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Create Your Own Shell in Python

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75 pointssupasate12 comments

the pythonic way of doing this is: https://docs.python.org/3/library/cmd.html

Reading the sorce code for cmd gave me my first "I am enlightened" moment when first programming in Python.

"You mean I just add a do_functionname method and that gives me a functionname command in the shell? And I can understand the 'refection' code that does this, and it dosen't look like incredibly verbose black-magiv-voodoo? I'm sold"

There are some youtube talks about python metaprogramming that you might enjoy later on. Your colleagues a little less.

The python prompt toolkit is very nice too, providing a lot more modern functionality (eg syntax highlighting, mouse support, suggestions). http://python-prompt-toolkit.readthedocs.io/

Another fun thing you can do is create a shell that only runs with the '-c' command line flag. If this is set as the default shell for a user, then you can use ssh as a simplistic rpc transport.

The funny thing is that I was literally just wondering about how shells are implemented right before getting on HN. Thanks for sharing, this should prove most useful :)

It's also shocking how simple pipes and redirection are. You just use the dup2 syscall to arrange file descriptors.

If you often wonder about such things, I highly recommend the book "Advanced programming in the UNIX environment" by Stevens/Rago: https://en.m.wikipedia.org/wiki/Advanced_Programming_in_the_...

It covers a lot of stuff in an authoritative way. For example, how one should implement a daemon properly (e.g., chdir to root to allow for unmounting the disk the program originally ran from, lots of stuff like that). I'll actually doubly recommend it, because it's so good.

Of course, a lot of the daemon boilerplate no longer really applies in the modern systemd world.

It didn't in fact apply to a sizeable portion of the pre-systemd world, either. systemd doco gives the quite false impression that this stuff is new, for "new-style daemons". Not using this boilerplate has been the right way to write daemons for many systems over the past quarter century, going back to the release of the IBM System Resource Controller at the beginning of the 1990s.

How does one implement logical connectives? (&&, ||)

There are many ways that one might; perhaps the simplest might be to just define them as a builtin to your shell that checks the exit code of the left-hand command, and executes or doesn't the right-hand command accordingly.

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