fetch-configlet: print instructions for installing completions

[ee7]

Closes: #635

---

@glennj I've taken the liberty of taking your commit from glennj@e55d775 to begin this PR, which should be attributed to you.

The first commit in this PR is just that commit, but rebased on main. Feel free to push to this PR's branch.

This PR is a follow-up to #629, where these changes were originally.

[ee7]

It's cool that we try to detect a user's shell. But I have these questions:

1. Is there a way to reliably detect the user's current shell when it is different from their default shell?
2. And if not, what do we think about "don't try to detect the user's shell, and just print the instructions for every supported shell"?

For example, a user might use fish as an interactive shell only, as discussed on the Arch wiki. Right now, for such a user,

getent passwd "${USER}" | cut -d: -f7

may return

/bin/bash

in which case the user sees instructions for a different shell. Or it may return, say:

/bin/zsh

in which case the user wouldn't see any instructions for installing completions, even though they're available.

Related recent post of Glenn's on unix.stackexchange.com:

$SHELL is the user's login shell. My login shell is fish: if I unset the SHELL variable and then launch bash, SHELL is /path/to/fish

[ErikSchierboom]

And if not, what do we think about "don't try to detect the user's shell, and just print the instructions for every supported shell"?

If there isn't a reliable, fool-proof way to detect the user's shell, I think we should print the instructions for every supported shell.

[glennj]

1. Is there a way to reliably detect the user's current shell when it is different from their default shell?

No, unless we parse ps to find the shell in the process hierarchy.

2. And if not, what do we think about "don't try to detect the user's shell, and just print the instructions for every supported shell"?

I think this was my original suggestion: let the user tell you what shell they want:

configlet completions -s fish

Perhaps if the -s option is not provided, list the available shells for which we have completions.

[ee7]

If there isn't a reliable, fool-proof way to detect the user's shell, I think we should print the instructions for every supported shell.

Let's do that.

No, unless we parse ps to find the shell in the process hierarchy.

Hah. I think that'd be trying to be much too clever.

I think this was my original suggestion: let the user tell you what shell they want:

configlet completions -s fish

We'll have this in the next configlet release (but with the subcommand called completion, not completions - see d10dcd6).

The help message will contain

Usage:
  configlet [global-options] <command> [command-options]

Commands:
  completion  Output a completion script for a given shell
  fmt         Format the exercise '.meta/config.json' files
  generate    Generate Concept Exercise 'introduction.md' files from 'introduction.md.tpl' files
  info        Print some information about the track
  lint        Check the track configuration for correctness
  sync        Check or update Practice Exercise docs, metadata, and tests from 'problem-specifications'.
              Check or populate missing 'files' values for Concept/Practice Exercises from the track 'config.json'.
  uuid        Output new (version 4) UUIDs, suitable for the value of a 'uuid' key

Options for completion:
  -s, --shell <shell>          Choose the shell type (required)
                               Allowed values: b[ash], f[ish], z[sh]

[...]

So the main remaining step is to make fetch-configlet explain how to install completions. I'd like for us to explain how to install the bash and zsh completions on a not-just-for-this-session basis.

I think this PR doesn't block the configlet release, though.