[NTLUG:Discuss] More shell scripting madness

[Chris Cox]

. Daniel wrote:
> I know it's hard to see, but I'm actually starting to learn this stuff 
> little by little.
> 
> Here's the purpose:
> 
> I upgraded the processor and video in my laptop.  Now it gets warmer than I 
> like, especially during game play, which slows down when the processor and 
> video heat up.  While I intend to improve the actual heatsink materials 
> (Copper is surprisingly hard to get in the form I need it... and EXPENSIVE) 
> I would like to keep the processor fans running.
> 
> As it turns out, the system is controlling the fans by itself based on 
> certain criteria.  But there is also a handy little utility to simply turn 
> on the fan!  It's called "i8kfan."  Run i8kfan without parameters and it 
> will simply return with two numbers indicating the mode of the fans at that 
> moment.  The values are 0, 1 and 2.  So if both fans are off, it will 
> return "0 0" and if both are on high speed it will return "2 2".
> 
> So I wrote a loop that checks the fan status every two seconds and if it's 
> not "2 2" it will set the fans to "2 2" and go back to sleep.  I then set 
> up an icon on my panel to start the process when I want to play a game so I 
> can prolong my "cool time."
> 
> Here's the problem though:  Gnome shortcuts no longer seem to want to 
> launch processes in terminal.  In earlier versions, there was a check box 
> to select this option.  It was a good option to use when you were debugging 
> because you could see error messages streaming by or whatever.  Now it's 
> gone and I can't run the script in a terminal window expect by hand... 
> which, of course, I don't wanna do.
> 
> So here's the script as simple as it is:
> 
> "i8kfanloop.sh"
> -----
> #!/bin/bash
> 
> loop=1
> 
> while [ loop ]
> do
>   status=`i8kfan`
>   if [ "$status" != "2 2" ]
>   then
>      i8kfan 2 2
>   fi
>   sleep 2
> done
> -----
> 
> That's some pretty simple bash code right?
> 
> Here's what I'd like to add to it.  I'd like to add some code that will 
> detect if a previous instance of this script is currently running and if it 
> is, kill it and exit.  If it's not running, just go into the loop.  So the 
> result would be, if I click the icon once, it runs the loop.  If I click 
> the icon again, it will stop the loop.
> 
> I imagine some code that will get the shell script's process ID number and 
> stores it somewhere, say "/tmp/$username_i8kfanloop.pid" or something like 
> that.  Then at the beginning of the script, it will look for said file and 
> compare the process ID recorded in the file with the process listing to see 
> if the process ID is there and actually the same script.  
> 
> If the filename doesn't exist, create the PID file and enter the loop.  
> If the filename exists but the process doesn't, it should erase the file, 
> create a new PID file and enter the loop.  
> If the filename exists and the process exists but the PID doesn't match up 
> with the script name, then erase the file, create a new PID file and enter 
> the loop.
> 
> I feel confident that if I messed with this for a few hours, I'd be able to 
> cobble up something that works.  But before I spend my work day on my 
> personal stuff, I'm willing to bet someone already has or knows about some 
> shell code out there that does this already.  And I'm hoping that someone 
> will be kind and generous enough to paste their code into a reply so that I 
> could learn and adapt my skills further. :)
> 

Consider a front end script called mydaemon-start.sh:

#!/bin/sh
if pid=$(pgrep mydaemon.sh); then
        kill -9 $pid
fi

exec /usr/local/bin/mydaemon.sh &

Where your script is what is inside of mydaemon.sh

You can always add a mydaemon-kill.sh that just does
the first part, or make the frontend understand
parameters so you can use the same script to 'start'
and 'stop' mydaemon.sh