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.
