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In my last entry, I introduced Python as a quick and easy way to create GUI programs for Ubuntu. However, Python has a super-handy module for creating your own command-line interfaces, known simply as cmd. Specifically, cmd contains a minimal class representing a command-line shell, called Cmd. To create your own shells, you must subclass cmd.Cmd and add your own functionality there. Here's a minimal example of how to create a command-line shell wth Python:

#!/usr/bin/env python

import os,sys,cmd,shlex,readline

class Shell(cmd.Cmd):
        def __init__(self,intro="Welcome to Shell):
                cmd.Cmd.__init__(self,intro)
        def do_prompt(self,line):
                "Set the command prompt"
                self.promptbackup="(Cmd) "
                if line=="":
                        self.prompt=self.promptbackup
                else:
                        self.prompt=line
        do_man=do_help
        def do_exit(self,line):
                "Exit Shell"
                return True

if __name__=="__main__":
        Shell().cmdloop()

As you can see, it's a fairly trivial affair. Actual commands are implemented as methods that begin with "do_". Exiting the shell requires a method that returns True. You can even create scripts for your shell, just like with any standard *nix shell or Windows batch file.