Support vi mode

[tmfink]

Feature description

Brush should support vi style line editing. Currently, brush only supports emacs line editing.

Use case / motivation

I (and other users I know) use vi mode in bash for more efficient line editing.
For many users vi-style line editing is a must-have feature for a shell.

As my standard .bashrc setup includes:

# Vi mode
set -o vi
bind -m vi-insert "\C-l":clear-screen

Proposed solution

Set switch behaviors:

set -o emacs # set vi OFF; set emacs ON
set +o emacs # set emacs OFF
set -o vi # set emacs OFF; set vi ON
set +o vi # set vi OFF

Set switches:

(emacs OFF, vi OFF) => no line editing
(emacs ON,  vi OFF) => emacs line editing
(emacs OFF, vi ON ) => vi line editing
(emacs ON,  vi ON ) => [NOT possible to set]

Line editing mode should be stored with something like this:

enum LineEditingMode {
    /// no line editing (emacs OFF, vi OFF)
    None,

    // Emacs style line editing (emacs ON, vi OFF)
    #[default]
    Emacs,

    // Vi style line editing (emacs OFF, vi ON)
    Vi,
}

Example session of bash --norc showing expected behavior:

bash-5.3$ alias m='set -o | rg -w "vi|emacs"; echo'
bash-5.3$ m # default (emacs)
emacs           on
vi              off

bash-5.3$ set -o vi # set to vi
bash-5.3$ m
emacs           off
vi              on

bash-5.3$ set -o emacs # set to back to emacs
bash-5.3$ m
emacs           on
vi              off

bash-5.3$ set +o emacs # disable line editing (tab inserts tab character)
bash-5.3$ m
emacs           off
vi              off

Additional context

Bash docs on vi mode:

While the Readline library does not have a full set of vi editing functions, it does contain enough to allow simple editing of the line. The Readline vi mode behaves as specified in the sh description in the POSIX standard.

You can use the set -o emacs and set -o vi commands (see The Set Builtin) to switch interactively between emacs and vi editing modes, The Readline default is emacs mode.

When you enter a line in vi mode, you are already placed in 'insertion' mode, as if you had typed an i. Pressing ESC switches you into 'command' mode, where you can edit the text of the line with the standard vi movement keys, move to previous history lines with k and subsequent lines with j, and so forth.

https://www.gnu.org/software/bash/manual/html_node/Readline-vi-Mode.html

[reubeno]

Thanks for filing the request, @tmfink, and also for the clear details and motivation.

That's a reasonable request--and thankfully the reedline crate we rely on for our readline-like input has added and been expanding its vi mode support over the last year+.

The main work needed in brush will be managing the mode setting as you call out, and ensuring that we sync the mode with reedline when we call down into it. There may be some targeted changes needed in our implementation of the bind builtin, but I'd hope they'd be minimal.