Basic implementation of a shell in Python. This is from a talk I gave at YipitData on August 29, 2018.

ysh.py

# REPL
# read, eval, print, loop program
import os
import subprocess
import shlex
import sys

last_returncode = 0

try:
    while True:
        # read
        command = raw_input("> ")
        command_args = []
        for arg in shlex.split(command):
            if arg.startswith('$'):
                env_var = arg[1:]
                if env_var in os.environ:
                    command_args.append(os.environ.get[env_var])
            else:
                command_args.append(arg)

        if command == 'EXIT':
            sys.exit(0)
        elif command == 'EXITCODE':
            print(last_returncode)
            continue
        elif command == 'PWD':
            print(os.getcwd())
            continue
        elif command_args[0] == 'CD':
            os.chdir(command_args[1])
            continue

        # eval
        proc = subprocess.Popen(
            command_args,
            stdin=sys.stdin,
            stdout=sys.stdout,
            stderr=sys.stderr)
        # print
        stdout, stderr = proc.communicate()
        last_returncode = proc.returncode

        # loop
except (KeyboardInterrupt, EOFError):
    sys.exit(0)