Mini Shell (Linux) – Custom Command Line Interpreter in C

📌 Project Overview

Mini Shell is a custom-built command line interpreter developed in C that replicates the core functionalities of a Linux terminal. The goal of this project is to understand and implement fundamental operating system concepts such as process creation, execution, inter-process communication, and signal handling.

This shell provides a user-friendly interface where users can execute both built-in and external commands, similar to a standard Unix/Linux shell.

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🎯 Objectives

• To design a simplified version of a Linux shell
• To understand process management using system calls
• To implement command parsing and execution
• To explore inter-process communication using pipes
• To handle signals like Ctrl+C and Ctrl+Z
• To gain hands-on experience with Linux system programming

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⚙️ Features

🔹 Built-in Commands

The shell supports several built-in commands:

• cd – Change directory
• pwd – Print current working directory
• echo – Display messages and environment variables
• exit – Exit the shell
• jobs – Display background jobs
• fg – Bring background process to foreground

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🔹 External Command Execution

• Executes system commands like ls, cat, wc, ps, etc.
• Uses fork() to create child processes
• Uses execvp() to replace child process image
• Parent process waits for child using waitpid()

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🔹 Pipe Implementation

• Supports single pipe (|)

• Supports multiple pipes (N-pipe execution)

• Enables command chaining:

  ls -l | grep .c | wc
  

• Uses pipe() and dup2() for inter-process communication

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🔹 Signal Handling

• Handles:

  • Ctrl + C (SIGINT) → Interrupt running process
  • Ctrl + Z (SIGTSTP) → Stop process and move to background

• Ensures shell does not terminate unexpectedly

• Custom signal handlers implemented

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🔹 Process Management

• Tracks background processes using linked list

• Implements:

  • jobs → View suspended/background processes
  • fg → Resume process in foreground

• Uses process IDs (PID) for management

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🔹 Environment Variables Support

• echo $$ → Displays shell PID
• echo $? → Displays last command exit status
• echo $SHELL → Displays shell path

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🔹 Custom Prompt

• User can change shell prompt dynamically:

  PS1=myshell
  

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🛠️ Technologies Used

• C Programming Language

• Linux Operating System

• System Calls:

  • fork(), execvp(), waitpid()
  • pipe(), dup2()
  • signal()

• File Handling APIs

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🧠 Key Concepts Learned

• Process creation and execution
• Parent-child process relationship
• Inter-process communication (IPC)
• Signal handling mechanisms
• Command parsing and tokenization
• Memory management in C
• Working of Unix/Linux shell internally

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📂 Project Structure

.
├── main.c
├── scan_input.c
├── command.c
├── signal.c
├── mini.h
├── external.txt
└── README.md

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▶️ How to Run

Step 1: Compile

gcc *.c

Step 2: Execute

./a.out

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🧪 Sample Execution

minishell$: ls -l | wc
     11      92     734

minishell$: echo $$
12345

minishell$: sleep 10
^C
minishell$:

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🚧 Limitations

• No support for input/output redirection (>, <)
• Limited error handling for invalid syntax
• Basic job control implementation

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🔮 Future Enhancements

• Add input/output redirection
• Support background execution (&)
• Improve job control (bg command)
• Add command history feature
• Implement auto-completion
• Improve parsing for complex commands

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🙌 Conclusion

This project provides a strong foundation in Linux system programming and demonstrates how a shell works internally. It significantly improved my understanding of operating system concepts and low-level programming in C.

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🔗 Connect With Me

If you liked this project or have suggestions, feel free to connect with me!