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Re: Detect whether attached or detached from screen session?

From: Peder Stray
Subject: Re: Detect whether attached or detached from screen session?
Date: Tue, 14 Oct 2008 22:30:14 +0200 (CEST)
User-agent: Alpine 2.00 (LRH 1167 2008-08-23)

On Tue, 14 Oct 2008, Mark Eichin wrote:

Peder Stray <address@hidden> writes:

On Tue, 14 Oct 2008, Trent W. Buck wrote:

"Sven Hazejager"
<address@hidden> writes:

I'm using screen to keep some applications open over a remote SSH
connection that sometimes is killed. What I would like to do, is to
somehow find out whether I am logged in (attached) or disconnected
from the screen session.

Reason is that I want procmail to send incoming mail to my BlackBerry
if I'm disconnected, but NOT do that when I'm SSH-ing into my Pine

I don't know how to achieve that, but perhaps there's a different
approach to the problem -- such as always forwarding all mail to an
IMAP server that you can access from both the blackberry and from

The screen-socket has execute permissions when attached, and none when
detached, like this:


ls -l /tmp/uscreens/S-peder/4598.irc
prwx------ 1 peder peder 0 Oct 14 00:00 /tmp/uscreens/S-peder/4598.irc

ls -l /tmp/uscreens/S-peder/4598.irc
prw------- 1 peder peder 0 Oct 14 07:06 /tmp/uscreens/S-peder/4598.irc

Guess you can test for that i procmail by something like:

*? perl -e 'exit ! grep { (stat $_)[2] & 0100 } </tmp/uscreens/S-peder/*>'

Parsing "screen -ls" output (for "Attached" or not) might be less
sensitive to implementation details, but that's a pretty clever test...

I'm not familiar with procmail, but it might be cleaner to use

 ! test -x /tmp/uscreens/S-peder/*

(to avoid starting perl on every message - I think both of these assume there's only one socket that matches the rule, though it's pretty easy to change if not...)

True, perl is bigger than test, but i guess you mailserver should be real slow for you to actually notice :) The perl test actually checks for any attaced screen, so it should work for cases where there is more than one socket in the dir. Using the test-line as above would break violently in that case since it expect only one argument. Parsing the output from screen -ls would of course be even more expensive that either perl or test.

Of course, all of this expects that screens tmp-dir is available on the mailserver, which it may not be. In that case, it may be easier to make a wrapper for screen that touches a statusfile before starting screen and maybe doing the perl-test after and remove the file if no screens are attached (so you actually can attached more than once to the screen)

  Peder Stray

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