What happens when there are 2 versions of a terminal command installed?

Question

For example, I have OpenJDK 6 and 7 both installed. If I run javac foo.java will is compile my program as a Java 6 program or a Java 7 program?

Multiple duplicates, i.e.: install jre-7 how to install java — user unknown, Jan 24 '12 at 00:01

score 4 · Accepted Answer · answered Jan 23 '12 at 23:22

Debian-based distributions have so-called "alternatives" system, which allows to have more than one program installed which perform the same function (this also includes different versions of the same program, like Java 6 and Java 7). As @user15936 points out, the system works by manipulating symlinks, so "javac" can point to the executable either from OpenJDK 6 or from OpenJDK 7

The command to manipulate the above mentioned symlinks (the process is in fact more complex than that as it includes some configuration, for example manpages change too etc.) - is called update-alternatives. You can learn how to use it by typing man update-alternatives in terminal.

To directly answer your question, another way to determine which javac executable will be invoked is by doing

update-alternatives --display javac

There is even the specialized update-java-alternatives. – user unknown Jan 23 '12 at 23:50 — user unknown, Jan 23 '12 at 23:50

score 1 · Answer 2 · edited Jan 23 '12 at 20:39

1

The answer is whichever is linked to /usr/bin/javac

That will be executed.

You can see this by going to /usr/bin/ and see the link javac is connected by doing ls -l javac.

edited Jan 23 '12 at 20:39

Bruno Pereira

73,643

answered Jan 23 '12 at 20:33

harish.venkat

266

score -1 · Answer 3 · edited Jan 23 '12 at 23:48

-1

You should type in a terminal javac -version and there you will get your answer...

edited Jan 23 '12 at 23:48

doug

17,026

answered Jan 23 '12 at 20:32

JuanToroMarty

111

There is nothing wrong with this answer, particularly for users where Oracle Java (JVM/JDK) is no longer provided as a .deb & they manually installed. Highly recommended reading, especially about the 'update java' script' which will set the various alternatives - http://www.webupd8.org/2011/09/how-to-install-oracle-java-7-jdk-in.html – doug Jan 23 '12 at 23:52
I didn't vote down, but java and javac might belong to different versions. – user unknown Jan 23 '12 at 23:53

What happens when there are 2 versions of a terminal command installed?

3 Answers3