Rewrite using an while loop

This is now able to:
- deal with cascaded symlinks like a->b, b->c
- deal with b being a symlink in a/b/c

realpath.sh

@@ -1,11 +1,39 @@
 #!/bin/sh
 p="${1%/}"
-target=$(readlink "$p")
+p="${p#/}"
 
-case "$target" in
-/*) p="$target";;
-'') ;;
-*) p="${p%/*}/$target";;
-esac
+r=""
 
-printf "%s\n" "$p"|sed -e 's@/[^/]*/\.\.\(/\|$\)@/@g' -e 's|/\.\(/\|$\)|/|g' -e 's|/$||g' -e '/^$/d'
+while [ -n "$p" ]; do
+  case "$p" in
+  */*)
+    seg="${p%%/*}"
+    p="${p#*/}"
+    ;;
+  *)
+    seg="$p"
+    p=""
+    ;;
+  esac
+  case "$seg" in
+  .)
+    r="${r%/*}"
+    continue
+    ;;
+  ..)
+    r="${r%/*/*}"
+    continue
+    ;;
+  esac
+  r="$r/$seg"
+  target="$(readlink "$r")"
+  case "$target" in
+    /*) p="${target}"
+        r="";;
+    '') ;;
+    *) p="${p%/*}/${target}";;
+  esac
+  p="${p#/}"
+done
+
+echo "$r"