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.
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.
alternatives --display unison
alternatives --set unison /usr/bin/unison-2.13
alternatives --set unison /usr/bin/unison-2.27
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