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This change adds support for generating a bash completion script. If
sourced, the shell will provide tab completions for the program's
arguments.
There are two possible approaches provided by clap for going about
generating shell completion functionality: either at build time, by
separately generating the clap parsers out-of-band, or at run time, as
an option to the main program itself. We are generally not too much in
favor of a run time approach, as it means less inspectability at
installation time and more overhead in the form of code crammed into the
main binary.
Hence, with this change we take the "build time" approach. Clap
recommends hooking the generation up in build.rs, but this seems like an
inflexible choice. For one, that is because it would mean
unconditionally generating this file or using some user-unfriendly
environment variable based approach for making the process conditional.
But there is also the fact that specifying the command for which to
generate the script should likely be configurable. That is a limitation
of the completion script that clap generates (see
https://github.com/clap-rs/clap/issues/1764).
In our version we provide a utility program that emits the completion
script to standard output, accepting regular command line options
itself. In doing so we allow for installation time generation of the
completion script or installation of the utility itself, the output of
which could be sourced on demand -- depending on the user's preference.
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