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esy-bash

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Cross-platform bash utilities - primed for Reason/OCaml

github.com/bryphe/esy-bash

bryphe/esy-bash

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alternatives 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 dif- ferent 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 par- ticular preference. The alternatives system aims to solve this problem. A generic name in the filesystem is shared by all files providing interchangeable func- tionality. 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 alterna- tive. Instead, it is a symbolic link to a name in the alternatives directory, which in turn is a symbolic link to the actual file refer- enced. 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 infor- mation 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. See `man alternatives' for more information.