🐚 Simple Shell in C

A lightweight custom shell built in C that supports:

✅ Command execution
✅ Input/Output redirection (<, >)
✅ Background processes (&)
✅ Built-in cd and exit commands
✅ Basic zombie process cleanup

This project is designed to help you understand how fork(), execvp(), and process management work in UNIX/Linux systems.

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📁 Features

Feature	Description
Command execution	Runs external commands using execvp()
Input redirection (<)	Reads input from a file instead of standard input
Output redirection (>)	Redirects output to a file instead of the terminal
Background processes (&)	Runs processes in the background (non-blocking)
Change directory (cd)	Built-in command to switch directories
Exit command (exit)	Gracefully terminates the shell
Zombie cleanup	Uses waitpid(-1, NULL, WNOHANG) to prevent zombie processes

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⚙️ Compilation

Use gcc to compile the program:

gcc src/main.c

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▶️ Running the Shell

After compilation, run your shell using:

./a.out

You'll see a prompt like:

=== Simple Shell with I/O Redirection ===
Commands:
  ls                    - list files
  ls > output.txt       - save output to file
  cat < input.txt       - read from file
  cat < in.txt > out.txt - read from in.txt, write to out.txt
  echo hello > file.txt - write to file
  ls &                  - background process
  cd <directory>        - change directory
  exit                  - quit shell

mysh:/home/user>

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💡 Examples

Command	Description
ls	List files in current directory
ls > files.txt	Save list of files to files.txt
cat < input.txt	Display content of input.txt
cat < input.txt > output.txt	Copy content of input.txt into output.txt
echo Hello > hello.txt	Write "Hello" to hello.txt
ls &	Run ls in background
cd /home	Change directory to /home
exit	Exit the shell

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🧠 How It Works

1. User Input

• Reads input using fgets() and removes the trailing newline.
• Detects background (&) and I/O redirection symbols (<, >).

2. Parsing

• Uses strtok() to split input into command tokens.
• Detects files for redirection and stores their names separately.

3. Execution

• Forks a new process:
  • Child: handles I/O redirection using dup2(), then runs command with execvp().
  • Parent: waits for the child unless it's a background process.

4. Zombie Cleanup

• Calls reap_zombies() before each prompt to clean up finished background processes.

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🧩 Code Highlights

🔁 Zombie Cleanup

void reap_zombies() {
    while (waitpid(-1, NULL, WNOHANG) > 0);
}

🔄 Input/Output Redirection

int fd_out = open(output_file, O_WRONLY | O_CREAT | O_TRUNC, 0644);
dup2(fd_out, STDOUT_FILENO);

🧍 Background Execution

if (isBackground == 0)
    waitpid(pid, NULL, 0);
else
    printf("[Background process started with PID %d]\n", pid);

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📚 Learning Goals

This project helps you learn:

• Process creation with fork()
• Executing programs with execvp()
• File descriptor manipulation using dup2()
• Waiting for processes (waitpid)
• Basic shell parsing logic