ubuntu下update-alternatives命令的使用
在ubuntu系统中，update-alternatives是专门维护系统命令链接符的工具，其可以对某个工具的多个软件版本进行管理，通过它可以很方便的设置系统默认使用哪个命令的哪个软件版本。
在命令行中直接输入update-alternatives --help命令，
[19:19:24]@~$ 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.
Options:
--altdir <directory> change the alternatives directory.
--admindir <directory> change the administrative directory.
--log <file> change the log file.
--force allow replacing files with alternative links.
--skip-auto skip prompt for alternatives correctly configured
in automatic mode (relevant for --config only)
--verbose verbose operation, more output.
--quiet quiet operation, minimal output.
--help show this help message.
--version show the version.
[19:19:28]@~$
其工作原理如下：系统路径下，/usr/bin/<name>这个软链接，指向了/etc/alternatives/<name>这个文件，其其实也是个软链接，指向了该<name>命令的实际可执行文件；如下：
lrwxrwxrwx 1 root root 22 5月 6 2015 /usr/bin/java -> /etc/alternatives/java*
lrwxrwxrwx 1 root root 70 10月 28 14:11 /etc/alternatives/java -> /home/likewise-open/SPREADTRUM/hunter.ding/tools/jdk1.8.0_112/bin/java*
可见，通过两次软链接，我们可以定位到实际的java文件；后面我们针对这个软件版本的管理都是通过改变/etc/alternatives/ --> /实际可执行文件 的软链接来进行的。
来看一个例子，在上面的java可执行组中，修改一下java的版本：
[19:37:17]@~$ java -version
java version "1.8.0_112"
Java(TM) SE Runtime Environment (build 1.8.0_112-b15)
Java HotSpot(TM) Server VM (build 25.112-b15, mixed mode)
[19:37:20]@~$ sudo update-alternatives --config java
There are 3 choices for the alternative java (providing /usr/bin/java).
Selection Path Priority Status
------------------------------------------------------------
* 0 /home/likewise-open/SPREADTRUM/hunter.ding/tools/jdk1.8.0_112/bin/java 1062 auto mode
1 /home/likewise-open/SPREADTRUM/hunter.ding/tools/jdk1.8.0_112/bin/java 1062 manual mode
2 /usr/lib/jvm/java-6-openjdk-amd64/jre/bin/java 1060 manual mode
3 /usr/lib/jvm/java-7-openjdk-amd64/jre/bin/java 1061 manual mode
Press enter to keep the current choice[*], or type selection number: 2
update-alternatives: using /usr/lib/jvm/java-6-openjdk-amd64/jre/bin/java to provide /usr/bin/java (java) in manual mode.
[19:37:24]@~$ java -version
java version "1.6.0_35"
OpenJDK Runtime Environment (IcedTea6 1.13.7) (6b35-1.13.7-1ubuntu0.12.04.2)
OpenJDK 64-Bit Server VM (build 23.25-b01, mixed mode)
[19:37:26]@~$
可见我们已经将java的版本从1.8切换到1.6了，这样我们来看一下软链接实例：
lrwxrwxrwx 1 root root 22 5月 6 2015 /usr/bin/java -> /etc/alternatives/java*
lrwxrwxrwx 1 root root 46 10月 28 19:37 /etc/alternatives/java -> /usr/lib/jvm/java-6-openjdk-amd64/jre/bin/java*
呵呵，/usr/bin/java -> /etc/alternatives/java* 的链接未改变，而改变的是/etc/alternatives/java -> /usr/lib/jvm/java-6-openjdk-amd64/jre/bin/java*。
主要的几个命令用法如下：
- --install <link> <name> <path> <priority>
向系统中添加一个新的alternatives组，
link：指向/etc/alternatives/<name>的符号引用
name：这个链接组的名称
path：这个命令对应的可执行文件的实际路径
priority：优先级，在auto模式下，数字较大的选项有较高的优先级
示例： sudo update-alternatives --install /usr/bin/java java /home/likewise-open/SPREADTRUM/hunter.ding/tools/jdk1.8.0_112/bin/java 1062
- --remove <name> <path> remove <path> from the <name> group alternative.
移除系统中注册的某个<name>的某个软件版本<path>
- --display <name> display information about the <name> group.
- --list <name> display all targets of the <name> group.
显示命令<name>的信息及目标文件
[19:53:34]@~$ update-alternatives --display java
java - manual mode
link currently points to /usr/lib/jvm/java-6-openjdk-amd64/jre/bin/java
/home/likewise-open/SPREADTRUM/hunter.ding/tools/jdk1.8.0_112/bin/java - priority 1062
/usr/lib/jvm/java-6-openjdk-amd64/jre/bin/java - priority 1060
/usr/lib/jvm/java-7-openjdk-amd64/jre/bin/java - priority 1061
Current 'best' version is '/home/likewise-open/SPREADTRUM/hunter.ding/tools/jdk1.8.0_112/bin/java
[19:53:49]@~$ update-alternatives --list java
/home/likewise-open/SPREADTRUM/hunter.ding/tools/jdk1.8.0_112/bin/java
/usr/lib/jvm/java-6-openjdk-amd64/jre/bin/java
/usr/lib/jvm/java-7-openjdk-amd64/jre/bin/java
- --config <name> show alternatives for the <name> group and ask the user to select which one to use.
配置命令<name>的版本，如下：
[19:55:48]@~$ sudo update-alternatives --config java
There are 3 choices for the alternative java (providing /usr/bin/java).
Selection Path Priority Status
------------------------------------------------------------
0 /home/likewise-open/SPREADTRUM/hunter.ding/tools/jdk1.8.0_112/bin/java 1062 auto mode
* 1 /home/likewise-open/SPREADTRUM/hunter.ding/tools/jdk1.8.0_112/bin/java 1062 manual mode
2 /usr/lib/jvm/java-6-openjdk-amd64/jre/bin/java 1060 manual mode
3 /usr/lib/jvm/java-7-openjdk-amd64/jre/bin/java 1061 manual mode
Press enter to keep the current choice[*], or type selection number: 0
[19:55:56]@~$
这样就使用java1.8作为java命令的默认版本。
