TinyShell

Overview

TinyShell is a minimal but real POSIX shell written in C. It supports the core features you’d expect from bash — command execution, pipes, I/O redirection, background jobs, and basic job control — implemented from scratch using fork, exec, pipe, and waitpid.

Features

• Pipelines — cmd1 | cmd2 | cmd3 with correct fd wiring
• Redirection — >, <, >>, 2> using dup2
• Background jobs — & suffix, jobs builtin, fg/bg control
• Builtins — cd, exit, echo, jobs, fg, bg
• Signal handling — SIGCHLD for zombie reaping, SIGINT/SIGTSTP forwarding

The Pipeline Challenge

The trickiest part is wiring pipes correctly in a multi-stage pipeline. Each process needs the read end of the previous pipe and the write end of the next. Getting the close() calls right to avoid deadlocks took several iterations.

// For n commands, create n-1 pipes
int pipes[n-1][2];
for (int i = 0; i < n-1; i++) pipe(pipes[i]);

for (int i = 0; i < n; i++) {
    pid_t pid = fork();
    if (pid == 0) {
        if (i > 0)   dup2(pipes[i-1][0], STDIN_FILENO);
        if (i < n-1) dup2(pipes[i][1],   STDOUT_FILENO);
        // close all pipe fds before exec
        execvp(cmds[i].argv[0], cmds[i].argv);
    }
}

What I Learned

• How a shell is fundamentally just a fork/exec/wait loop
• Why SIGCHLD handling is necessary to avoid zombie processes
• The subtlety of process groups and sessions for proper job control
• How dup2 replaces file descriptors to wire up pipes invisibly to child programs