Is your “java” command working

$ java -version

java version "1.8.0_161"
Java(TM) SE Runtime Environment (build 1.8.0_161-b12)
Java HotSpot(TM) 64-Bit Server VM (build 25.161-b12, mixed mode)

but JAVA_HOME is not set?

$ echo $JAVA_HOME

$

There are some simple ways how to find out, where your JRE/JDK is located. Like which

$ which java

/c/ProgramData/Oracle/Java/javapath/java

or alternatives

$ alternatives java

but what if they do not work? E.g. which might return /usr/bin/java, which is a link to one of the alternatives

$ which java
/usr/bin/java

$ ls -la /usr/bin/java
lrwxrwxrwx. 1 root root 22 Mar 27 12:43 /usr/bin/java -> /etc/alternatives/java

The question is: is there a generic way how to set your JAVA_HOME based on java command in your PATH. This can be interesting when scripting your infrastructure as code, for instance.

Let me show

$ java -XshowSettings:properties -version
Property settings:
    awt.toolkit = sun.awt.windows.WToolkit
    file.encoding = Cp1252
...
    java.home = C:\Program Files\Java\jre1.8.0_161
...

As showSettings uses stderr for output, we need to redirect and grep for line containing java.home

java -XshowSettings:properties -version 2>&1 | grep java.home

Now we just need to extract the value after the equal sign. Either using awk

export JAVA_HOME=$(java -XshowSettings:properties -version 2>&1 | grep java.home | awk -F"=" '{print $NF}')

or, if no awk available in your system, then using cut

export JAVA_HOME=$(java -XshowSettings:properties -version 2>&1 | grep java.home | cut -d= -f2)

This should be now the result

$ echo $JAVA_HOME
C:\Program Files\Java\jre1.8.0_161

Enjoy!

Appendix #1

Just noticed that the proposed solution does not work for NiFi

$ bin/nifi.sh start
nifi.sh: JAVA_HOME is not valid:  /usr/java/jdk1.8.0_162/jre

Obviously, NiFi requires JAVA_HOME without jre folder

$ export JAVA_HOME=/usr/java/jdk1.8.0_162/
$ echo $JAVA_HOME
/usr/java/jdk1.8.0_162/
$ bin/nifi.sh status

Java home: /usr/java/jdk1.8.0_162/
NiFi home: /home/vagrant/nifi-1.5.0

Bootstrap Config File: /home/vagrant/nifi-1.5.0/conf/bootstrap.conf

2018-03-27 13:28:07,403 INFO [main] org.apache.nifi.bootstrap.Command Apache NiFi is currently running, listening to Bootstrap on port 51373, PID=3377

So how to get arouns this in a generic way?

export JAVA_HOME=$(java -XshowSettings:properties -version 2>&1 | grep java.home | \
  cut -d= -f2 | xargs -I % sh -c 'if [ "$(basename %)" == "jre" ]; then dirname %; else echo %; fi')

Fairly clumsy, I admit, but it works. Should you have a more elegant solution, please let me know.