Podczas pisania javana mojej konsoli następuje łącze symboliczne /usr/bin/java.
which java
/usr/bin/javaTen symboliczny link wskazuje na /etc/alternatives/java.
file /usr/bin/java
/usr/bin/java: symbolic link to `/etc/alternatives/java' / etc / alternatives / java jest linkiem symbolicznym, który wskazuje /usr/lib/jvm/jdk1.8.0_20/bin/java
Dlaczego odbywa się taki objazd? Dlaczego nie javawskazuje wprost /usr/lib/jvm/jdk1.8.0_20. Nie widzę żadnej przewagi.
Odpowiedzi:
Odpowiedź, której szukasz, znajduje się w odpowiedzi AB, a dokładniej - w tekście strony podręcznika, akapitach drugim i trzecim. Mówiąc prościej, system alternatyw działa jako baza programów domyślnych, co pozwala nadać priorytet kilku różnym programom w tym samym celu
OK, ale dlaczego objazd? Dlaczego nie /usr/bin/gnome-terminaluruchomić /usr/bin/xterm? Aby utrzymać oprogramowanie systemowe. Załóżmy, że masz zainstalowany zarówno Oracle, jak i Open-JDK. Ustawiłeś Oracle Java jako najwyższy priorytet, a pewnego dnia postanowiłeś ją usunąć. Jeśli java, a raczej /usr/bin/javabyłby link do dowolnego pliku binarnego jdk Oracle, musiałbyś ręcznie zmienić ten link. System alternatyw umożliwia automatyczne przełączanie łącza do następnego elementu na liście priorytetów.
Odkryłeś system alternatyw w Debian / Ubuntu:
Wypróbuj to
% 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:Od 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 (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, 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.