Re: Screen capturing:

[Nikhil Nair <nn201@cus.cam.ac.uk>]

On Sat, 30 Nov 1996, Luke Davis wrote:

> Ok, I'm running a few consoles, and a terminal . . .
> [...]
> Fine, but that's not what I wanted . . .
> 
> What I want, is to be able to copy what's already on the console/terminal
> screen, to a terminal/file/console.
> 
> Can that be done?

Yes.  Unfortunately, it's very operating system dependent.  What follows
works only on Linux, and requires kernel 1.1.92 or later.

There are some devices called /dev/vcs* and /dev/vcsa*.  The numbers after
those prefices are 0 or none for current VC, or any other number for that
number VC, i.e. /dev/vcsa or /dev/vcsa0 uses the current VC, /dev/vcsa3
uses VC 3.

the vcs versions are the simplest.  If you don't have them, then as root
you could type:

mknod -m o= /dev/vcs$n c 7 $n
chown root.tty /dev/vcs$n
chmod 660 /dev/vcs$n

where $n is between 0 and 63.

The vcs devices contain one character per screen position, going in the
order you'd expect (left to right then top to bottom).

WARNING: these devices may not use the character set you expect!  BEcause
of a quirk in Linux, chances are you won't be able to recover accent signs
correctly, without some manual translation, anyway.

The vcsa devices are more powerful, in that they give screen size, cursor
position and attribute information.  They also take more interpreting, of
course.

To create them, simply add 128 to the minor device number of the
corresponding vcs device, i.e.

mknod -m o= /dev/vcsa$n c 7 $[n+128]
chown root.tty /dev/vcsa$n
chmod 660 /dev/vcsa$n

If you want to know about the file format, and if you can't find the
documentation, ask me.  For some sample code using it, you could look at
the LiveScreen class in scrdev.cc from brltty-1.0.2:
   ftp://sunsite.unc.edu/pub/Linux/system/Access/brltty-1.0.2.tar.gz

I didn't know about setterm until just now, but remember that, unless the
last octal digit of your umask is 7 (which it wouldn't normally be), using
this is a potential security hole, as other users could read the contents
of the console through the contents of that file.  Use that with care!

Hope this helps,

Nikhil.

--
Nikhil Nair
Trinity College, Cambridge, England
Tel.: +44 1223 368353
Email: nn201@cus.cam.ac.uk
       nnair@debian.org