In a makefile, how to get the relative path from one absolute path to another?

Question

An example to illustrate my question:

Top level makefile

rootdir = $(realpath .)
export includedir = $(rootdir)/include
default:
    @$(MAKE) --directory         


        

Answer 1

Didier's answer is the best one, but the following might give you some ideas:

includedir=/a/b/c/d
currentdir=/a/b/e/f/g
up=; while ! expr $includedir : $currentdir >/dev/null; do up=../$up; currentdir=`dirname $currentdir`; done; relative=$up`expr $includedir : $currentdir'/*\(.*\)'`
echo "up=$up  currentdir=$currentdir, relative=$relative"

Sorted!

(no-one said it had to be pretty...)