Just start Minecraft. Does not download assets.

Prerequisites:

All the stuff you need to play Minecraft

Java, etc...

7-zip (for extracting natives)

Debian: apt install p7zip
Windows: https://www.7-zip.org/download.html - Copy 7z.exe/7z.dll to the same folder as the launcher.

Python 3.x

(3.4 is supported, lower may work)
Debian: apt install python3
Windows: https://www.python.org/downloads/ (use 3.8.x on Win7, 3.4.4 on WinXP)

Distutils

Debian: apt install python3-distutils
Windows: comes pre-installed.

Colorama

(optional, for color)
Debian: apt install python3-colorama
Windows: python -m pip install colorama

Simple usage:

• Extract the .py and .ini to .minecraft
• Edit offline-minecraft-launcher.ini to your liking
• Start offline-minecraft-launcher.py
• Enter data
• Profit

.ini Configuration:

• name: Your player name
• version: The ID of the version you wanna start (Example: b1.8.1)
• console: Whether to show console output (1/0)
• gamePath: Where the game is located
• profileFolder: Whether to make a profile folder (profiles/$name/$version/.minecraft) in gamePath to store user-specific data in, 0 recommended for compatibility (1/0)
• osName: Override your OS in case it isn't detected (windows/linux/macos)
• osVersion: Override your OS version in case it isn't detected
• jvmArch: Override the architecture of your JVM in case it isn't detected (amd64/x86). If your JVM is neither amd64/x86, pick amd64 if it's 64-bit and x86 if it's 32-bit, for the best results
• jvmArguments: The arguments to use to launch your JVM, json formatted - You may need to escape ", if you're changing this value from the console
• java: The path to the java executable. Try not to append a file ending, as w will be appended to it, if console is 0

The script will parse variables within settings values, the variables are sourced from the script itself. $+var$ for global variables, $var$ for local variables. $+sp$ gives you the script's path, for example.

Commandline - Flags:

• -downloadonly: Only download the required files for the specified version, don't extract natives, don't launch the game.
• -verifydata: Verify the file-integrity of libraries, natives and assets
• -nocolor: Disable text color, if preferred or issues occur.

Commandline - Overriding settings:

You can automatize entering data or override settings by using commandline arguments. Each argument works like this: key=value - These act like the settings in the ini, and will override the corresponding settings. However, settings you set via this method will not have the aforementioned variable support enabled, use console variables instead.

Example: offline-minecraft-launcher.py name=fier version=1.16.5 - Launch Minecraft 1.16.5 as fier.

Other:

• Installing Forge