A configurable alternative for update-alternatives.
update-alternatives, at its core, is a symlink management system. This
system is then mostly used as means for administrators of a system to
select a default application. For example, if we have vim or emacs
installed, the admin can then make one accessible as an editor
application.
The simple principle behind update-alternatives also is its main
drawback. Since it only manages symlinks, the preferred configuration
is only managed per system and cannot be altered per user. Furthermore,
if an application requires additional processing prior to execution via
the default handler, this is outside the domain of update-alternatives.
Another major drawback of update-alternatives is that it is a state
system and this state is modified or updated at installation time. If
the state of the running system is to be kept separate from the installation
state (eg. multiple snapshots of parts of the system), the symlinks'
state can become stale or simply wrong without a possibility of a
fallback mechanism.
libalternatives provides an alternative, not a replacement, to this
symlink management system. Instead of symlinks, the preferred executable
is executed directly. Which executable is executed depends on the
available alternatives installed on the system and the system and/or
user configuration files. By default, the preferred alternative with the
highest priority is executed. If an alternative is selected by the user
or the system administrator, this alternative will be executed instead.
If the user or system selection is unavailable, the system will fall
back to the default, highest priority, installed alternative.
alts is a simple wrapper around two config files. First is a
config file provided by packages that describe the alternatives
provided. The second config file is either a system or a user
preferences file that overrides default alternative preference.
Packages need to ship their package preferences in the following format,
/usr/share/libalternatives/<binary>/<pref>.conf
where
- binary is the name of the alternative provided
- pref is a numeric priority
The config file must be of the following format:
binary=<absolute path to binary>
man=<list of manpages>
group=<list of binaries>
options=KeepArgv0
where the binary entry is mandatory and list consists of a comma (,) separated items. All other entries are optional
Typically, the binary can then be a symlink to /usr/bin/alts that will exec() the configured binary with largest pref number, forwarding all the arguments to the configured binary.
In normal cases, argv[0] contains name of the executed target by the alts binary. In the example below, this would be vim. If the called process needs the name of for argv[0], set the option options=KeepArgv0 in the config file.
Note: Only these two options for argv[0] are available. Some called processes expect other names which cannot be handled by libalternatives.
For example:
binary=/usr/bin/vim
man=vim.1
options=KeepArgv0
alts is also the configuration utility that allows system or user override of the default binary. See alts(1) for details.
At times, multiple alternatives should be controlled as a group.
Altering the preferred binary in one should alter the entire group. To
facilitate this, one should use the group entry in the package
preferences to list all the binaries that are to be altered together.
It is important that all the binaries have the same group entries in
their package preference files and have the same priorities. It is also
important that no higher priority alternative exists unless it also
forms the same sized group. The alts binary will warn you when reading
the state of one of the executables with a group entry.
libalternatives is NOT aimed to be a replacement for
update-alternatives. It is meant as an alternative that tends to stay
out of the way and provides more flexibility and stability for users and
the package maintainers.
- alts(1)