[Discuss] cron notifications to GUI front end

[Derek Martin]

I neglected to mention that the cron script also needs to set DISPLAY,
most likely to ":0", and then export it.  That's probably not a
surprise to any veteran X users though...

On Fri, Mar 29, 2019 at 03:02:42PM -0500, Derek Martin wrote:
> On Thu, Mar 28, 2019 at 11:41:09AM -0400, Rich Pieri wrote:
> > On Thu, 28 Mar 2019 11:23:26 -0400
> > Jerry Feldman <gaf.linux at gmail.com> wrote:
> > 
> > > One idea I had was to write a small GUI that would cancel any
> > > currently scheduled shutdown and reschedule and notify the user when
> > > the shutdown is imminent. If the user logs off, the shutdown would
> > > still be in effect.
> > 
> > I don't think that's going to work the way you expect. One big problem
> > is that a process running from cron can't directly access your X server
> > so it can't send GUI notices.
> 
> That's not actually true, though it will take a little bit of hackery
> to make it work.
> 
> Modern Linux systems depend on MIT-MAGIC-COOKIE-1 authentication to
> allow access to the X display.  You control this through the xauth
> command.  Once you're logged in, you can see the cookies you have,
> like so:
> 
> $ xauth list
> myhostname/unix:0  MIT-MAGIC-COOKIE-1 1234567890abcdef1234567890abcdef
> 
> [You may have more than one, but chances are the /unix:0 entry is the
> one you want.]
> 
> So when you log in, e.g. in your .xsession file or what have you, dump
> this to a file, say $HOME/tmp/xauth, and make sure it's only readable
> by you (and of course root).  Then change your cron job to a script
> that adds that line to roots xauth, and runs your gui thingy.  You can
> do that with something like:
> 
> xauth add `cat ~myuser/tmp/xauth | grep "unix:0"`
> 
> FWIW I tested this by logging into a VC on my desktop as root, setting
> up xauth essentially as above, and then running xeyes.  Worked like a
> charm. :)
> 
> Then, in your cron script, if the gui thingy fails (e.g. because
> there's no display to connect to, or auth failed, etc.), execute the
> shutdown command.  Otherwise don't.  Obviously the gui widget thingy
> needs to exit with a corresponding exit code so you can test it
> appropriately...
> 
> Personally, I would never do this, but if I were to want to do this, I
> kinda like this approach--though I think Rich's is fairly equivalent.
> I suspect there's not a ton of difference in the code you'd need to
> write for either solution, though my guess is the client-server
> solution would require slightly more, depending on the language and
> GUI toolkit you chose.  I would definitely use Python, but then I'd
> need to re-research GUI options for the easiest.
> 
> -- 
> Derek D. Martin    http://www.pizzashack.org/   GPG Key ID: 0xDFBEAD02
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Derek D. Martin    http://www.pizzashack.org/   GPG Key ID: 0xDFBEAD02
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