This project was created as part of the 42 curriculum by chgomez.

pipex

Overview

pipex is a C project that recreates the behavior of a Unix shell pipeline.

The program takes an input file, executes two commands connected by a pipe, and redirects the final output to an output file.

It reproduces the following shell behavior:

< infile cmd1 | cmd2 > outfile

Using the program:

./pipex infile "cmd1" "cmd2" outfile

Description

The goal of pipex is to understand how Unix processes communicate through pipes and how command execution works at a low level.

The project uses system calls such as fork(), pipe(), dup2(), execve(), open(), close() and waitpid() to create two child processes, redirect file descriptors and execute external commands.

The first child process reads from the input file and sends its output through the pipe. The second child process reads from the pipe and writes the final result into the output file.

Example

Shell behavior:

< infile grep "hello" | wc -l > outfile

Equivalent pipex usage:

./pipex infile "grep hello" "wc -l" outfile

Features

• Recreates a basic Unix pipeline.
• Executes two commands using execve().
• Searches command paths through the environment variable PATH.
• Redirects input and output using file descriptors.
• Connects processes with pipe().
• Creates child processes with fork().
• Handles command arguments with a custom ft_split.
• Includes error handling for files, commands and system calls.
• Builds the executable using a custom Makefile.

Technologies

• C language
• Unix system calls
• File descriptors
• Process creation
• Pipes
• Environment variables
• Makefile
• Compilation with cc
• Flags: -Wall -Wextra -Werror

Project Structure

.
├── Makefile
├── en.subject.pdf
├── include/
│   └── pipex.h
└── src/
    ├── main.c
    ├── child1.c
    ├── child2.c
    ├── execute.c
    ├── errors.c
    ├── ft_split.c
    └── utils.c

How to Compile

Compile the project:

make

This generates the executable:

pipex

Remove object files:

make clean

Remove object files and the executable:

make fclean

Rebuild the project:

make re

How to Run

General syntax:

./pipex infile "cmd1" "cmd2" outfile

Example:

./pipex infile "grep hello" "wc -l" outfile

This should behave like:

< infile grep hello | wc -l > outfile

Testing Example

Create a simple input file:

echo -e "hello\nworld\nhello 42" > infile

Run pipex:

./pipex infile "grep hello" "wc -l" outfile

Check the result:

cat outfile

Expected output:

2

You can compare it with the shell behavior:

< infile grep hello | wc -l > expected
diff outfile expected

If diff produces no output, both results are the same.

Implementation Highlights

• main.c validates arguments, creates the pipe and manages child processes.
• child1.c handles the first command and redirects the input file to the pipe.
• child2.c handles the second command and redirects the pipe output to the output file.
• execute.c resolves command paths and executes commands with execve().
• errors.c centralizes error handling.
• utils.c contains helper functions used across the project.
• ft_split.c provides custom string splitting for command parsing.

What I Learned

• How Unix pipes work internally.
• How to create and manage child processes with fork().
• How to redirect standard input and output with dup2().
• How to execute external programs with execve().
• How to work with environment variables such as PATH.
• How file descriptors are inherited by child processes.
• How to close unused file descriptors to avoid leaks or blocked processes.
• How to structure a small Unix system programming project in C.

Resources

• man 2 pipe
• man 2 fork
• man 2 dup2
• man 2 execve
• man 2 open
• man 2 waitpid
• 42 pipex subject
• Unix process and file descriptor documentation

Notes

This project follows the constraints and style expected in the 42 curriculum. The implementation is intentionally written in C and focuses on Unix process management, pipes, command execution and file descriptor redirection.

Author

Christian Gómez
Junior Software Developer in training at 42 Barcelona
GitHub: github.com/chgomez04