Set $JAVA_HOME to installed JDK.

set_jdk_JAVA_HOME.sh

if [ -x "$(command -v java)" ]; then
  case "$(uname)" in
    CYGWIN*)
      #Assumes %JAVA_HOME% was set in Windows
      #Convert $JAVA_HOME to unix-like path
      JAVA_HOME=$(cygpath --unix "$JAVA_HOME")
      ;;
    Darwin*)
      if [ -z "$JAVA_HOME" ]; then
        if [ -x "/usr/libexec/java_home" ]; then
          JAVA_HOME=$(/usr/libexec/java_home)
        elif [ -d "/System/Library/Frameworks/JavaVM.framework/Versions/CurrentJDK/Home" ]; then
          export JAVA_HOME=/System/Library/Frameworks/JavaVM.framework/Versions/CurrentJDK/Home
        fi
      else
        echo Warning: \$JAVA_HOME was already set. No change made.
      fi
      ;;
    Linux*)
      if [ -z $JAVA_HOME ]; then
        #Note: this assumes gnu readlink (darwin and perhaps others use BSD
        #readlink where -f has different meaning)
        #Sometimes java is pointing to JRE inside a JDK
        # - if so, JAVA_HOME is set to the JDK root folder
        # - otherwise, JAVA_HOME is just 2 folders up from java command
        JAVA_HOME=$(readlink -f "$(command -v java)" | sed 's:\(/jre\)\?/bin/java$:/:')
      else
        echo Warning: \$JAVA_HOME was already set. No change made.
      fi
      ;;
    MINGW*)
      #Assumes %JAVA_HOME% was set in Windows
      #Convert $JAVA_HOME to unix-like path
      JAVA_HOME="$(cd "$JAVA_HOME" || exit; pwd)"
      ;;
  esac
  echo \$JAVA_HOME="$JAVA_HOME"
  export JAVA_HOME
fi

See similar solution for Windows: https://gist.github.com/darcyparker/7121448