Linux命令---- update-alternatives

 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

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