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How does the Debian Alternatives systems work ?

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Old Unix and Linux 06-08-2016
sreyan32 sreyan32 is offline
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Debian How does the Debian Alternatives systems work ?

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)


Code:
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
)-:
Quote:
The generic name is not a direct symbolic link to the selected
alternative. Instead, it is a symbolic link to a name in the
alternatives directory, which in turn is a symbolic link to the actual
file referenced. This is done so that the system administrator's
changes can be confined within the /etc directory: the FHS (q.v.) gives
reasons why this is a Good Thing.

Why is good that the alternatives is confined to the /etc/ directory ? Didn't find anything FHS.


Referenced Documentation:
Ubuntu Manpage:

DebianAlternatives - Debian Wiki
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