Символические ссылки и смысл / etc / alternatives

При вводе java в мою консоль сопровождается символьная ссылка в /usr/bin/java .

 which java /usr/bin/java 

Эта символическая ссылка указывает на /etc/alternatives/java .

 file /usr/bin/java /usr/bin/java: symbolic link to `/etc/alternatives/java' 

/ etc / alternatives / java – это символическая ссылка, которая указывает на /usr/lib/jvm/jdk1.8.0_20/bin/java

Почему такой обход? Почему java не указывает прямо на /usr/lib/jvm/jdk1.8.0_20 . Я не вижу никакого преимущества.

Ответ, который вы ищете, находится в ответе AB, точнее – в тексте справочной страницы, второго и третьего абзацев. Но, говоря проще, система альтернатив выступает в качестве базы данных программ по умолчанию, что позволяет приоритезировать несколько разных программ с той же целью

Хорошо, но почему обход? Почему бы не запустить /usr/bin/gnome-terminal или /usr/bin/xterm ? Для поддержки системного программного обеспечения. Скажем, у вас установлены оба Oracle и Open-JDK. Вы установили Oracle Java как наивысший приоритет, а затем в один прекрасный день решили удалить его. Если java , или, вернее, /usr/bin/java была ссылкой на любой бинарный файл jdk Oracle, вам придется вручную изменить эту ссылку. Альтернативная система позволяет автоматически переключать связь на следующий элемент в списке приоритетов.

Вы обнаружили альтернативную систему в Debian / Ubuntu:

Попробуйте

 % sudo update-alternatives --config java There are 3 choices for the alternative java (providing /usr/bin/java). Selection Path Priority Status ------------------------------------------------------------ 0 /usr/lib/jvm/java-8-oracle/jre/bin/java 1082 auto mode 1 /usr/lib/jvm/java-7-openjdk-i386/jre/bin/java 1071 manual mode 2 /usr/lib/jvm/java-8-openjdk-i386/jre/bin/java 1081 manual mode * 3 /usr/lib/jvm/java-8-oracle/jre/bin/java 1082 manual mode Press <enter> to keep the current choice[*], or type selection number: 

От man update-alternatives

 update-alternatives - maintain symbolic links determining default commands DESCRIPTION update-alternatives creates, removes, maintains and displays information about the symbolic links comprising the Debian alternatives system. 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 for an editor to invoke if the user has not specified a particular preference. Debian's 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 (qv) gives reasons why this is a Good Thing. When each package providing a file with a particular functionality is installed, changed or removed, update-alternatives is called to update information about that file in the alternatives system. update-alternatives is usually called from the postinst (configure) or prerm (remove and deconfigure) scripts in Debian packages. It is often useful for a number of alternatives to be synchronized, 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. update-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 retain the choice of the administrator and avoid changing the links (except when something is broken). 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 update-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, update-alternatives will list all of the choices for the link group of which given name is the master alternative name. The current choice is marked with a '*'. You will then be prompted for your choice regarding this link group. Depending on the choice made, the link group might no longer be in auto mode. You will need to use the --auto option in order to return to the automatic mode (or you can rerun --config and select the entry marked as automatic). If you want to configure non-interactively you can use the --set option instead (see below). Different packages providing the same file need to do so cooperatively. In other words, the usage of update-alternatives is mandatory for all involved packages in such case. It is not possible to override some file in a package that does not employ the update-alternatives mechanism.