Re: TTY management

[Sebastian Gniazdowski <sgniazdowski@gmail.com>]

On Fri, 8 Feb 2019 at 01:16, Bart Schaefer <schaefer@brasslantern.com> wrote:
> First let's talk about "vim | cat".  Programs that expect to use
> capabilities of an interactive terminal nearly always make their own
> determination of whether they can do so, but how this is done varies a
> lot.  Vim in particular works very hard to find a terminal - it will
> look at both stdin and stdout, and if either of those is a terminal it
> will try to interact with that (assuming the descriptor is open for
> both reading and writing).

I think this works differently, and I have proof for this:

% vim | cat > file

doesn't yield a working vim like the `vim | cat' does. This means that
vim doesn't look for a terminal on stderr or /dev/tty, but that
instead it just streams its terminal-codes to the redirected stdout.
If the redirected output is still a terminal, then things work, if not
(e.g. the >file above), then things don't work.

That's why I wanted to just make the isatty() test pass with True.

> Regarding isatty() -- as has been explained, this is a system call.
> It examines an ioctl that is restricted to terminal devices.
(...)
> Terminal devices operate
> using (what to the OS appear to be) hardware-level drivers to mediate
> the bidirectional data flow.

Ahso, pity then this would mean that one cannot simply pretend that a
descriptor is a tty.

> There are actually security-related reasons for this -- think 1980/90s
> technology -- you don't want a man-in-the-middle attack capturing a
> login exchange from a physical terminal in a computer lab, for
> example.  Virtual terminal devices came along later and have to
> address this differently.

Ahso, interesting.

> The upshot of this is that, as PWS said, the core shell is not the
> place to handle this.  If someone wants to find open-source of the
> "script" command and turn it into a module, though, that might be
> worth considering.

What exactly would this module do?

-- 
Sebastian Gniazdowski
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