gistfile1.txt

       set [--abefhkmnptuvxBCHP] [-o option] [arg ...]
--8<-------
              -x      After expanding each simple command, for command, case command, select
                      command, or arithmetic for command, display the expanded value of PS4,
                      followed by the command and its expanded arguments or associated  word
                      list.
--8<-------
              The options are off by default unless otherwise noted.  Using + rather than  -
              causes  these  options to be turned off.  The options can also be specified as
              arguments to an invocation of the shell.  The current set of  options  may  be
              found  in  $-.   The  return status is always true unless an invalid option is
              encountered.
--8<-------
       PS4    The  value  of this parameter is expanded as with PS1 and the value is printed
              before each command bash displays during an execution trace.  The first  char-
              acter  of PS4 is replicated multiple times, as necessary, to indicate multiple
              levels of indirection.  The default is ``+ ''.