Basically, you setup the standard minecraft service and then create a "socket" i...

Basically, you setup the standard minecraft service and then create a "socket" in systemd to use as stdin for the process(relevant documentation in systemd.socket and systemd.exec).

For me this looks like

-- /etc/systemd/system/minecraft.service --

  [Unit]                                    
  Description="Minecraft server service"
  
  [Service]
  Environment=JAVA_HOME="/usr/lib/jvm/java-22-openjdk"
  WorkingDirectory=/home/steam/minecraft/1.20.6/
  ExecStart=/usr/lib/jvm/java-22-openjdk/bin/java -Xmx4096M -Xms1024M -jar /home/steam/minecraft/1.20.6/server.jar nogui
  User=steam
  Group=steam

  Sockets=minecraft.socket
  StandardInput=socket
  StandardOutput=journal
  StandardError=journal

  [Install]
  WantedBy=multi-user.target

-- /etc/systemd/system/minecraft.socket --

  [Socket]                                  
  ListenFIFO=%t/minecraft.stdin         
  Service=minecraft.service

-------------

What this will do is add a systemd dependency on minecraft.service to start minecraft.socket first(which creates the fifo `/run/minecraft.stdin`) then setup minecraft.service to listen to this socket for it's StandardInput(while leaving stdout and stderr pointing towards the journal).

The service can then be started and set to automatically start on boot(`systemctl daemon-reload && systemctl enable --now minecraft`). While running, data can be written to the socket file with `echo` and redirection(e.x. `echo "help" > /run/minecraft.stdin`), and the output will be visible in the journal(`journalctl -xef --unit minecraft.service`)

If you set stderr/out to go over the socket as well, then you can attach something like `screen` to it and use it like a typical TTY(or `telnet`).

This uses the file `/run/minecraft.stdin` as the socket, but the documentation for systemd.socket shows that this can also be a TCP port to listen for connections(and systemd.service shows using regular files, but then you have to manually set them up).