Custom Unix Shell

A lightweight Unix-like shell implemented from scratch in C, using low-level POSIX system calls.
This project demonstrates core operating-system concepts such as process creation, inter-process communication, file descriptor management, and signal handling.

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Overview

This shell supports executing Unix commands, pipelines, I/O redirection, background processes, and basic job control.
It is designed to be educational, minimal, and close to how real shells work internally.

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Features

Command Execution

• Execute standard Unix commands
• Argument parsing and PATH-based executable lookup
• Implemented using fork() and execvp()

Pipelines

• Supports multi-stage pipelines using |
• Implemented with pipe() and dup2()

Example: ls -l | grep txt | wc -l

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I/O Redirection

• Input redirection: <
• Output redirection: >
• Append redirection: >>

Example:

cat input.txt | grep error > output.txt

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Background & Foreground Jobs

• Background execution using &
• Foreground execution with blocking waits
• Job tracking using process groups

Example:

sleep 30 & jobs fg 1

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Built-in Commands

• cd — change working directory
• exit — exit the shell
• export — set environment variables
• jobs — list background jobs
• fg / bg — job control

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Signal Handling

• Handles SIGINT (Ctrl+C)
• Handles SIGTSTP (Ctrl+Z)
• Handles SIGCHLD (child termination)

The shell itself does not terminate on Ctrl+C / Ctrl+Z.

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Environment Variable Support

• Environment variable expansion ($VAR)
• PATH-based command resolution

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Technologies Used

• Language: C (GNU C / C17)
• System Calls & APIs:
  • fork, execvp
  • pipe, dup2
  • waitpid
  • kill
  • setpgid
  • signal
• Headers: unistd.h, signal.h, fcntl.h, sys/wait.h

Build Instructions

Compile using Makefile

make

Compile manually

gcc -std=gnu11 -O2 -Wall -Wextra shell.c -o mysh

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Running the Shell

./mysh

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Example Usage

ls ls -l | grep cpp cat input.txt | grep error > output.txt sleep 10 & jobs fg 1 export PATH=/usr/local/bin:$PATH cd /tmp exit

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If you find this project useful, feel free to ⭐ the repository!