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, except 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 effect 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
       unfreezing 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
       external commands and/or freezing the tty around a single command.
