process A running my code (in Python unless i need to use C) starts process B (in a directory process A created) which execs some other binary executable. process B opens a file for output in the current directory that will always be a new file. i want to have process A learn about this event and learn the name of that file ASAP. is there a better way to do this than repeatedly getting the change time of this directory, detecting a change, and getting a list of file names to see what's new? my intent is to watch the growth of this file, looking for a period of no growth or the exit of process B. process A will kill process B if this pause is detected and start another instance and track that one. each time a file is complete, it will be moved to another directory where another process archives it (and all the other instances of A/B pairs that will run concurrently). the various pairs will be downloading different information (what program B does for just one case). the problem i am having is that process B stalls frequently. i am trying to work around that. i will need to work good timing values.

inotify - seems to be python bindings.

process A stays running my Python code. however, process B execs a binary i did not develop. i might be able to get its source code and mod it to send notification. can inotify just do this on its own based on the system calls the tracked process performs? i do intend to run a separate process A for each of these cases so i do not need to make process A do multithreading so there will be as many process As as there are separate cases. so if inotify cannot do complex multi-waiting for all the various cases, that's OK. i'll go try to find it after this posts.

looks like process B will need to be written in Python to do this. but it comes in binary. specifically, the program to run in process B is "ffmpeg" being given the URL of a video to download. it uses the file name given by the server and my process A code does not know what that name is. once it is aware the file is started, it will rescan the names in the directory to see what is new (it is probably the only file). i just want a faster way to know the file has been created rather that doing lstat() on the directory every 1/4 second.

You can do that with incron and have it trigger a script when new files are created in a specific directory.

Edit: As per the next comment:
https://linux.die.net/man/5/incrontab
https://linux.die.net/man/1/incrontab

Say what ?.Go back and read the inotify doco then. You set a watch on the target directory and wait to be told what has just happened. No-one cares what the perpetrator is - the kernel will see the event and tell you.

i was expecting the watched app to send the notification. i thought of Python because that's what i have been doing things with. i was thinking about a different notification tool.

OK. looks like the right way to do this. thanks.