Use realpath of scripts to determine relative locations

[andrew-christianson]

Just using paths relative to ${BASH_SOURCE} fails if the scripts are
symlinked to a location, as that variable reports the path at which
the script is invoked, not where it actually resides. This commit
implements a simple version of the realpath functionality in order to
determine the path to the more comprehensive realpath script

This should resolve my issue #307

[j9ac9k]

I think this issue and associated PR would fix this downstream issue: Homebrew/homebrew-core#40810

[bosr]

Is there any news? This prevents the use of homebrew-installed pyenv-virtualenv for managing miniconda3. Thanks!

[frankhinek]

I have the same issue with Homebrew-installed pyenv-virtual not working for managing miniconda3. The issue with this PR, at least for me, is that it depends on a user having installed GNU coreutils. For other reasons I don't have this package installed and the readlink version at /usr/bin/readlink doesn't support the -f parameter so the suggested fix fails with:

/usr/bin/readlink: illegal option -- f
usage: readlink [-n] [file ...]

[j9ac9k]

Having coreutils installed I would suspect is fine on its own, however if we mandate they be added in the PATH with their non "g" prefix, this causes all sorts of other issues.

Here is a stack overflow post for getting equivalent behavior of readlink -f on macOS

https://stackoverflow.com/questions/1055671/how-can-i-get-the-behavior-of-gnus-readlink-f-on-a-mac

[Arbitrageur]

Please review and merge PR from @andrew-christianson

[andrew-christianson]

@frankhinek at least in my understanding, pyenv expects macOS users to have coretutils installed but not linked, as there are other points in the main project where the same READLINK=$(type ...) construction is used.

@j9ac9k I'm not good enough shell to determine whether the linked answer is sufficiently portable for this project - I'd rather defer to a system- or homebrew-provided readlink instead of re-implementing.

@yyuu could you merge this? I've been my version since this PR, and haven't had issues with the change.