Linux 命令 alternatives和update-alternatives

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参考：

http://lionbule.iteye.com/blog/717722

http://manpages.ubuntu.com/manpages/oneiric/man8/update-alternatives.8.html

http://blog.chinaunix.net/uid-9185047-id-445077.html

http://blog.csdn.net/heyutao007/article/details/5441482

======================================================

1. alternatives

2. update-alternatives

3、实例

举个使用例子吧，改变系统bin默认java的指向：

4、总结

1、alternatives和update-alternatives其实一个东东，都指向alternatives。

2、网上找了一个说明（也不知道对不对）

用于存放系统的一些默认打开程序的信息和配置, 比如默认的编辑器、默认的网络浏览器、 默认的图形登陆器、默认的鼠标指针  等。

=====================================================================

Linux 发展到今天，可用的软件已经非常多了。这样自然会有一些软件的功能大致上相同。例如，同样是编辑器，就有 nvi、vim、emacs、nano，而且我说的这些还只是一部分。大多数情况下，这样的功能相似的软件都是同时安装在系统里的，可以用它们的名称来执行。例如，要执行 vim，只要在终端下输入 vim 并按回车就可以了。不过，有些情况下我们需要用一个相对固定的命令调用这些程序中的一个。例如，当我们写一个脚本程序时，只要写下 editor，而不希望要为“编辑器是哪个”而操心。Debian 提供了一种机制来解决这个问题，而 update-alternatives 就是用来实现这种机制的。
首先要介绍的参数是 --display。它使我们可以看到一个命令的所有可选命令。执行：
update-alternatives --display editor
可以看到我的机器上的所有可以用来被 editor 链接的命令。
--config。这个选项使我们可以选择其中一个命令程序来作为editor,执行：
update-alternatives --config editor

首先，update-alternatives 在一般情况下是由postinst 和 prerm 这样的安装脚本自动调用的，所以一个 alternative 的状态有两种：自动和手动。每个 alternative 的初始状态都是自动。如果系统发现管理员手动修改了一个 alternative，它的状态就从自动变成了手动，这样安装脚本就不会更新它了。如果你希望将一个 alternative 变回自动，只要执行代码:
update-alternatives --auto editor

general name -- 这是指一系列功能相似的程序的“公用”名字（包括绝对路径），比如 /usr/bin/editor。
link -- 这是指一个 alternative 在 /etc/alternative 中的名字，比如 editor。
alternative -- 顾名思义，这是指一个可选的程序所在的路径（包括绝对路径），比如 /usr/bin/vim。

-auto，--display 和 --config 跟的都是 link。我们要说的第三个概念是优先级。这个 比较简单，当然优先级越高的程序越好啦（在大多数情况下，我不想争论）最后一个概 念是主和从的 alternative。想想看，你将 /usr/bin/editor 链接到了 vim，可是当你执 行 man editor 时看到的却是 emacs 的 manpage，你会做何感想呢？这就引出了主和从 alternative 的概念了：当更新主的 alternative 时，从的 alternative 也会被更新。

另外两个选项：
第一个是 --install。它的格式是：  代码:

gen，link，alt，pri 分别是我们上面说过的。如果需要从的 alternative，你可以用--slave 加在后面。如果你在向一个已经存在的 alternative 组中添加新的 alternatives，该命令会把这些 alternatives 加入到这个已经存在的 alternative 组的 列表中，并用新的可选命令作为新的命令；否则，将会建立一个新的自动的 alternative组。

呜呼！我加入了一个错误的 alternative。我不想要这个 alternative 了。在这种情况
下，可以执行下面的命令：代码:

name 是一个在 /etc/alternatives 中的名字，也就是上面的 link，而 path 是希望删除 的可选程序名的绝对路径名（放心，这样只是从列表中删除了这个程序，并不会真的从硬盘 上删除程序的可执行文件）。如果从一个 alternative 组中删除了一个正在被链接的程序并且这个组仍然没有变成空的，update-alternatives 会自动用一个具有其他优先级的可选程序代替原来的程序。如果这个组变成空的了，那么连这个 alternative 组都会被移 除。如果删除的程序没有被链接，则只有有关这个程序的信息会被移除。

=====================================================================

As @Tommy suggested, you should use update-alternatives.
It assigns values to every software of a family, so that it defines the order in which the applications will be called.

