On macOS 14 and newer, `/usr/libexec/path_helper` supports setting a
`PATH_HELPER_ROOT` environment variable.
With this set, `path_helper` checks `$PATH_HELPER_ROOT/etc/paths` and
`$PATH_HELPER_ROOT/etc/paths.d` in the same way it checks `/etc/paths`
and `/etc/paths.d`.
We can use this to simplify management of the user's `PATH` variable
when they do `brew shellenv`. In particular, if their system supports
it, we delegate setting the `PATH` environment variable to `path_helper`
instead of our own code. We also write a default `etc/paths` file if one
is not already present.
This is nicer because it simplifies management of the user's `PATH`
variable. For example, if a user wants a keg-only formula to be in their
`PATH`, they can simply add the necessary path to `/etc/paths` or
`/etc/paths.d` without having to do something like `brew link --force`
or adding to `PATH` themselves.
When `/usr/libexec/path_helper` is not available, we just fall back to
the existing code.
The main class you should look at is the {Formula} class (and classes linked from there). That's the class that's used to create Homebrew formulae (i.e. package descriptions). Assume anything else you stumble upon is private.
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