Question

I created a client-server application and now I would like to deploy it.

While development process I started the server on a terminal and when I wanted to stop it I just had to type “Ctrl-C”.

Now want to be able to start it in background and stop it when I want by just typing:

/etc/init.d/my_service {stop|stop}

I know how to do an initscript, but the problem is how to actually stop the process ?

I first thought to retrieve the PID with something like:

ps aux | grep "my_service"

Then I found a better idea, still with the PID: Storing it on a file in order to retrieve it when trying to stop the service.

Definitely too dirty and unsafe, I eventually thought about using sockets to enable the “stop” process to tell the actual process to shut down.

I would like to know how this is usually done ? Or rather what is the best way to do it ?

I checked some of the files in the init.d and some of them use PID files but with a particular command “start-stop-daemon”. I am a bit suspicious about this method which seems unsafe to me.

Answer 1

If you have a utility like start-stop-daemon available, use it.

start-stop-daemon is flexible and can use 4 different methods to find the process ID of the running service. It uses this information (1) to avoid starting a second copy of the same service when starting, and (2) to determine which process ID to kill when stopping the service.

• --pidfile: Check whether a process has created the file pid-file.
• --exec: Check for processes that are instances of this executable
• --name: Check for processes with the name process-name
• --user: Check for processes owned by the user specified by username or uid.

The best one to use in general is probably --pidfile. The others are mainly intended to be used in case the service does not create a PID file. --exec has the disadvantage that you cannot distinguish between two different services implemented by the same program (i.e. two copies of the same service). This disadvantage would typically apply to --name also, and, additionally, --name has a chance of matching an unrelated process that happens to share the same name. --user might be useful if your service runs under a dedicated user ID which is used by nothing else. So use --pidfile if you can.

For extra safety, the options can be combined. For example, you can use --pidfile and --exec together. This way, you can identify the process using the PID file, but don’t trust it if the PID found in the PID file belongs to a process that is using the wrong executable (it’s a stale/invalid PID file).

I have used the option names provided by start-stop-daemon to discuss the different possibilities, but you need not use start-stop-daemon: the discussion applies just as well if you use another utility or do the matching manually.