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To get the player ID or see the player list you can use command players. Output would look similar to the following table:

ID	Player name	IP
*0*	CaTGC-Server	*127.0.0.1*
*1*	баба-габа	*123.123.123.123*
*2*	Bluestar	*456.456.456.456*

You can also use the command id <playername> to directly display the player id.
Example command: id bluestar
Output in this case is: bluestar id is *2*

Player IDs are quite often used for multiple commands from setting player skins to mute and kick [More info] commands.

More commands are listed:
Player directed commands: [More info]
Player skin commands: [More info]
Server commands: [More info]
Multiplayer list, and server hosting: [More info]