Show full example of getting XDG user directory

[laniakea64]

Per XDG base directory specification, getting an XDG user directory involves a bit more than reading an environment variable:

All paths set in these environment variables must be absolute. If an implementation encounters a relative path in any of these variables it should consider the path invalid and ignore it.
...
$XDG_CONFIG_HOME defines the base directory relative to which user-specific configuration files should be stored. If $XDG_CONFIG_HOME is either not set or empty, a default equal to $HOME/.config should be used.

This PR expands the example to fully show how to get an XDG user directory in a justfile.

[casey]

Interesting!

This does at least slightly make the case that getting the XDG dirs correctly is painful, and should be better supported by just.

I wonder if this is something that users should actually care about. I.e., if the XDG_CONFIG_HOME isn't absolute, the user should really fix that, and the justfile shouldn't try to compensate, even if it is acting according to the spec.

Because if so you could do:

xdg_config_dir := env('XDG_CONFIG_HOME', home_directory() / '.config')

Which is much nicer.

[laniakea64]

I wonder if this is something that users should actually care about. I.e., if the XDG_CONFIG_HOME isn't absolute, the user should really fix that, and the justfile shouldn't try to compensate, even if it is acting according to the spec.

The justfile is likely seeking to match other XDG-directory-using programs.

Testing a few applications in Xubuntu 22.04, they don't seem to care about relative paths. However, they do care about the env vars being set to empty values: most apps ignore the empty env var and use the default, a few apps proceed as if there is no user directory available.

So even when a justfile wants to be relaxed about the XDG specification, getting an XDG user directory still involves more than a single env() call for the most likely case:

xdg_config_dir := if env('XDG_CONFIG_HOME', '') != '' {
  env('XDG_CONFIG_HOME')
} else {
  home_directory() / '.config'
}

[casey]

Gotcha. And that isn't much shorter than the fully correct version which uses =~ '^/', so we might as well do that.