A Unix-style shell written in C++, implemented natively on both Windows and Linux — not emulated, not wrapped, two real process models understood and built against.
lsh is a shell — the thing that reads a command, runs it, and gives you a prompt back — built to actually understand what a shell is doing underneath, not just to have one working. It's inspired by Stephen Brennan's classic "Write a Shell in C", rebuilt in C++, and extended past the tutorial in a specific way: it runs natively on both Windows and Linux, using each OS's own process-creation model rather than papering over the difference with a compatibility layer.
That distinction matters more than it sounds. Unix gives you fork() + execvp() — clone the calling process, then overwrite the clone with a new program. Windows has no equivalent primitive; process creation there is a one-shot CreateProcess()-family call. Most "make it cross-platform" shortcuts reach for an emulation layer (MSYS2/Cygwin) that fakes fork() on top of Windows. This project doesn't — it implements both real models side by side.
(Recording coming once the Unix build is finished — see Roadmap below.)
| Command | Description |
|---|---|
cd <dir> |
Change working directory. Supports .. to move up one level. |
cwd |
Print the current working directory. |
tree |
Recursively list files/directories from the current path, hidden files excluded. |
tree_all |
Same as tree, but includes hidden files. |
man <command> |
Print usage info for any builtin. |
help |
List all builtin commands. |
exit |
Terminate the shell. |
Input is read line by line (lsh_read_line), then tokenized on whitespace into arguments (lsh_split_line) — no shell-metacharacter parsing (pipes, redirects, quoting) yet; see Roadmap.
Builtins are wired through two parallel arrays matched by index — a name table (builtin_str[]) and a function-pointer table (builtin_func[]). lsh_execute checks the typed command against every name in the table; on a match, it calls the function at the same index in the function table. No typed command matches a builtin, it falls through to lsh_launch, which hands the command off to the OS to run as an external program.
On Linux:
fork() → clone the calling process (copy-on-write)
↓
execvp() → child overwrites its own memory image with the new program
↓
waitpid() → parent blocks until the child terminates, retrieves exit status
On Windows:
_spawnvp(_P_WAIT, ...) → CreateProcess() under the hood: builds a brand-new
process directly from the executable, no intermediate
"clone" state — then blocks until it exits
There's no Windows equivalent to the fork-then-exec split — _spawnvp fuses process creation, program loading, and waiting into a single call, because Windows never exposed a "duplicate my own running process" primitive the way Unix's copy-on-write fork() does.
g++ -std=c++17 -o shell.exe lsh.cpp
./lsh.exeg++ -std=c++17 -o shell lsh.cpp
./lshRequires a C++17-capable compiler (<filesystem> is used for tree/cwd).
- Core read → parse → execute loop
- Builtins:
cd,cwd,exit,help,man -
cd ..support -
tree/tree_all - Windows process launch via
_spawnvp - Native Unix process launch via
fork/execvp/waitpid - Signal handling (Ctrl+C shouldn't kill the shell itself)
- I/O redirection (
>,<) - Pipes (
|) - Command history
Every one of these problems is solved better by bash. That's not the point. The point was going one level below the tools I use daily — understanding what a shell is actually doing when it runs a command, what a process is at the OS level, and where Windows and Unix genuinely diverge in how they model something as basic as "run a program." That's the actual deliverable; the shell is just the artifact that proves it.
As the famous quote goes
What I cannot create I do not understand.
Built on the shoulders of Stephen Brennan's "Write a Shell in C" — the canonical starting point for this exact project, extended here into a cross-platform, C++ rewrite with additional builtins and a from-scratch Windows process-launch path. Working on creating more features and learning more everyday.
Built by Waasila Asif