POSIX-compliant implementation of relative_path.sh

contrib/relative_path.sh

@@ -1,49 +1,31 @@
-#!/usr/bin/env bash
+#!/bin/sh
 # This file is a part of Julia. License is MIT: http://julialang.org/license
 
 # both $1 and $2 are absolute paths beginning with /
 # returns relative path to $2/$target from $1/$source
 
-# Source copied from http://stackoverflow.com/a/12498485/230778
-
-source=$1
-target=$2
-
-if [[ ! "$source" == /* ]] || [[ ! "$target" == /* ]]; then
-    echo "ERROR: paths must be absolute paths, they must start with a forward slash!"
-    exit 1
-fi
-
-common_part=$source # for now
-result="" # for now
-
-while [[ "${target#$common_part/}" == "${target}" ]] && [[ "$common_part" != "/" ]]; do
-    # no match, means that candidate common part is not correct
-    # go up one level (reduce common part)
-    common_part="$(dirname $common_part)"
-    # and record that we went back, with correct / handling
-    if [[ -z $result ]]; then
-        result=".."
-    else
-        result="../$result"
-    fi
-done
-
-if [[ "$common_part" == "/" ]]; then
-    # special case for root (no common path)
-    result="$result/"
-fi
-
-# since we now have identified the common part,
-# compute the non-common part
-forward_part="${target#$common_part}"
-
-# and now stick all parts together
-if [[ -n $result ]] && [[ -n $forward_part ]]; then
-    result="$result$forward_part"
-elif [[ -n $forward_part ]]; then
-    # extra slash removal
-    result="${forward_part:1}"
-fi
-
-echo $result
+relpath () {
+    [ $# -ge 1 ] && [ $# -le 2 ] || return 1
+    current="${2:+"$1"}"
+    target="${2:-"$1"}"
+    [ "$target" != . ] || target=/
+    target="/${target##/}"
+    [ "$current" != . ] || current=/
+    current="${current:="/"}"
+    current="/${current##/}"
+    appendix="${target##/}"
+    relative=''
+    while appendix="${target#"$current"/}"
+        [ "$current" != '/' ] && [ "$appendix" = "$target" ]; do
+        if [ "$current" = "$appendix" ]; then
+            relative="${relative:-.}"
+            echo "${relative#/}"
+            return 0
+        fi
+        current="${current%/*}"
+        relative="$relative${relative:+/}.."
+    done
+    relative="$relative${relative:+${appendix:+/}}${appendix#/}"
+    echo "$relative"
+}
+relpath "$@"