|Linux / Unix Command: alternatives|
NAMEalternatives - maintain symbolic links determining default commands
SYNOPSISalternatives [options] --install link name path priority [--slave link name path]... [--initscript service]
alternatives [options] --remove name path
alternatives [options] --set name path
alternatives [options] --auto name
alternatives [options] --display name
DESCRIPTIONalternatives creates, removes, maintains and displays information about the symbolic links comprising the alternatives system. The alternatives system is a reimplementation of the Debian alternatives system. It was rewritten primarily to remove the dependence on perl; it is intended to be a drop in replacement for Debian's update-dependencies script. This man page is a slightly modified version of the man page from the Debian project.
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 of editor to invoke if the user has not specified a particular preference.
The 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.
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.
When each package providing a file with a particular functionality is installed, changed or removed, alternatives is called to update information about that file in the alternatives system. alternatives is usually called from the %post or %pre scripts in RPM packages.
It is often useful for a number of alternatives to be synchronised, so that they are changed as a group; for example, when several versions of the vi(1) editor are installed, the man page referenced by /usr/share/man/man1/vi.1 should correspond to the executable referenced by /usr/bin/vi. alternatives handles this by means of master and slave links; when the master is changed, any associated slaves are changed too. A master link and its associated slaves make up a link group.
Each link group is, at any given time, in one of two modes: automatic or manual. When a group is in automatic mode, the alternatives system will automatically decide, as packages are installed and removed, whether and how to update the links. In manual mode, the alternatives system will not change the links; it will leave all the decisions to the system administrator.
Link groups are in automatic mode when they are first introduced to the system. If the system administrator makes changes to the system's automatic settings, this will be noticed the next time alternatives is run on the changed link's group, and the group will automatically be switched to manual mode.
Each alternative has a priority associated with it. When a link group is in automatic mode, the alternatives pointed to by members of the group will be those which have the highest priority.
When using the --config option, alternatives will list all of the choices for the link group of which given name is the master link. You will then be prompted for which of the choices to use for the link group. Once you make a change, the link group will no longer be in auto mode. You will need to use the --auto option in order to return to the automatic state.
TERMINOLOGYSince the activities of alternatives are quite involved, some specific terms will help to explain its operation.
- generic name
- A name, like /usr/bin/editor, which refers, via the alternatives system, to one of a number of files of similar function.
- Without any further qualification, this means a symbolic link in the alternatives directory: one which the system administrator is expected to adjust.
- The name of a specific file in the filesystem, which may be made accessible via a generic name using the alternatives system.
- alternatives directory
- A directory, by default /etc/alternatives, containing the symlinks.
- administrative directory
- A directory, by default /var/lib/alternatives, containing alternatives' state information.
- link group
- A set of related symlinks, intended to be updated as a group.
- master link
- The link in a link group which determines how the other links in the group are configured.
- slave link
- A link in a link group which is controlled by the setting of the master link.
- automatic mode
- When a link group is in automatic mode, the alternatives system ensures that the links in the group point to the highest priority alternatives appropriate for the group.
- manual mode
- When a link group is in manual mode, the alternatives system will not make any changes to the system administrator's settings.
OPTIONSExactly one action must be specified if alternatives is to perform any meaningful task. Any number of the common options may be specified together with any action.
- Generate more comments about what alternatives is doing.
- Don't generate any comments unless errors occur. This option is not yet implemented.
- Don't actually do anything, just say what would be done. This option is not yet implemented.
- Give some usage information (and say which version of alternatives this is).
- Tell which version of alternatives this is (and give some usage information).
- --altdir directory
- Specifies the alternatives directory, when this is to be different from the default.
- --admindir directory
- Specifies the administrative directory, when this is to be different from the default.
- --install link name path pri [--slave slink sname spath] [--initscript service]...
- Add a group of alternatives to the system. name is the generic name for the master link, link is the name of its symlink, and path is the alternative being introduced for the master link. sname, slink and spath are the generic name, symlink name and alternative for a slave link, and service is the name of any associated initscript for the alternative. NOTE: --initscript is a Red Hat Linux specific option. Zero or more --slave options, each followed by three arguments, may be specified.
If the master symlink specified exists already
in the alternatives system's records,
the information supplied will be added as a new
set of alternatives for the group.
Otherwise, a new group, set to automatic mode,
will be added with this information.
If the group is in automatic mode,
and the newly added alternatives' priority is higher than
any other installed alternatives for this group,
the symlinks will be updated to point to the newly added alternatives.
If --initscript is used, the alternatives system will manage the initscript associated with the alternative via chkconfig, registering and unregistering the init script depending on which alternative is active.
NOTE: --initscript is a Red Hat Linux specific option.
- --remove name path
- Remove an alternative and all of its associated slave links. name is a name in the alternatives directory, and path is an absolute filename to which name could be linked. If name is indeed linked to path, name will be updated to point to another appropriate alternative, or removed if there is no such alternative left. Associated slave links will be updated or removed, correspondingly. If the link is not currently pointing to path, no links are changed; only the information about the alternative is removed.
- --set name path
- The symbolic link and slaves for link group name set to those configured for path, and the link group is set to manual mode. This option is not in the original Debian implementation.
- --auto name
- Switch the master symlink name to automatic mode. In the process, this symlink and its slaves are updated to point to the highest priority installed alternatives.
- --display name
- Display information about the link group of which name is the master link. Information displayed includes the group's mode (auto or manual), which alternative the symlink currently points to, what other alternatives are available (and their corresponding slave alternatives), and the highest priority alternative currently installed.
SEE ALSOln(1), FHS, the Filesystem Hierarchy Standard.
Important: Use the man command (% man) to see how a command is used on your particular computer.