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linux下指定程序或版本——update-alternatives

发表于 更新于 分类于 Disqus：

系统为Ubuntu16.04
使用fabric编写自动部署脚本，想使用python3
而系统中安装了2.7、3.5等多个版本，而python命令默认是2.7
为了使默认python版本为3.5，使用了update-alternatives命令来实现
Debian系的发行版自带？
功能大概就是windows的“打开方式”吧

用法

照例，先是文档

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$ update-alternatives --help
Usage: update-alternatives [<option> ...] <command>

Commands:
  --install <link> <name> <path> <priority>
    [--slave <link> <name> <path>] ...
                           add a group of alternatives to the system.
  --remove <name> <path>   remove <path> from the <name> group alternative.
  --remove-all <name>      remove <name> group from the alternatives system.
  --auto <name>            switch the master link <name> to automatic mode.
  --display <name>         display information about the <name> group.
  --query <name>           machine parseable version of --display <name>.
  --list <name>            display all targets of the <name> group.
  --get-selections         list master alternative names and their status.
  --set-selections         read alternative status from standard input.
  --config <name>          show alternatives for the <name> group and ask the
                           user to select which one to use.
  --set <name> <path>      set <path> as alternative for <name>.
  --all                    call --config on all alternatives.

<link> is the symlink pointing to /etc/alternatives/<name>.
  (e.g. /usr/bin/pager)
<name> is the master name for this link group.
  (e.g. pager)
<path> is the location of one of the alternative target files.
  (e.g. /usr/bin/less)
<priority> is an integer; options with higher numbers have higher priority in
  automatic mode.

显然，需要通过--install参数建立一个组，包含同功能的不同程序或不同的版本，从该组中选择默认程序即可
即相当于建立了一个中间层，通过软链接来指定到最终的程序

实例

  1. 找到要管理的所有程序

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    $ which python 
    /usr/bin/python

    $ which python3
    /usr/bin/python3

  2. 选定默认方式
    很显然，是python，即/usr/bin/python

  3. 创建alternatives group
    组名为python

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    $ sudo update-alternatives --install /usr/bin/python python /usr/bin/python2 100

    $ sudo update-alternatives --install /usr/bin/python python /usr/bin/python3 200
    update-alternatives: using /usr/bin/python3 to provide /usr/bin/python (python) in auto mode

    # 由于python3的优先级更高，所以当前执行``python``，就是python3了
    $ python -V
    Python 3.5.2

  4. 手动指定程序

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    $ update-alternatives --config python
    There are 2 choices for the alternative python (providing /usr/bin/python).

      Selection    Path              Priority   Status
    ------------------------------------------------------------
    * 0            /usr/bin/python3   200       auto mode
      1            /usr/bin/python2   100       manual mode
      2            /usr/bin/python3   200       manual mode

    Press <enter> to keep the current choice[*], or type selection number:

    输入对应数字即可完成自定义

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