ttyctl [ -fu ]
       The  -f option freezes the tty (i.e. terminal or terminal emulator),
       and -u unfreezes it.  When the tty is frozen, no changes made to the
       tty settings by external programs will be honored by the shell,  ex-
       cept  for  changes  in the size of the screen; the shell will simply
       reset the settings to their previous values as soon as each  command
       exits  or is suspended.  Thus, stty and similar programs have no ef-
       fect when the tty is frozen.  Freezing the tty does  not  cause  the
       current state to be remembered: instead, it causes future changes to
       the state to be blocked.

       Without options it reports whether the terminal is frozen or not.

       Note that, regardless of whether the tty is frozen or not, the shell
       needs  to  change  the  settings when the line editor starts, so un-
       freezing the tty does not guarantee settings  made  on  the  command
       line  are  preserved.   Strings  of commands run between editing the
       command line will see a consistent tty state.  See  also  the  shell
       variable STTY for a means of initialising the tty before running ex-
       ternal commands and/or freezing the tty around a single command.
