Converting relative path into absolute path?

The most reliable method I’ve come across in unix is readlink -f:

$ readlink -f /x/y/../../a/b/z/../c/d
/a/b/c/d

A couple caveats:

1. This also has the side-effect of resolving all symlinks. This may or may not be desirable, but usually is.
2. readlink will give a blank result if you reference a non-existant directory. If you want to support non-existant paths, use readlink -m instead. Unfortunately this option doesn’t exist on versions of readlink released before ~2005.