Hulk BASH!

Carapace supports all major shells. It does not rely on shell-specific functions and even has its own custom file completion. This way it can provide features that not only are consistent across shells. But exceed what they natively provide.

Bash

Take Bash for example. What a nasty little bugger. By default Bash has no support for descriptions. But with some trickery, these can be added.

Then there are more complicated topics. Like Wordbreaks, where you must return a substring of the value to be inserted. And Redirects, which are passed unfiltered to the completion function.

Carapace had to resort to its Lexer to handle these correctly.

You really don't want to write completions for Bash. I went all green on this one.

Fish

Fish is a bit more advanced and has a modal completion menu with descriptions.

It also takes care of correctly escaping special characters in the values. And provides a basic framework for creating completion scripts.

But replicating a complex argument structure isn't easy and subcommand determination is rather optimistic.

function __fish_seen_subcommand_from
    set -l cmd (commandline -pxc)
    set -e cmd[1]
    for i in $cmd
        if contains -- $i $argv
            return 0
        end
    end
    return 1
end

Carapace relies on the work done in spf13/cobra. It is a widely adopted argument parser that does this without guesswork.

Zsh

Between shells, Zsh so far has the most sophisticated framework for writing completions.

But it might be a bit too complex. Learning it takes some effort and there are limitations and traps as well. Coloring for example is done using complicated and slow regex matching. It is thus mostly used for file completion (LS_COLORS) and rarely seen in custom scripts.

Carapace adopted Styles from Elvish. A human-readable abstraction of ANSI Escape Sequences.

See Puking Rainbows for how this works in detail.