Execute commands at different time intervals,

• Wait for processes to finish, and

• Handle errors gracefully.

6. Signal Handling: Unix shells should also handle signals, such as Ctrl+D, Ctrl+Z, Ctrl+C. (Do make an extensive research to know how to handle these and what exactly they do). We can use the custom function _signal() function to register signal handlers and define the desired behavior for each signal.

7. Prompt and Loop:

• We create an interactive shell, and our objective is to make sure it is not running in non-interractive mode. we can continuously prompt the user for input after executing a command. This allows users to enter multiple commands one after another. We can decide to use a loop to repeat the processes:
  • Reading input,
  • xecuting commands, and
  • Displaying the prompt until the user chooses to exit.(EOF)

8. How to use:

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Conclusion: Building this simple Unix shell program provided a great opportunity to learn about:

• Process management,
• Command execution, and
• User interaction in a Unix-like environment.

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ALX SE