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The situation is as follows.

I have a multiuser desktop machine with Debian Linux 6.0 and an ATI videocard with one monitor connected. I have root access to it. There can be several KDE sessions started, like this:

$ w
 21:51:30 up ? days,  4:22,  ? users,  load average: 1.72, 1.68, 1.67
USER     TTY      FROM              LOGIN@   IDLE   JCPU   PCPU WHAT
u1       pts/0    :0               Sat18    4days  0.00s 11.68s kdeinit4: kded4 [kdeinit]                      
u2       pts/5    :1               Mon17    2days  0.00s  6.65s kdeinit4: kded4 [kdeinit]

So, two virtual terminals are in use, tty7 and tty8:

$ ps aux|grep /usr/bin/[X]
root      2944  3.1 12.4 670040 1019904 tty7   Ss+  Aug27 187:52 /usr/bin/X :0 vt7 -br -nolisten tcp -auth /var/run/xauth/A:0-??????
root      5507  0.9  3.7 425136 309676 tty8    Ss+  Aug29  29:38 /usr/bin/X :1 vt8 -br -nolisten tcp -auth /var/run/xauth/A:1-??????

But of course only one of them is active at any given moment, i.e. displayed on the monitor. Someone sitting at the keyboard can switch between them using Ctrl+Alt+F[78]

So, I connect via ssh from a remote host. I need to know which X DISPLAY is active now. Is it possible? I have googled all over the place and can't find the answer.

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spinics.net/lists/linux-api/msg01144.html seems to be a rejected patch.. – kagali-san Aug 31 '11 at 16:12
@mhambra I'm not quite familiar with Linux kernel patch acceptance process, but how can we determine that it's really rejected, and not just forgotten? – Double-V Aug 31 '11 at 16:48
@mhambra Anyway, thanks a lot. Maybe I'll have to rebuild my kernel to accomplish this task, but I would be very-very glad to find another solution... Maybe some coding, but without touching the kernel. – Double-V Aug 31 '11 at 16:50
just use fgconsole, as Pablo said; checked with ssh login to my box, it works – kagali-san Aug 31 '11 at 18:04
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1 Answer

up vote 2 down vote accepted

fgconsole is what you are looking for

http://linux.die.net/man/1/fgconsole

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Sorry, but it does not do what I want. It just prints Couldnt get a file descriptor referring to the console. Any ideas of what might be the reason? – Double-V Sep 1 '11 at 14:21
I was stupid enough to not use sudo :) With sudo, it works like a charm. Thanks a lot, problem solved! – Double-V Sep 1 '11 at 14:29
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