🔗 C-Project: Pipex

💥 Sixth project in the 42 Common Core
I am . . . f i n e ( ._. )

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🧠 Project Overview

Pipex is a reproduction of how Unix handles pipes (|) in the shell.

You will build a program that mimics the shell behavior for piping commands like:

< infile cmd1 | cmd2 > outfile

Implemented in C using:

• pipe()
• fork()
• dup2()
• execve()

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🔧 Usage

Build

make

Run

./pipex infile cmd1 cmd2 outfile

This is equivalent to:

< infile cmd1 | cmd2 > outfile

Example:

./pipex input.txt "grep hello" "wc -l" output.txt

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📁 File Structure

File	Purpose
pipex.c	Main logic for parsing and executing
utils.c	String splitting, path resolving
pipex.h	Header with macros and prototypes
Makefile	Builds your program

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📚 Unix Concepts Recap

📄 Everything is a File

In Unix:

• Files = text, devices, sockets, etc.
• You use file descriptors (FDs) to access them

🔁 Pipes (|)

A pipe sends the stdout of one command to the stdin of another.

Example:

echo "Hello" | wc -c  # Outputs: 6

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🔍 How Pipes Work in C

🔨 Code Example

int fd[2];
pipe(fd);
if (fork() == 0)
{
    close(fd[0]);           // Close read end
    dup2(fd[1], STDOUT_FILENO); // Redirect stdout
    execve(cmd1_path, cmd1_args, envp);
}
else
{
    close(fd[1]);           // Close write end
    dup2(fd[0], STDIN_FILENO);  // Redirect stdin
    execve(cmd2_path, cmd2_args, envp);
}

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🧵 Processes in Unix

Every program is a process with:

• A PID
• A Parent
• Possibly Children

Creating a new process:

pid_t pid = fork();

• pid == 0 → child
• pid > 0 → parent

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⚙️ Core System Calls Used

pipe(int fd[2])

Creates a pipe: fd[0] for reading, fd[1] for writing.

fork()

Creates a new process.

dup2(oldfd, newfd)

Redirects file descriptors.

dup2(fd[1], STDOUT_FILENO); // Redirect stdout to write end

execve()

Runs a command.

char *cmd[] = {"/bin/ls", NULL};
execve(cmd[0], cmd, envp);

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🧪 Testing

Basic Test

./pipex infile "cat" "wc -l" outfile

Compare to Shell Output

diff <(./pipex in "grep hello" "wc -l" out) <( < in grep hello | wc -l > out )

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🛠 Bonus Ideas

• Support more than 2 commands (multi-pipe)
• Handle quotes and escaped characters
• Implement your own split() and get_path()

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🧠 Educational Notes

Imperative Programming

Pipex teaches core Unix concepts like:

• File descriptors
• Process control
• Inter-process communication
• Executing binaries

Plus, you'll get better at debugging with:

• strace ./pipex ...
• valgrind ./pipex ...

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🧵 Debugging Tip

Use these to see what’s happening behind the scenes:

valgrind ./pipex input "ls -l" "wc -l" output
strace ./pipex input "ls" "wc" output

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🧼 Clean

make fclean

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🧊 Sample Output

$ cat infile
hello
hi there

$ ./pipex infile "grep h" "wc -l" outfile

$ cat outfile
2

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🌐 Resources

• man execve
• man pipe
• fork() visualizer

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💬 Final Words

This project is where you:

• Really understand how a shell works
• Learn the cost of forking badly
• Fall into execve segfault despair

But then... you emerge stronger 💪

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Made with 🍝, 🧠, and a little bit of crying by your local wizard.

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