shell.py

#!/usr/bin/env python3

import re
import sys
import os
import signal
from typing import List
from subprocess import call


BUILTINS = {
    "cd": lambda path, *_args: os.chdir(path),
    "pwd": lambda *_args: print(os.getcwd()),
    "help": lambda *_args: print("commands:\n" + "\t".join(BUILTINS.keys()))
}


def sigint_handler(_signo, _stack_frame):
    print()
    sys.exit(0)


def try_builtin(tokens: List[str]) -> bool:
    """Try to execute tokens as a builtin."""
    try:
        BUILTINS[tokens[0]](*tokens[1:])
        return True
    except KeyError:
        return False


def try_command(tokens: List[str]) -> bool:
    """Try to execute tokens as a command."""
    try:
        status = call(tokens, stdout=sys.stdout, stderr=sys.stderr)
        return True
    except FileNotFoundError as e:
        print(f"{e.filename}: command not found", file=sys.stderr)
        return False


def main() -> None:
    """Python Shell."""
    signal.signal(signal.SIGINT, sigint_handler)

    while True:
        try:
            command = input(f"{os.getcwd()}$ ")
        except EOFError:
            print()
            sys.exit(0)

        tokens = re.findall(r"\S+", command)

        if len(tokens) == 0:
            continue

        try_builtin(tokens) or try_command(tokens)


if __name__ == "__main__":
    main()