http://blog.sina.com.cn/s/blog_3fc8570201000cny.html
对于一些程序, Debian提供了这些程序的多种不同实现. 如awk提供了gawk,nawk, java提供了gij, sun-java5, sun-java6等. 同时Debian也提供了update-alternatives来切换多个不同的实现.
不使用任何参数运行update-alternatives命令, 则显示命令的帮助
| blueice:~# update-alternatives update-alternatives: need --display, --config, --set, --install, --remove, --all, --remove-all or --auto 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. --list <name> display all targets of the <name> group. --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. --test don't do anything, just demonstrate. --verbose verbose operation, more output. --quiet quiet operation, minimal output. --help show this help message. --version show the version. |
update-alternatives --list <command> 列出<command>可用的候选命令, 例如
| blueice:~# update-alternatives --list java /usr/bin/gij-wrapper-4.0 /usr/bin/gij-wrapper-4.1 /usr/lib/jvm/java-gcj/jre/bin/java /usr/lib/jvm/java-6-sun/jre/bin/java |
update-alternatives --config <command>可用来显示和修改<command>实际指向的候选命令, 例如
| blueice:~# update-alternatives --config java There are 4 alternatives which provide `java'. Selection Alternative ----------------------------------------------- 1 /usr/bin/gij-wrapper-4.0 2 /usr/bin/gij-wrapper-4.1 + 3 /usr/lib/jvm/java-gcj/jre/bin/java * 4 /usr/lib/jvm/java-6-sun/jre/bin/java Press enter to keep the default[*], or type selection number: 4 Using `/usr/lib/jvm/java-6-sun/jre/bin/java' to provide `java'. |
其中, '+'表示推荐的候选命令, '*'表示实际指向的候选命令.
update-alternatives --all则可以配置系统上所有的候选命令.
事实上, update-alternatives通过修改/etc/alternatives下的链接来实现候选命令的更改.
blueice:~# ls -al /usr/bin/java
lrwxrwxrwx 1 root root 22 Aug 30 2005 /usr/bin/java -> /etc/alternatives/java
blueice:~# ls -al /etc/alternatives/java
lrwxrwxrwx 1 root root 36 Jul 24 22:21 /etc/alternatives/java -> /usr/lib/jvm/java-6-sun/jre/bin/java
blueice:~# s -al /usr/lib/jvm/java-6-sun/jre/bin/java
-rwxr-xr-x 1 root root 47116 Jun 15 07:26 /usr/lib/jvm/java-6-sun/jre/bin/java
