How Bash Completion Works

Re:How Bash Completion Works by vunie on Monday October 07, 2019 @10:13AM (#21181005)

You raise a good point about solvable completion, and one that I agree with. However, automatic completion support doesn't have to cover every use case. It only needs to be good enough. Complex applications can fall back to providing their own completion scripts.

One point I'm not clear on is why you think such a scheme wouldn't be feasible for a large number of commands. Completion wouldn't be invoked until the fist argument (the command's name) is complete just like how bash doesn't scan the system's completion directory until it knows what command is being invoked.

And as for completion for the command's arguments, the command is only ran once to generate the list of completion candidates.

Note that there are projects that already do exactly this like [1] .

[1]:https://github.com/kislyuk/argcomplete

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Re:How Bash Completion Works by shakna on Monday October 07, 2019 @11:04AM (#21181688)

> One point I'm not clear on is why you think such a scheme wouldn't be feasible for a large number of commands. Completion wouldn't be invoked until the fist argument (the command's name) is complete just like how bash doesn't scan the system's completion directory until it knows what command is being invoked.

Simply because some command pipelines are extremely complicated. For example, with ffmpeg, some flags can disable earlier flags, or re-enable them. Completion isn't a simple left-to-right thing. There's other programs that can be similar.

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Re:How Bash Completion Works by vunie on Monday October 07, 2019 @02:45PM (#21184503)

> Completion isn't a simple left-to-right thing.

Sorry, I still don't understand why that's relavent. It's probably my fault for not being clear enough. Please be patient with me as I try to further elaborate:

If the command is in charge of both generating completions and also for parsing arguments, wouldn't it be aware of all inner-dependencies between flags? Wouldn't such awareness make it possible to generate candidates based on previous flags?

I'm not familiar with ffmpeg, but let's assume that there was another similar program called ggmpeg. Let's assume that it accepts the flag '--input-format' and then either the flag '--repeat-video-range' or '--repeat-image-frames', depending on whether the format is an video or an image.

That is, only the following flag sequences are valid:

    ggmpeg --input-format=video/webm --repeat-video-range=.. ...

    ggmpeg --input-format=image/png --repeat-image-frames=.. ...

Let's also assume that ggmpeg signals to the shell that it is responsible for generating its own completions by passing the flag --_completion to it.

Now, if the user types the following command

    ggmpeg --input-format=image/png --repea[CURSOR]

and then presses tab, thereby invoking the shell's auto completion routine, which in turn ultimately executes

    "ggmpeg" "--_completion" "--input-format=image/png" "--repea"

wouldn't ggmpeg have all the necessary information to only suggest '--repeat-image-frames' and not '--repeat-video-range'?

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Re:How Bash Completion Works by shakna on Monday October 07, 2019 @09:55PM (#21187914)

Ok, here's a simpler example that might demonstrate why it can't always be solved.

    ffmpeg -i input.avi -metadata author=shakna output.mp4

What metadata keys are allowed, if at all, actually depends on the output file. If I'd chosen to make an ogv instead, then that metadata flag would become disabled, because it doesn't accept most sets of metadata.

I can't complete:

    ffmpeg -i input.avi -metadata a[TAB]

Because the entire set of CLI commands isn't simple left-to-right solveable.

However, if I do:

    ffmpeg -i input.avi -metadata author=shakna output.mp4 output.ogv

That means I'm outputting one file with metadata, output.mp4, and one file without it, output.ogv. So the flag is conditionally enabled or disabled.

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Re:How Bash Completion Works by vunie on Tuesday October 08, 2019 @08:19PM (#21198695)

I can now see what you meant by completion not being left-to-right solvable. Thanks for taking the time to lay out a clear example.

While completion isn't be solvable in general, surly we can agree that its usefulness (when solvable) merits making it more readily available to users.

Having the shell implementation automatically deduce suggestions from the command itself (when available/possible) would go a long way towards increasing availability in my opinion.

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