RESERVED WORDS
       Reserved words are words that have a special meaning to the shell.  The
       following words are recognized as reserved when unquoted and either the
       first  word of a command (see SHELL GRAMMAR below), the third word of a
       case or select command (only in is valid), or the third word of  a  for
       command (only in and do are valid):

       !  case   coproc   do  done elif else esac fi for function if in select
       then until while { } time [[ ]]