Debian alternatives system: programs with the same name and priorities

alternativesdebian

I can't understand from the update-alternatives documentation how the priorities system works.

Suppose that I want /usr/bin/node to point to nodejs for nodejs applications and to an amateur radio program, called node, in other cases. I have to say:

update-alternatives --install /usr/bin/node nodejs /usr/bin/nodejs 100 --slave /usr/share/man/...

Suppose I have 2 programs that expect /usr/bin/node to be different things, first – nodejs, second – amateur radio. How would they determine which one to use?

What's the role of priority in this process?

ADDITION: Note, that npm, nodejs package manager, expects nodejs command to be called node, thus I have to install nodejs as node.

Best Answer

At any time, /usr/bin/node can only be one of the programs. The alternatives mechanism is a way to choose which one it is.

Priorities control which one is the default when both are installed. They don't offer a way to somehow have both and select between the two based on what other program called this one.

When programs call each other by name, there can only be a single program by a given name that comes first on the search path. You should disambiguate the node command in your programs. Following Debian, use nodejs for Node.js and ax25-node for the HAM program. If there's one that it would be really annoying to change, install either the node package (which makes /usr/bin/node an alias for ax25-node) or the nodejs-legacy package (which makes /usr/bin/node an alias for nodejs).

If you really have a lot of programs that call node and that can't be changed easily, run them with different PATH variables, one containing a directory containing a symbolic link to ax25-node and one containing a directory containing a symbolic link to nodejs:

mkdir -p /usr/local/etc/nodejs/bin /usr/local/etc/ax25/bin
ln -s ../../../../bin/nodejs /usr/local/etc/nodejs/bin/node
ln -s ../../../../bin/ax25-node /usr/local/etc/ax25/bin/node

PATH=/usr/local/etc/ax25/bin:$PATH program-using-ax25
PATH=/usr/local/etc/node/bin:$PATH program-using-nodejs

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Debian Kernel Module Error – Cannot Create ‘Hello World’ Module with NVIDIA and VirtualBox

SOLVED!

Simple as that: /root/.bashrc had this inside:

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Changed it to:

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...and restarted the root shell (of course; do not omit this step). Everything started working again. Both NVIDIA and VirtualBox kernel modules built from the first try. I am so happy! :-)

Then again though, I am slighly disappointed by the kernel build tools. They should know better and pass --color=never everywhere they use grep; or rather, store the old value of GREP_OPTIONS, override it for the lifetime of the building process, then restore it.

I am hopeful that my epic one-week battle with this problem will prove valuable both to the community and the kernel build tools developers.

A very warm thanks to the people who were with me and tried to help.

(All credits go here: http://forums.gentoo.org/viewtopic-p-4156366.html#4156366)

Ubuntu – How does APT deal with different programs with the same name

On Debian-derivatives, it's handled through the alternatives system:

$ ls -l /usr/bin/vim
lrwxrwxrwx 1 root root 21 Jun 11  2010 /usr/bin/vim -> /etc/alternatives/vim
$ ls -l /etc/alternatives/vim
lrwxrwxrwx 1 root root 18 Jun 11  2010 /etc/alternatives/vim -> /usr/bin/vim.gnome

The package post install script (the thing that runs when dpkg says "configuring package X") told the alternative system about a new alternative for vim. The new alternative had a higher priority, so it was picked.

You can run them directly as vim.tiny, vim.full, vim.gnome, etc.

You can override the default using the update-alternatives command.

(Actually, I think vim.tiny works somewhat as a special case, as its not really intended to be used except when space is at a huge premium. At least, it doesn't show as an alternative here.)

Best Answer

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