The alternatives command (man page) is a tool to create, remove, maintain and display information about the symbolic links comprising the alternatives system. So, you make system-wide changes with it and, thus, you need a user with elevated privileges. You either have to sudo update-alternatives (if sudo is installed and the user is a sudoer) or login as root with su - and then run said command. This answer to a question about how one runs commands that need root permissions may be helpful to you.

The command you are looking for is update-alternatives. So you should be running sudo update-alternatives --config java.

See 'air-dex' example on this post: https://askubuntu.com/questions/159575/how-do-i-make-java-default-to-a-manually-installed-jre-jdk . Note: sometimes people install Java JDK's into '/usr/lib/jvm', and as you can see in the output on my blog post, the system came with default JVMs in this location, but personally I don't recommend that for a JDK. Maybe I would choose '/usr/lib/jvm' if it was a JRE I was installing. If you wanted an alternative location for JDKs and JREs, I would choose either:

   1. /usr/java/
   2. /opt/java/

If I were comparing to windows, I might say:

JDKs: /usr/java = C:\Java
JREs: /usr/lib/jvm = C:\Program Files\Java
Alt:  /opt/java = E:\Java

Fedora/CentOS does not use alternatives to select the compiler. It is true that Fedora/CentOS has and uses the alternatives package. However alternatives does not automatically manage every installed software application, and each software package has to be explicitly set up to use alternatives, and in Fedora/CentOS this is simply not done for gcc.

Building gcc is no small feat. By comparison, using an explicit path to your custom gcc build seems like an insignificant triviality. Or, put your custom built gcc's location in your PATH environment variable first, and move on to something else.

It appears than the option --list doesn't take arguments, it's a bug in the manual synopsis.Moreover for manage my java versions I should try env variables.

(The bug has been reported as https://github.com/fedora-sysv/chkconfig/issues/11)

Add python2.6 to alternatives:

sudo alternatives --install /usr/bin/python python /usr/local/bin/python2.6 5

Then set python3.9 as defaut:

sudo alternatives --config  python

Or override the symlink with the desired python version:

sudo ln -fs /usr/bin/python3.9 /usr/bin/python

I was trying to set the java path while installation of Glassfish on Ubuntu 14.04.

The command goes like in CentOS as

" alternatives --install /usr/bin/java java /opt/java/jdk/bin/java 1"

“alternatives --config java”

Well, the problem is that in Ubuntu the “alternative” command does not work.

So, what can be done regarding this ?

I was recommended to run update-alternatives --auto php or update-alternatives --config php which did not work for me. What worked however was this:

1. I manually deleted the faulty symlink under /etc/alternatives/php
2. I ran sudo update-alternatives --install /usr/bin/php php /usr/bin/php8.0 1 which, in contrast to the other commands, created a new and working symlink to the corresponding php version.

This restored normal functionality of update-alternatives and made me able to use php from cmd again.

You presumably have /usr/local/bin ahead of /usr/bin in your PATH, and since there’s a php binary in /usr/local/bin, your shell uses that. The php alternative configured by update-alternatives ends up having no effect — at least, on anything based on that PATH.

To fix this, in your case, I recommend removing the PHP installation in /usr/local. A simpler approach would be to re-order your PATH, but that might affect other binaries you rely on in /usr/local/bin.