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Oils Headless Mode: For Alternative UIs

A GUI or TUI process can start Oils like this:

osh --headless

and send messages to it over a Unix domain socket. In this mode, the language and shell state are decoupled from the user interface.

This is a unique feature that other shells don't have!

Note: This doc is in progress. Join the #shell-gui channel on Zulip for current information.

The General Idea

The UI process should handle these things:

• Auto-completion. It should use Oils for parsing, and not try to parse shell itself!
• History: Allow the user to retrieve commands typed in the past.
• Cancelling commands in progress.
• Optional: multiplexing among multiple headless shells.

The shell process handles these things:

• Parsing and evaluating the language
• Maintaining state (shell options, variables, etc.)

How to Write a Client for a Headless Shell

Implement the FANOS Protocol

FANOS stands for File descriptors and Netstrings Over Sockets. It's a control protocol that already has 2 implementations, which are very small:

• client/py_fanos.py: 102 lines of code
• cpp/fanos_shared.c: 215 lines of code

Send Commands and File Descriptors to the "Server"

List of commands:

• EVAL. Parse and evaluate a shell command. The logic is similar to the eval and source builtins.
  • It can be used for both user-entered commands and "behind the scenes" functions for the shell UI.
  • The stdin, stdout, and stderr of the shell and its child processes will be redirected to the descriptors you pass.
  • There's no history expansion for now. The UI can implement this itself, and Oils may be able to help.

TODO: More commands.

Query Shell State and Render it in the UI

You may want to use commands like these to draw the UI:

• echo ${PS1@P} -- render the prompt
• echo $PWD $_ -- get the current directory and current status

You can redirect them to a pipe, rather than displaying them in the terminal.

Remember that a fundamental difference between a REPL and a GUI is that a GUI shows state explicitly. This is a good thing and you should take advantage of it!

Example Code

See client/headless_demo.py. This is pure Python code that's divorced from the rest of Oils.

Related Links

Feel free to edit these pages:

• Headless Mode on the wiki. We want there to be a rich ecosystem of interactive shells built upon Oils.
• Interactive Shell on the wiki. Be inspired by these nice projects, many of which have screenshots!