How to find relative path given two absolute paths?

Find the longest common path (in this case, /a/path/to) and delete it from both absolute paths. That would give:

• /a
• /somewhere/else

Now, replace each path component in the starting path with ../ and prepend the result to the destination path. If you want to go from directory else to directory a, that would give you:

../../a

If you want to go the other way, you'd instead have:

../somewhere/else

I answered a similar question here: Resolving a relative path without referencing the current directory on Windows.

There is no standard function for this. There is a function in vi-like-emacs for this purpose. A quick check of apropos relative shows me few other programs which likely implement this: revpath for example).

It could be done as a string-manipulation (no need to compute working directories):

• start by finding the longest common prefix which ends with a path-separator.
• if there is no common prefix, you are done
• strip the common prefix from (a copy of...) the current and target strings
• replace each directory-name in the current string with ".."
• add that (with a path-separator) in front of the target string
• return that combined string

The "done" in the second step presumes that you want to use a relative path to shorten the result. On the other hand, you might want to use a relative pathname regardless of the length. In that case, just skip the step (the result will be longer, but relative).