For identifying JRE, use below command in Debian:
JAVA_HOME=$(readlink -f /usr/bin/java | sed "s:bin/java::")
For identifying JDK, use below command in Debian:
JAVA_HOME=$(readlink -f /usr/bin/javac | sed "s:/bin/javac::")
You can also try below bash command:
export JAVA_HOME=$(dirname $(dirname $(readlink -f /usr/bin/java)))