How Interactive Shells Work

• Back to Interactive Shell (a page of ideas)

Let's figure out how interactive shells work! bash, zsh, fish, and Oil are significantly different. The goal is to figure out:

• What interactive features Oil should have. (Right now, it's mostly bash-like)
• What hooks/APIs it should have so that others can implement nice interactive experiences (like zsh plugins and ble.sh on top of bash)

Please add links or hand-written descriptions in the sections below, or talk to us on #oil-discuss at https://oilshell.zulipchat.com/ (log in with Github)

Parts of the Interactive Shell

• The prompt.
  • In POSIX shell this is $PS1 and $PS2.
  • bash has extensions.
  • Does a shell have to know how wide the prompt is to draw correctly? (note: this depends on unicode chars in the propmt)
• Shell Autocompletion -- when you hit TAB, what happens
• Autosuggestions -- does it display suggestions on the prompt line automatically? That is, there is no prompting.
• Spell correct? zsh has this I think.
• History. What format is the history stored in?
  • in bash it's a text file, which gives it an odd "two level store" behavior: see issue 324

Research

note: feel free to split this into more wiki pages

How Bash Works

• It uses GNU Readline. bash and readline are maintained together.

How Zsh Works

• TODO: What are the common plugins and what do they expect from the shell?

How Fish Works

Features

• autosuggestions

How ble.sh Works

TODO

• is there a "main loop" ?
• what does it expect from bash?
  • $LINES, $COLUMNS
  • it needs to trap SIGWINCH (todo: test this signal in Oil)
• how does it execute commands?

How Oil Works

Oil is mostly bash-like now. I added a zsh-like completion interface that doesn't scroll every time you hit TAB, but it has some bugs, like #257.

• Search for \x1b to see all the escape codes Oil uses: https://github.com/oilshell/oil/blob/master/core/comp_ui.py