Sometimes the best way to perform complex behavior in a callback is to put that behavior into an external script which can take the job ID as an argument.
The following is an example of a callback that runs for every frame when the job completes. Admittedly, it may be more efficient to run it once and have the external script iterate over every frame, but this is for demonstration purposes.
Use Popen.subprocess in the callback
Please note the use of Popen.subprocess() to run the external script in the callback. This returns immediately and allows the callback to continue running, rather than blocking and waiting for the external script to complete; this ties up the supervisor process for the duration of the external script's execution.
Do not use os.system() to run the external script, as this call will block until the external script exits. When a large number of callbacks tie up supervisor processes at the same time, your supervisor performance will suffer.
Never use sys.exit() in a callback
Do not call sys.exit() at the end of the callback code, this kills the calling supervisor process.
In line 41 of the above script, using "python" assumes that python is in the supervisor user's PATH environment. If not, use the full path to python. If the python script can be invoked on its own (without calling it as an argument to the python executable itself), then you can leave out "python" all together.
Remember, callbacks are executed on the supervisor by the user who is running the supervisor (typically the local system account or "root"). Keep this in mind when reading from or writing to the shared file system.