ttyctl [ -fu ]
       The  -f option freezes the tty (i.e. terminal or terminal emula-
       tor), 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 sim-
       ilar  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.
