How do I change player name in minecraft multiplayer in offline mode in Linux?

I don't have access to Minecraft on Linux at the moment, but that command line looks suspicious. Give this a try:

java -Xms512m -Xmx1024m -cp "$HOME/.minecraft/bin/*" -Djava.library.path="$HOME/.minecraft/bin/natives" net.minecraft.client.Minecraft "$USER"

If it still isn't working for you, knowing what does happen (error messages, lets you in but has wrong name, …) would be helpful for further troubleshooting. :-)

Edited: Work on Ubuntu 12.04 x32

*P.S.: after "$USER" you can also add "$PASSWORD" "$SERVER" *

TL;DR: Use a launcher which can modify the name. Or use some long-winded terminal command to manipulate it.

Long answer:

The offline mode in the Minecraft server does not require players to authenticate with the Minecraft account database. However, when you:

• Directly run the minecraft.jar file
• Use the "Play Offline" function of the original Minecraft launcher.

Your in-game name would be "Player".

The only way to change this is to:

• Use an un-offical launcher which can change the name (eg. MinecraftSP)
• The command line
• Editing the source code and recompiling it

When you do any of these methods successfully, your in-game name will then be different, without requiring to authenticate with the Minecraft account database for verification. This verification is done with servers with online-mode=true, thus people who use these methods are unable to join those servers.

Please note that when you use any of these methods in conjunction with the Minecraft files (minecraft.jar and others), it is illegal if you did not buy the game.

Although the title only mentions linux, the OP mentions Windows, and searching brought me here.

I thought I might share my Windows offline name changing solution which I use for testing my plugins when I need multiple clients connected all under different names, naturally.

@SET /P IGN=IGN=
@java -Xms512m -Xmx512m -cp "%APPDATA%/.minecraft/bin/*" -Djava.library.path="%APPDATA%/.minecraft/bin/natives" net.minecraft.client.Minecraft %IGN%

I put the above into a file named Offline.bat When I double click that file, it will open a command window and prompt you to enter the In-Game Name (IGN) you wish to use. Simply enter the name you want to use in and press enter to launch Minecraft in offline mode with that name.