Shell Writing Roadmap

Phase 1 — Basics (DONE)

Goal: Run a single external command from your shell prompt.

1. Process creation
   • Learn fork(), execvp(), waitpid()
   • Understand return values of fork() and process IDs
2. REPL loop
   • Prompt → read input → parse into argv[] → execute
3. Builtins
   • Implement cd (chdir()), exit
4. String parsing
   • Tokenize input using strtok or manual parsing

Phase 2 — File Operations & Redirection

Goal: Handle <, >, >> redirections.

1. File descriptors
   • stdin = 0, stdout = 1, stderr = 2
2. Opening/closing files
   • open(), close() from <fcntl.h>
   • Flags: O_RDONLY, O_WRONLY | O_CREAT | O_TRUNC, O_WRONLY | O_CREAT | O_APPEND
3. Replacing file descriptors
   • dup2() to redirect stdin/stdout before execvp()
4. Parsing for redirection symbols
   • Remove <, >, >> from argv[], open the files, apply dup2()

Phase 3 — Pipes

Goal: Support cmd1 | cmd2 | cmd3

1. Create pipes
   • pipe() returns two fds: read end and write end
2. Connect processes
   • Use dup2() to attach pipe ends to stdin/stdout
3. Multiple stages
   • Close unused pipe ends in each process
   • Fork once per stage of the pipeline