How to run a program and get its PID in the background

Question

I have a Minecraft server startup script that looks like this:

#!/bin/bash
cd "$(dirname "$0")"
exec java -Xmx4096M -Xms4096M -jar minecraft_server.jar

How do I get java process's PID while being able to enter input into the Java process?

if I change the exec line to

exec java -Xmx4096M -Xms4096M -jar minecraft_server.jar & echo $! > pid

it won't let me input any text into the Minecraft server java process.

Solution

As you start the java process with exec, its PID will be the same as the one of the shell script. So you can do

echo $$ > pid
exec java ...

Comments

[user1984103]

Have you tried backgrounding the process, grabbing the PID, and then bringing the process back into the foreground?

[sean_m]

Do you still need to communicate with the server process?

[Ivan]

Yes, I need to communicate with the server process (I use screen -x mc-vanilla -X stuff "say doing something blah blahboaweohv $(printf '\r')" to input things into my minecraft server.

[Ivan]

The reason I didn't want to do that was because I have multiple minecraft servers running at the same time. I wanted to get the PID of the process so I could kill that process and not the other minecraft servers.

[sean_m]

Gotcha. We use a lot of Tomcat servers at my work and use the ps aux method but use grep to parse the full path of the jar file to differentiate between server instances.

[Mark K Cowan]

Finally, I've been wondering how to run in foreground and export PID for ages!