java콘솔에 입력 할 때 심볼릭 링크 /usr/bin/java가 이어집니다.
which java
/usr/bin/java이 심볼릭 링크는을 가리 킵니다 /etc/alternatives/java.
file /usr/bin/java
/usr/bin/java: symbolic link to `/etc/alternatives/java' / etc / alternatives / java는 다음을 가리키는 기호 링크 자체입니다. /usr/lib/jvm/jdk1.8.0_20/bin/java
왜 그런 우회가 이루어 집니까? 왜하지 않습니다 java직접 가리 킵니다 /usr/lib/jvm/jdk1.8.0_20. 나는 어떤 이점도 보지 못한다.
답변:
당신이 찾는 대답은 AB의 대답, 더 구체적으로-맨 페이지의 텍스트, 두 번째 및 세 번째 단락에 있습니다. 그러나 이것을 간단히 말하면 대안 시스템은 기본 프로그램의 데이터베이스 역할을하므로 동일한 목적으로 여러 가지 다른 프로그램의 우선 순위를 지정할 수 있습니다
그래, 왜 우회인가? 왜 /usr/bin/gnome-terminal시작 하지 /usr/bin/xterm않습니까? 시스템 소프트웨어를 유지합니다. Oracle과 Open-JDK가 모두 설치되어 있다고 가정하십시오. Oracle java를 최우선 순위로 설정 한 후 언젠가는 제거하기로 결정했습니다. 경우 java, 또는 오히려이 /usr/bin/java오라클의 JDK 무엇이든 바이너리에 대한 링크했다, 당신은 수동으로 링크를 변경해야 할 것입니다. 대체 시스템은 링크가 우선 순위 목록의 다음 항목으로 자동 전환되도록합니다.
데비안 / 우분투에서 대안 시스템을 발견했습니다.
사용해보십시오
% 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:에서 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.