A lightweight Bash script to manage and switch Java environment versions.

javaenv.bash

# Function to manage Java environments
function javaenv() {
    local jvm_dir="/usr/lib/jvm"
    local config_file="$HOME/.config/javaenv"
    local action="$1"

    case "$action" in
        "list")
            # List directory names under /usr/lib/jvm that contain a java executable
            if [ ! -d "$jvm_dir" ]; then
                echo "Directory $jvm_dir does not exist."
                return 1
            fi
            
            echo "Available Java environments:"
            echo "  system (OS Default)"
            for dir in "$jvm_dir"/*; do
                if [ -d "$dir" ] && [ -x "$dir/bin/java" ]; then
                    echo "  $(basename "$dir")"
                fi
            done
            ;;

        "load")
            local env_name="$2"
            
            # If no environment name is specified, fallback to 'default'
            if [ -z "$env_name" ]; then
                env_name="default"
            fi

            # Handle 'default' alias by reading from the config file
            if [ "$env_name" = "default" ]; then
                if [ -f "$config_file" ]; then
                    env_name=$(cat "$config_file")
                else
                    env_name="system"
                fi
            fi

            # Handle 'system' environment explicitly
            if [ "$env_name" = "system" ]; then
                if [ -n "$JAVA_HOME" ]; then
                    unset JAVA_HOME
                fi
                export PATH="$(echo "$PATH" | sed -E -e "s|/usr/lib/jvm/[^/]+/bin:?||g" -e "s|:$||")"
                echo "Loaded system default Java environment."
                java -version
                return 0
            fi

            local target_dir="$jvm_dir/$env_name"

            # Validate the target directory and java executable
            if [ -d "$target_dir" ] && [ -x "$target_dir/bin/java" ]; then
                export JAVA_HOME="$target_dir"
                # Remove existing /usr/lib/jvm paths first, then prepend new path
                export PATH="$JAVA_HOME/bin:$(echo "$PATH" | sed -E "s|/usr/lib/jvm/[^/]+/bin:?||g")"
                
                echo "Loaded environment: $env_name"
                java -version
            else
                echo "Error: Invalid Java environment '$env_name'"
                echo "Use 'javaenv list' to see available environments."
                return 1
            fi
            ;;

        "unload")
            # Unset JAVA_HOME and remove any /usr/lib/jvm paths from PATH
            if [ -n "$JAVA_HOME" ]; then
                unset JAVA_HOME
                echo "Unset JAVA_HOME."
            fi
            
            # Clean up PATH
            export PATH="$(echo "$PATH" | sed -E -e "s|/usr/lib/jvm/[^/]+/bin:?||g" -e "s|:$||")"
            echo "Removed Java from PATH."
            ;;

        "default")
            local target_env="$2"

            # If no argument is provided, display the currently configured default
            if [ -z "$target_env" ]; then
                if [ -f "$config_file" ]; then
                    echo "Current default: $(cat "$config_file")"
                else
                    echo "Current default: system"
                fi
                return 0
            fi

            # Handle 'current' keyword to save the currently active session environment
            if [ "$target_env" = "current" ]; then
                if [ -n "$JAVA_HOME" ]; then
                    target_env=$(basename "$JAVA_HOME")
                else
                    target_env="system"
                fi
            fi

            # Handle 'system' case: do not save to config (remove config if it exists)
            if [ "$target_env" = "system" ]; then
                if [ -f "$config_file" ]; then
                    rm -f "$config_file"
                fi
                echo "Default environment set to 'system'. Configuration file removed/not updated."
                return 0
            fi

            # Validate if the specified environment exists before saving
            if [ ! -d "$jvm_dir/$target_env" ] || [ ! -x "$jvm_dir/$target_env/bin/java" ]; then
                echo "Error: '$target_env' is not a valid Java environment."
                return 1
            fi

            # Ensure config directory exists and save
            mkdir -p "$(dirname "$config_file")"
            echo "$target_env" > "$config_file"
            echo "Saved '$target_env' as the default Java environment."
            ;;

        *)
            echo "Usage:"
            echo "  javaenv list          - List available Java environments"
            echo "  javaenv load [name]   - Load specific Java environment (defaults to 'default')"
            echo "  javaenv unload        - Unload Java environment and clear variables"
            echo "  javaenv default       - Show current default environment"
            echo "  javaenv default <name>- Save specific environment (or 'current' / 'system') as default"
            return 1
            ;;
    esac
}

# Automatically load the default Java environment on shell startup if configured
if [ -f "$HOME/.config/javaenv" ]; then
    javaenv load default > /dev/null 2>&1
fi