diff --git a/z.sh b/z.sh
index 883ed3d..9a0271c 100644
--- a/z.sh
+++ b/z.sh
@@ -217,7 +217,7 @@ elif complete &> /dev/null; then
  complete -o filenames -C '_z --complete "$COMP_LINE"' ${_Z_CMD:-z}
  [ "$_Z_NO_PROMPT_COMMAND" ] || {
   # bash populate directory list. avoid clobbering other PROMPT_COMMANDs.
-  echo $PROMPT_COMMAND | grep -q "_z --add" || {
+  grep -q "_z --add" <<< "$PROMPT_COMMAND" || {
    PROMPT_COMMAND='_z --add "$(pwd '$_Z_RESOLVE_SYMLINKS' 2>/dev/null)" 2>/dev/null;'"$PROMPT_COMMAND"
   }
  }