Linux script to start java minecraft with a custom username directly without running the launcher

minecraft-start

#!/usr/bin/env bash

# Function to display help message
show_help() {
    cat << EOF
Usage: minecraft-start [options] <username>

Launch Minecraft with the specified username.

Arguments:
  username              The Minecraft username to use

Options:
  -h, --help            Show this help message and exit

Requirements:
  - jq must be installed (sudo apt-get install jq)
  - Minecraft must be installed in the default location (~/.minecraft)

Example:
  minecraft-start MyMinecraftUser

This script launches Minecraft using the latest installed release version.
It automatically detects the appropriate Java runtime and natives directory.
EOF
}

# Check if jq is installed
if ! command -v jq &> /dev/null; then
    echo "Error: jq is not installed. Please install it (e.g., sudo apt-get install jq) and try again." >&2
    exit 1
fi

# Initialize variables
USERNAME=""
SHOW_HELP=false

# Parse arguments
while [[ $# -gt 0 ]]; do
    case $1 in
        -h|--help)
            SHOW_HELP=true
            shift
            ;;
        *)
            if [ -z "$USERNAME" ]; then
                USERNAME="$1"
            else
                echo "Error: Unexpected argument '$1'" >&2
                echo "Use 'minecraft-start --help' for more information." >&2
                exit 1
            fi
            shift
            ;;
    esac
done

# Show help if requested or if no username provided
if $SHOW_HELP; then
    show_help
    exit 0
fi

if [ -z "$USERNAME" ]; then
    echo "Error: Please provide a username as an argument." >&2
    echo "Use 'minecraft-start --help' for more information." >&2
    exit 1
fi

MINECRAFT_DIR="$HOME/.minecraft"

# Function to find the latest release Minecraft version
find_latest_version() {
    local manifest_file="$MINECRAFT_DIR/versions/version_manifest_v2.json"
    if [ ! -f "$manifest_file" ]; then
        echo "Error: version_manifest_v2.json not found!" >&2
        exit 1
    fi
    
    jq -r '.latest.release' "$manifest_file"
}

# Get the latest release version
VERSION=$(find_latest_version)
echo "Using Minecraft version: $VERSION"

# Create the custom log4j2 configuration file
cat > "${MINECRAFT_DIR}/custom_log4j2.xml" << EOL
<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="WARN">
  <Appenders>
    <Console name="Console" target="SYSTEM_OUT">
      <PatternLayout pattern="%d{HH:mm:ss} [%t] %-5level %logger{36} - %msg%n"/>
    </Console>
    <File name="File" fileName="minecraft_custom.log">
      <PatternLayout pattern="%d{HH:mm:ss} [%t] %-5level %logger{36} - %msg%n"/>
    </File>
  </Appenders>
  <Loggers>
    <Root level="info">
      <AppenderRef ref="Console"/>
      <AppenderRef ref="File"/>
    </Root>
  </Loggers>
</Configuration>
EOL

# Dynamically build the classpath
CP=$(find "${MINECRAFT_DIR}/libraries" "${MINECRAFT_DIR}/versions/${VERSION}" -name "*.jar" | tr '\n' ':')

# Remove the trailing colon
CP=${CP%:}

# Find the Java runtime
JAVA_RUNTIME=$(find "$MINECRAFT_DIR/runtime" -name "java" | head -n 1)
if [ -z "$JAVA_RUNTIME" ]; then
    echo "Java runtime not found in Minecraft directory. Using system Java."
    JAVA_RUNTIME="java"
fi

# Find the natives directory
NATIVES_DIR=$(find "${MINECRAFT_DIR}/bin" -type d -exec sh -c 'ls -1 "$0"/*.so 2>/dev/null | grep -q .' {} \; -print | head -n 1)
if [ -z "$NATIVES_DIR" ]; then
    echo "Warning: Natives directory not found. The game may not work correctly." >&2
    NATIVES_DIR="${MINECRAFT_DIR}/bin"
fi

echo "Using natives directory: ${NATIVES_DIR}"

"$JAVA_RUNTIME" \
-Xmx2G -Xms2G \
-Djava.library.path="${NATIVES_DIR}" \
-Dlog4j.configurationFile="${MINECRAFT_DIR}/custom_log4j2.xml" \
-cp "${CP}" \
net.minecraft.client.main.Main \
--username "${USERNAME}" \
--version ${VERSION} \
--gameDir ${MINECRAFT_DIR} \
--assetsDir ${MINECRAFT_DIR}/assets \
--assetIndex 17 \
--accessToken 0 \
--userProperties {} \
--userType legacy

This bash script provides a flexible and user-friendly way to launch Minecraft from the command line. It automatically detects the latest installed Minecraft release version and sets up the necessary environment for launching the game.

Key features:

• Automatically detects and uses the latest installed Minecraft release version
• Supports custom usernames for LAN play
• Dynamically builds the classpath and finds the appropriate Java runtime
• Locates the correct natives directory for proper game functionality
• Implements custom logging for better debugging

Requirements:

• jq must be installed (e.g., sudo apt-get install jq)
• Minecraft must be installed in the default location (~/.minecraft)

Usage:
./minecraft-start [options]

Example:
./minecraft-start MyMinecraftUser