I have a python script that can run for long time in the background, and am trying to find a way of getting a status update from it. Basically we’re considering to send it a SIGUSR1 signal, and then have it report back a status update.
Catching the signal in Python is not the issue, lots of information about that.
But how to get back information to the process initiating the signal? It seems that there is no way to figure out the pid of the initiating process by the receiving process, which could provide a way to send information back. A single reply message is enough here (in the tune of ‘busy uploading; at 55% now; will finish at such a time’); a continuing update would be fantastic but not necessary.
What I’ve come up with is to write this data to a temporary file with predetermined name – has the issue of leaving stale files behind, and need some kind of clean-up routine then. But that sounds like a hack. Is there anything better available?
The way the running process is signalled doesn’t matter, it doesn’t have to be kill -SIGUSR1 pid. Any way to communicate with it would do. As long as the communication can be initiated from a new process that’s started after the main process runs, possibly running under as different user.
Signals are not designed to be general inter-process communication mechanisms that allow for passing data. They can’t do much more than provide a notification. What the target process does in response can be fairly general (generating output to a particular file that the sender then knows to go look at, for example), but passing data directly back to the sender would require a different mechanism like a pipe, shared memory, message queue, etc. Also note that, in general, a process receiving a signal can’t really determine who sent the signal, so it wouldn’t know where to send a response anyway.