当我在我的控制台中输入内容时，会出现java一个符号链接。/usr/bin/java
which java
/usr/bin/java
该符号链接指向/etc/alternatives/java。
file /usr/bin/java
/usr/bin/java: symbolic link to `/etc/alternatives/java'
/etc/alternatives/java 本身是一个符号链接，指向/usr/lib/jvm/jdk1.8.0_20/bin/java
为什么要绕这么远？为什么不直接java指向/usr/lib/jvm/jdk1.8.0_20。我看不出有什么好处。
答案1
您发现了 Debian/Ubuntu 中的替代系统：
试试看
% 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:
从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.
答案2
您要寻找的答案就在 AB 的答案中，更具体地说，在手册页的文本中，第二段和第三段。但简单地说，备选系统充当默认程序的数据库，允许对具有相同目的的多个不同程序进行优先排序
好的，但为什么要绕道而行？为什么不启动/usr/bin/gnome-terminal或/usr/bin/xterm？为了维护系统软件。假设您同时安装了 Oracle 和 Open-JDK。您已将 Oracle java 设置为最高优先级，然后有一天决定将其删除。如果java，或者更确切地说/usr/bin/java是指向 Oracle 的 jdk 的任何二进制文件的链接，您必须手动更改该链接。替代系统允许将链接自动切换到优先级列表中的下一个项目。