tty?

/*
 *  call-seq:
 *     ios.isatty   => true or false
 *     ios.tty?     => true or false
 *  
 *  Returns <code>true</code> if <em>ios</em> is associated with a
 *  terminal device (tty), <code>false</code> otherwise.
 *     
 *     File.new("testfile").isatty   #=> false
 *     File.new("/dev/tty").isatty   #=> true
 */

static VALUE
rb_io_isatty(io)
    VALUE io;
{
    OpenFile *fptr;

    GetOpenFile(io, fptr);
    if (isatty(fileno(fptr->f)) == 0)
        return Qfalse;
    return Qtrue;
}