What exactly does `update-alternatives` do?

Question

What is the command update-alternatives used for?

Take this example:

sudo update-alternatives --install /usr/bin/java java /usr/local/java/jre1.7.0_09/bin/java 1

What does it do? How is it different from adding jdk to the path?

Let's say that I have run the command. How would I revert back to the original state?

Solution 1

It updates the links in /etc/alternatives to point to the program for this purpose. There's lots of examples, like x-www-browser, editor, etc. that will link to the browser or editor of your preference. Some scripts or system tools may want you to edit a file manually (e.g. configuration conflict in dpkg) and they'll look into the alternatives to give you the editor of choice. For java, this is the Java runtime environment - Oracle's, OpenJRE, etc.

The links in /etc/alternatives are just symbolic links. You can see them using for example

ls -l /etc/alternatives

Moreover, the regular /usr/bin binaries are also symlinks. E.g.:

ls -l /usr/bin/java
  lrwxrwxrwx 1 root root 22 Aug 14 10:33 /usr/bin/java -> /etc/alternatives/java
ls -l /etc/alternatives/java
  lrwxrwxrwx 1 root root 46 Aug 14 10:33 /etc/alternatives/java -> /usr/lib/jvm/java-6-openjdk-amd64/jre/bin/java

So, no PATH has to be modified. It just uses symbolic links.

Solution 2

While @gertvdjik's answer is a good explanation of how alternatives work at the lower level, it doesn't explicitly say how to revert the original state.

I find it easier to use the corresponding GUI galternatives which is available as a package. To install it, just run:

sudo apt-get install galternatives

Then managing alternatives becomes much easier. For java in particular, you have a lot of auxiliary binaries which you'll have to update and it's faster to overview them in the GUI.

Comments

[Admin]

You can also use sudo update-alternatives --config java and javac and javaws to choose between installed versions.

[Developer Android]

So in other words I can type in a single command and it will prompt me for choices. That is the only added advantage over setting a path right. Also lets say I have a different vendor of java also installed How would I update java to point to both the two different vendors?

[Developer Android]

Okay. That clears things up. I know I am coming back to paths but I just want to understand the difference between them . In case of a path we have to explicitly specify the location but in this case it just sym links so that when we type java it searches /etc/alternatives first. Or am i wrong in making this assumption?