1. echo $SHELL can show what shell you are using.
$ echo $SHELL
/bin/bash
2. What is /usr/libexec/java_home
2.1 On Mac OS X 10.5 or later, we can use /usr/libexec/java_home to return the location of the default JDK.
$ /usr/libexec/java_home
/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home
2.2 Also, find all installed JDKs.
$ /usr/libexec/java_home -V
Matching Java Virtual Machines (2):
1.8.0_91, x86_64: "Java SE 8" /Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home
1.7.0_79, x86_64: "Java SE 7" /Library/Java/JavaVirtualMachines/jdk1.7.0_79.jdk/Contents/Home
/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home
2.3 Also, run a specified JDK command.
$ /usr/libexec/java_home -v1.8
/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home
3. Set JAVA_HOME
3.1 Open the ~/.bash_profile
vi ~/.bash_profile
3.2 add the following content
export JAVA_HOME=/System/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home
$ source ~/.bash_profile
$ echo $JAVA_HOME
/System/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home
4. Switch between different JDK versions
4.1 if you want to switch between different JDK, you can see this article.
How to Set $JAVA_HOME environment variable on macOS
