Function to get relative path give two absolute paths: a base and a target

relativepath.zsh

#!/bin/zsh

## This function exists because on a Mac the `realpath` program doesn't have a `--relative-to` option like GNU does.
## This file can both be used as a standalone command and as a sourced file to use in other scripts.

relativepath()
{
	local RelativeTo=$1 Path=$2

	if [[ $RelativeTo != /* || $Path != /* ]]; then
		return 1
	fi

	local -a RelativeToComponents=(${(@s:/:)RelativeTo})
	local -a PathComponents=(${(@s:/:)Path})

	while [[ ${#RelativeToComponents} -gt 0 && ${#PathComponents} -gt 0 ]]; do
		if [[ $RelativeToComponents[1] != $PathComponents[1] ]]; then
			break
		fi
		shift RelativeToComponents
		shift PathComponents
	done

	RelativeToComponents=(${RelativeToComponents//*/..})

	local Result=${(j:/:)${RelativeToComponents:-.}}/${(j:/:)PathComponents}

	echo $Result
}

if [[ $zsh_eval_context[-1] == toplevel ]]; then
	relativepath $@
fi