Handle empty PROMPT_COMMANDS

z.sh

@@ -217,8 +217,8 @@ 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 $PROMPT_COMMAND | grep -q "_z --add" || {
-   PROMPT_COMMAND="${PROMPT_COMMAND%;}"';_z --add "$(pwd '$_Z_RESOLVE_SYMLINKS' 2>/dev/null)" 2>/dev/null;'
+  grep -q "_z --add" <<< "$PROMPT_COMMAND" || {
+   PROMPT_COMMAND="${PROMPT_COMMAND%;}${PROMPT_COMMAND:+;}"'_z --add "$(pwd '$_Z_RESOLVE_SYMLINKS' 2>/dev/null)" 2>/dev/null;'
   }
  }
 fi