Alternative management for Java

alternative.bash

#!/usr/bin/env bash

# Inspired by:
# https://stackoverflow.com/a/53902806/4905625

set -e

usage () {
    echo "Usage: $0 {register|install} JAVA_DIR [PRIORITY]"
    echo
    echo "Register or activate an alternative Java version"
    echo
    echo "NOTE: Probably requires superuser privileges - run as sudo"
}

if [ -z "$1" ] || [ "$1" = "-h" ] || [ "$1" = "--help" ]  || [ -z "$2" ] || [ ! -z "$4" ]
then
    usage
fi

DIRECTIVE="$1"
JAVA_DIR="$2"

case "$DIRECTIVE" in
    register)
        PRIORITY="$3"
        if [ -z "$PRIORITY" ]; then usage; echo "ERROR: register command requires PRIORITY"; exit 1; fi
        for binary in $JAVA_DIR/bin/*
        do
            binary_path=`realpath $binary`
            if [ -x "$binary_path" ]
            then
                binary_name=`basename "$binary_path"`
                update-alternatives --install "/usr/bin/$binary_name" "$binary_name" "$binary_path" "$PRIORITY"
            fi
        done
        ;;

    activate)
        for binary in $JAVA_DIR/bin/*
        do
            binary_path=`realpath $binary`
            if [ -x "$binary_path" ]
            then
                binary_name=`basename "$binary_path"`
                update-alternatives --set "$binary_name" "$binary_path"
            fi
        done
        ;;

    *)
        usage
        exit 2
        ;;
esac