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.
