How can I detect what other copy of Python script is already running

Question

I have a script. It uses GTK. And I need to know if another copy of scrip starts. If it starts window will extend.

Please, tell me the way I can detect it.

Answer 1

The new copy could search for running copies, fire a SIGUSER signal and trigger a callback in your running process that then handles all the magic.

See the signal library for details and the list of things that can go wrong.

Answer 2

You can use a PID file to determine if the application is already running (just search for "python daemon" on Google to find some working implementations).

If you detected that the program is already running, you can communicate with the running instance using named pipes.

Answer 3

You could use a D-Bus service. Your script would start a new service if none is found running in the current session, and otherwise send a D-Bus message to the running instace (that can send "anything", including strings, lists, dicts).

The GTK-based library libunique (missing Python bindings?) uses this approach in its implementation of "unique" applications.

Answer 4

I've done that using several ways depending upon the scenario

1. In one case my script had to listen on a TCP port. So I'd just see if the port was available it'd mean it is a new copy. This was sufficient for me but in certain cases, if the port is already in use, it might be because some other kind of application is listening on that port. You can use OS calls to find out who is listening on the port or try sending data and checking the response.
2. In another case I used PID file. Just decide a location and a filename, and everytime your script starts, read that file to get a PID. If that PID is running, it means another copy is already there. Otherwise create that file and write your process ID in it. This is pretty simple. If you are using django then you can simply use django's daemonizer: "from django.utils import daemonize". Otherwise you can use this script: http://www.jejik.com/articles/2007/02/a_simple_unix_linux_daemon_in_python/