Creating a Custom Linux Shell: Command-Line Interface Development in C

For this project, I designed and implemented a custom shell program for a Linux environment Written in C, this shell replicates core features of common command-line interfaces, providing both built-in and external command execution.

The shell supports several built-in commands, including exit, pid, ppid, cd, pwd, help, and jobs, each designed to function similarly to well-known Linux shell commands. For non-built-in commands, the shell uses the execvp() function, allowing it to fork new processes for executing external programs such as ls, vim, and others. It also handles background processes using job control, adding and tracking background jobs seamlessly.

Design Features

1. Command Execution and Parsing: Commands entered by the user are tokenized and parsed to identify if they are built-in or require external execution. For external commands, the shell forks a child process and uses execvp() to run the desired utility.

2. Background Job Handling: A key feature of the shell is its ability to manage background processes. By identifying if a command ends with an &, the shell can execute it asynchronously, allowing the user to continue entering new commands while background jobs run.

3. Error Handling and Feedback: Extensive error handling is integrated, ensuring that invalid commands, directory changes, or failed background jobs are reported clearly to the user.

   
   

This project demonstrates the power of C for systems-level programming and provides insight into the inner workings of a Linux shell. Below are images of the shell in operation, accompanied by source code snippets for deeper understanding of its functionality.

Source Code with extensive Comments for your convenance:

MyShell.txt

Download TXT • 5KB

Main():

In the image above, you can see the main function. This function is responsible for initializing the shell, parsing user input, and running the main while loop that continuously processes commands while the program remains active.

Execute_Command():

The execute_command function serves as the core of the shell, responsible for parsing commands, executing built-in functions, or passing commands to execvp() for execution. It determines whether the command is built-in or external, and appropriately handles each case by either invoking the respective built-in function or forking a new process to run the external command.

My Takeaway

In this lab, I designed and implemented a custom shell program in C, gaining a deep understanding of systems-level programming within the Linux operating system. Working closely with the OS, I developed skills in command parsing, process control, and managing background jobs. Using execvp(), I was able to execute external commands, while also replicating core Linux shell features. This project highlighted the importance of both C programming and operating system interactions, especially in building robust command-line interfaces.