POSIX-compliant implementation of relative_path.sh

[ghost]

This is an alternative implementation of relative_path.sh, authored by @simonair on http://stackoverflow.com/a/14914070
Known limitations: it doesn't clean multiple slashes and doesn't resolve symlinks.
Let's see if it passes Travis CI.

[tkelman]

"based on code from" - and add this file to the exception list in contrib/add_license_to_files.jl

[tkelman]

seems to work on travis. does the existing version handle symlinks or multiple slashes? how does this handle the problem cases from #14097 / #14103?

[tkelman]

there's also a comment on SO that "Doesn't work when the first path ends in / I'm afraid." not sure what to make of that or how general we need this to be.

[ghost]

there's also a comment on SO that "Doesn't work when the first path ends in / I'm afraid."

I did a quick test and it appears to work. Maybe that comment refers to another implementation, there are 3 of them in simon's answer.
Can you test in your machine too?

About multiple slashes, if it's really a concern, we can pipe the output of relpath through sed s#//*#/#g
But I don't know how to deal with symlinks. But if the current version of relative_path.sh doesn't deal with symlinks, then we're fine to merge.

[tkelman]

there were failure cases linked above that we want to be sure we're not reintroducing

[tkelman]

If we decide this version is okay it would also be worth asking @simonair if he would be willing to relicense his code snippet as MIT.

[simonheimlicher]

I'm honored to see my little code snippet be of use. I would be happy to license it under the MIT license – what do I need to do?

[StefanKarpinski]

Saying so is all you need to do. Thanks!

[tkelman]

Yep, thanks much! We can then remove the license special-casing for that file, and give it the standard same MIT banner we give everything else. More convenient to have fewer exceptions.

[ghost]

With licensing solved, we just have to wait for @Keno to give the green-light and it can be merged.

[ghost]

Two simple questions.

• Do we need to deal with multiple slashes?
  If so, the solution is a one line change.
  -echo "$relative"
  +echo "$relative" | sed s#//*#/#g
• Do we need to handle symlinks?
  I don't know. Apparently the current version doesn't?
  If so we should be fine to merge.

Can someone answer these questions?

[Keno]

Did you check that it correctly handles the two corner cases we had issues for above, i.e.
/foo/bar relative to /foo/bar-baz? and
/foo relative to /bar?

[ghost]

Is this what you ask?

$ ./relative_path.sh /foo/bar /foo/bar-baz
../bar-baz
$ ./relative_path.sh /foo /bar
../bar

[Keno]

Yes, those were the two failure cases in the old script that we had to fix. LGTM then.

[ghost]

Do you think it's necessary to deal with multiple slashes?
-echo "$relative"
+echo "$relative" | sed s#//*#/#g

Or can it be merged already?

[tkelman]

Doesn't look like the current script does so. Everywhere we call it should be getting passed off to things that will deal with multiple slashes as expected.