Luke Maurits

Here is an absolute bare-minimum Unix shell written in just 36 lines of Python
(including a few blank lines). It doesn’t do pipelines, input/output redirection,
tab-completion or have a command history, it’s just a really light wrapper around
the fork, execv and wait system calls, using the os
module. The only internally implemented function is cd because a shell
is a painful thing to use without it.

from os import chdir, execv, fork, getenv, wait
from os.path import exists
from sys import exit

while True:

    # Get input
    prompt = getenv("PS1", "$ ")
    try:
        input = raw_input(prompt)
    except EOFError:
        exit()

    # Parse input
    argv = input.split()
    command = argv[0]

    # Do inbuilts
    if command == "cd":
        if len(argv) == 1:
           chdir(getenv("HOME", "/"))
        else:
           chdir(argv[1])
    # Do external
    else:
        found = False
        path = getenv("PATH")
        for dir in path.split(":"):
            if exists("/".join((dir, command))):
                found = True
                if(fork() == 0):
                    execv("/".join((dir, command)), argv)
                else:
                    wait()
    if not found:
        print "%s: Not found" % command

This works quite smoothly, it doesn’t feel laggy or anything when you’re
actually using it. I suppose this isn’t unusual because the Python code in use
probably just directly calls the underlying C system calls. It’s really pretty
neat, if not slightly silly.

I have a somewhat more complicated version working, but at the moment it’s still
looking a bit too ugly to post in a blog entry – it handles interpolation of
environment variables, multiple semi-colon delimited commands on a line and has the
beginnings of pipeline and redirection support.

I think I’ll end up writing an article that explains how Unix shells work,
using a Python shell as an example (because it’s much easier for the average
person to read than C), and put the complete shell up on my
software page.

On an unrelated note, here’s Super Mario World implemented in Javascript. Wow.