update-alternatives命令zz

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 [ ...] Commands:   --install     [--slave ] ...                            add a group of alternatives to the system.   --remove    remove from the group alternative.   --remove-all       remove group from the alternatives system.   --auto             switch the master link to automatic mode.   --display          display information about the group.   --list             display all targets of the group.   --config           show alternatives for the group and ask the                            user to select which one to use.   --set       set as alternative for .   --all                    call --config on all alternatives. is the symlink pointing to /etc/alternatives/.   (e.g. /usr/bin/pager) is the master name for this link group.   (e.g. pager) is the location of one of the alternative target files.   (e.g. /usr/bin/less) is an integer; options with higher numbers have higher priority in   automatic mode. Options:   --altdir      change the alternatives directory.   --admindir    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 列出可用的候选命令, 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 可用来显示和修改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