Shell from Scratch

The first "real" program that crosses Low-Level Atlas I's systems-programming theory into code you use every day. A shell is a small, sharp lens on fork/exec, process groups, signals, and file descriptors — all things Atlas I covered conceptually and this branch makes load-bearing.

Builds on Atlas I's systems programming and pointers & memory branches — this note assumes you already know what a syscall is and how fork() works; it does not re-explain them.

Planned notes

• The read–parse–execute loop and where a shell fits between you and the kernel
• Tokenizing a command line: quoting, escaping, and word splitting
• Building an AST for pipelines, redirections, and &&/||/;
• fork() + exec(): why two syscalls, not one
• Wiring pipes between processes (pipe(), dup2())
• I/O redirection: >, >>, <, and file descriptor bookkeeping
• Built-ins vs external commands: cd, exit, export, why cd can't be a subprocess
• Signal handling: SIGINT, SIGTSTP, and not killing the shell itself
• Job control: foreground/background (&), Ctrl-Z, fg/bg, process groups and setpgid
• Environment variables and $PATH resolution
• A minimal .shellrc and command history
• Comparing against dash/bash source for what we deliberately left out

Core sources

• "Build Your Own Shell" (codecrafters) — staged, testable milestones for this exact build.
• Advanced Programming in the UNIX Environment (Stevens & Rago) — process control, signals, and job-control chapters.
• The Linux Programming Interface (Kerrisk) — the canonical reference for every syscall this shell touches.

Connects to: Concurrency in Practice · Craftsmanship Low-Level II