How to add Java environment variable on Linux.

root@kabali:~$ which java  /usr/bin/java

root@kabali:~$ ll /usr/bin/java  lrwxrwxrwx 1 root root 22 2009-01-06 19:08 /usr/bin/java -> /etc/  alternatives/java 

root@kabali:~$ ll /etc/alternatives/java  lrwxrwxrwx 1 root root 36 2009-01-14 08:38 /etc/alternatives/java -> /usr/  lib/jvm/java-6-sun/jre/bin/java

  root@kabali:~$ which javac  /usr/bin/javac

root@kabali:~$ ll /usr/bin/javac   lrwxrwxrwx 1 root root 23 2009-01-14 00:24 /usr/bin/javac -> /etc/  alternatives/javac

root@kabali:~$ ll /etc/alternatives/javac  lrwxrwxrwx 1 root root 37 2009-01-14 08:25 /etc/alternatives/javac -> /  usr/lib/jvm/java-6-openjdk/bin/javac  

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Install new java location by following command:

 root@kabali:/usr/local/java/jdk1.8.0_05/jre#update-alternatives --install "/usr/bin/java" "java" "/usr/local/java/jdk1.8.0_05/bin/java" 1
 root@kabali:/usr/local/java/jdk1.8.0_05/jre#update-alternatives --install "/usr/bin/javac" "javac" "/usr/local/java/jdk1.8.0_05/bin/javac" 1
 root@kabali:/usr/local/java/jdk1.8.0_05/jre#update-alternatives --install "/usr/bin/javaws" "javaws" "/usr/local/java/jdk1.8.0_05/bin/javaws" 1ws

Set
root@kabali:/usr/local/java/jdk1.8.0_05/jre# update-alternatives --config java

There are 3 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                            Priority   Status
------------------------------------------------------------
* 0            /usr/lib/jvm/java-6-openjdk-amd64/jre/bin/java   1061      auto mode
  1            /usr/lib/jvm/java-6-openjdk-amd64/jre/bin/java   1061      manual mode
  2            /usr/lib/jvm/java-7-openjdk-amd64/jre/bin/java   1051      manual mode
  3            /usr/local/java/jdk1.8.0_05/bin/java             1         manual

root@kabali:/usr/local/java/jdk1.8.0_05/jre# update-alternatives --config javac
There are 2 choices for the alternative javac (providing /usr/bin/javac).

  Selection    Path                                         Priority   Status
------------------------------------------------------------
* 0            /usr/lib/jvm/java-6-openjdk-amd64/bin/javac   1061      auto mode
  1            /usr/lib/jdk1.8.0_05/bin/javac                1         manual mode
  2            /usr/lib/jvm/java-6-openjdk-amd64/bin/javac   1061      manual mode

root@kabali:/usr/local/java/jdk1.8.0_05/jre# update-alternatives --config javaws
There are 3 choices for the alternative javaws (providing /usr/bin/javaws).

  Selection    Path                                              Priority   Status
------------------------------------------------------------
* 0            /usr/lib/jvm/java-6-openjdk-amd64/jre/bin/javaws   1061      auto mode
  1            /usr/lib/jvm/java-6-openjdk-amd64/jre/bin/javaws   1061      manual mode
  2            /usr/lib/jvm/java-7-openjdk-amd64/jre/bin/javaws   1060      manual mode
  3            /usr/local/java/jdk1.8.0_05/bin/javaws             1         manual mode

root@kabali:/usr/local/java/jdk1.8.0_05/jre# java -version
java version "1.8.0_05"
Java(TM) SE Runtime Environment (build 1.8.0_05-b13)
Java HotSpot(TM) 64-Bit Server VM (build 25.5-b02, mixed mode)