[launchd-dev] Restrict so only one instance runs

[Crawford Kyle]

On Aug 27, 2008, at 3:29 PM, Dave Zarzycki wrote:

>
> On Aug 27, 2008, at 11:28 AM, Crawford Kyle wrote:
>
>> Hello,
>>
>> I often need to schedule scripts to run at an interval, but I don't
>> know how long that script will take to complete and I don't want the
>> script to run again at its normal interval unless the script isn't
>> running.
>>
>> I've done this with pid files and grepping through ps lists to exit
>> the script if another instance is running, but I was wondering if
>> there is something built into launchd to handle this.
>>
>> I tried combining LaunchOnlyOnce with StartInterval, but with
>> LaunchOnlyOnce, it seems that the job gets unloaded as soon as it
>> runs, so the interval becomes pretty meaningless.
>>
>> Is there a way to do this with launchd?  Should there be a way to do
>> this with launchd?
>
>
> What you want is implicit with launchd. LaunchOnlyOnce will  
> eventually get renamed to "HopefullyNeverExits" Just setup a  
> StartInterval and you're set. If the job is still running, launchd  
> will not start a second copy.

Thanks Dave. I must be missing something.  Here is my job plist.  The  
shell script is just a test that logs to system log.

I see a message in system log only once though. And the job no longer  
shows in launchctl list after it runs.

I tried this with and without OnDemand.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd 
">
<plist version="1.0">
<dict>
     <key>Label</key>
     <string>com.company.test</string>
     <key>ProgramArguments</key>
     <array>
         <string>/etc/testscript.sh</string>
     </array>
     <key>StartInterval</key>
     <integer>30</integer>
     <key>LaunchOnlyOnce</key>
     <true/>
</dict>
</plist>