Alright I have read all the documentation regarding the alternatives system present in Debian. And I still can't get my head around it.
First,
Correct me if I am wrong, the /etc/alternatives directory is nothing but a directory of symlinks right ? So for example java will have a symlink pointing to the actual executable to be executed when java is called from the command line. Like it is described in this answer in SO (What exactly does `update-alternatives` do? - Ask Ubuntu).
So let me get this straight, some programs that has a generic name is assigned a symlink in the /etc/alternatives/ directory. So like /usr/bin/java is assigned a symlink in /etc/alternatives called java which points to the actual file. So it is <generic_name>---> <alternatives_symlink> ---> <actual_file> right ?
But I have seen systems like <alternatives_symlink> ---> <generic_name> ---> <actual_file>. Isn't that the wrong implementation of the alternatives system ?
Second,
How are priorities maintained in the alternatives system ? I mean a symlink can only point to one file at a time so then how can the alternatives system keep track of multiple programs under the same name ? For example, java maybe openjdk or oracle jdk. So once I have uninstalled oracle jdk I want the symlink in the alternatives directory to point to openjdk, how does this happen automatically ?
Thirdly,
How to master and slave links work ? What's the intuition behind them ? For example lets take the following command example taken from this post (How to switch GCC version using update-alternatives - Code Yarns)
sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-4.8 100 --slave /usr/bin/g++ g++ /usr/bin/g++-4.8
So the above command is telling gcc to point to version 4.8, also it says that along with gcc please update g++. Right ? So AFAIK master and slave links signify that if the master is updated please update the slave also. But here both of them ARE UPDATED simultaneously. So why am I saying that update slave if master is updated even when IT HAS ALREADY BEEN UPDATED.
Also lets say I change the gcc alternatives to something else next what will the slave be updated to ? I mean this is really needlessly confusing.
Lastly,
Taking a line from the Ubuntu documentation([Ubuntu Manpage:
update-alternatives - maintain symbolic links determining default](http://manpages.ubuntu.com/manpages/trusty/man8/update-alternatives.8.html)\)-:
Why is good that the alternatives is confined to the /etc/ directory ? Didn't find anything FHS.
Referenced Documentation:
Ubuntu Manpage:
DebianAlternatives - Debian Wiki