符号链接和 /etc/alternatives 的意义

Question

在java我的控制台中输入时，/usr/bin/java会跟随一个符号链接。

which java
/usr/bin/java

此符号链接指向/etc/alternatives/java.

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

/etc/alternatives/java 本身就是一个符号链接，指向 /usr/lib/jvm/jdk1.8.0_20/bin/java

为什么要走这样的弯路？为什么不java直接指向/usr/lib/jvm/jdk1.8.0_20. 我看不出有什么优势。

Answer 1

您已经在 Debian/Ubuntu 中发现了替代系统：

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.

Answer 2

您寻求的答案在 AB 的答案中，更具体地说 - 在手册页的第二段和第三段的文本中。但简单地说，替代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级排序

好吧，但是为什么要绕路呢？为什么不/usr/bin/gnome-terminal推出或/usr/bin/xterm？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，则您必须手动更改该链接。替代系统允许链接自动切换到优先级列表上的下一个项目。