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.

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.

I would try cleaning it up manually. I've never done this so make sure you backup beforehand.

• Remove the link from /etc/alternatives
• Remove the relevant file from the admin directory
  • /var/lib/dpkg/alternatives/ on ubuntu (debian may be the same but check the man pages under the FILES section)
  • /var/lib/alternatives/ on CentOS 6&7

Basically it says to your machine to use this alternative of Jave JDK instead of the default one, which, in Linux systems, is OpenJDK.

A brief extract from the man page is better than any answer I could write:

update-alternatives creates, removes, maintains and displays information about the symbolic links comprising the Debian alternatives system.

It is possible for several programs fulfilling the same or similar functions to be installed on a single system at the same time. For example, many systems have several text editors installed at once. This gives choice to the users of a system, allowing each to use a different editor, if desired, but makes it difficult for a program to make a good choice for an editor to invoke if the user has not specified a particular preference.

Debian's alternatives system aims to solve this problem. A generic name in the filesystem is shared by all files providing interchangeable functionality. The alternatives system and the system administrator together determine which actual file is referenced by this generic name. For example, if the text editors ed(1) and nvi(1) are both installed on the system, the alternatives system will cause the generic name /usr/bin/editor to refer to /usr/bin/nvi by default. The system administrator can override this and cause it to refer to /usr/bin/ed instead, and the alternatives system will not alter this setting until explicitly requested to do so.

With --install you specified a link, "/usr/bin/java" a name "java" and a path "/usr/lib/java/JDK...." and you add a group of alternatives to the system. link is the generic name for the master link, name is the name of its symlink in the alternatives directory, and path is the alternative being introduced for the master link.

I hope to be clear enough, Here there is a post regarding java alternatives.

For the complete usage list I suggest to look at the same manual page, typing man update-alternatives on your OS shell;

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

On Debian (but not Fedora or RHEL), to see a list of all "master alternative names":

update-alternatives --get-selections

--get-selections list master alternative names and their status.

And for each of those listed, you can run --list $ALTERNATIVE_NAME, e.g.

update-alternatives --list editor

--list name Display all targets of the link group.

If you would like to see a list of all alternatives in their respective groups, you could run the following in fish shell:

for alternative in (update-alternatives --get-selections)
    echo $alternative 
    update-alternatives --list (echo $alternative | cut -d" " -f1)
    echo
end | pager

The (ba|z)?sh syntax should be something similar.

To change the alternatives, run sudo update-alternatives --config $ALTERNATIVE_NAME

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)

Does your version have --set?

--set name path
Set the program path as alternative for name. This is equivalent to --config but is non-interactive and thus scriptable.