It is used to maintain different versions of the same software on a system. In your case, you will be able to use several declinations of gcc, and one will be favoured.

To figure out the current priorities of gcc, type in the command pointed out by @tripleee's comment:

update-alternatives --query gcc

Now, note the priority attributed to gcc-4.4 because you'll need to give a higher one to gcc-3.3.
To set your alternatives, you should have something like this (assuming you gcc installation is located at /usr/bin/gcc-3.3, and gcc-4.4's priority is less than 50):

update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-3.350

--edit--

Finally, you can also use the interactive interface of update-alternatives to easily switch between versions. Type update-alternatives --config gcc to be asked to choose the gcc version you want to use among those installed.

--edit 2 --

Now, to fix the CXX environment variable systemwide, you need to put the line indicated by @DipSwitch's in your .bashrc file (this will apply the change only for your user, which is safer in my opinion):

echo 'export CXX=/usr/bin/gcc-3.3'>>~/.bashrc

Here's a complete example of jHackTheRipper's answer for the TL;DR crowd. :-) In this case, I wanted to run g++-4.5 on an Ubuntu system that defaults to 4.6. As root:

apt-get install g++-4.5
update-alternatives --install /usr/bin/g++ g++/usr/bin/g++-4.6100
update-alternatives --install /usr/bin/g++ g++/usr/bin/g++-4.550
update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-4.6100
update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-4.550
update-alternatives --install /usr/bin/cpp cpp-bin /usr/bin/cpp-4.6100
update-alternatives --install /usr/bin/cpp cpp-bin /usr/bin/cpp-4.550
update-alternatives --set g++/usr/bin/g++-4.5
update-alternatives --set gcc /usr/bin/gcc-4.5
update-alternatives --set cpp-bin /usr/bin/cpp-4.5

Here, 4.6 is still the default (aka "auto mode"), but I explicitly switch to 4.5 temporarily (manual mode). To go back to 4.6:

update-alternatives --auto g++
update-alternatives --auto gcc
update-alternatives --auto cpp-bin

(Note the use of cpp-bin instead of just cpp. Ubuntu already has a cpp alternative with a master link of /lib/cpp. Renaming that link would remove the /lib/cpp link, which could break scripts.)

=====================================================================

AME

update-alternatives - maintain symbolic links determining default
commands

SYNOPSIS

update-alternatives [options] command

DESCRIPTION

update-alternatives creates, removes, maintains and displays
information about the symbolic links comprising the Debian alternatives
system. It is possible for several programs fulfilling the same or similar
functions to be installed on a single system at the same time. For
example, many systems have several text editors installed at once.
This gives choice to the users of a system, allowing each to use a
different editor, if desired, but makes it difficult for a program to
make a good choice for an editor to invoke if the user has not
specified a particular preference. Debian's alternatives system aims to solve this problem. A generic
name in the filesystem is shared by all files providing interchangeable
functionality. The alternatives system and the system administrator
together determine which actual file is referenced by this generic
name. For example, if the text editors ed(1) and nvi(1) are both
installed on the system, the alternatives system will cause the generic
name /usr/bin/editor to refer to /usr/bin/nvi by default. The system
administrator can override this and cause it to refer to /usr/bin/ed
instead, and the alternatives system will not alter this setting until
explicitly requested to do so. The generic name is not a direct symbolic link to the selected
alternative. Instead, it is a symbolic link to a name in the
alternatives directory, which in turn is a symbolic link to the actual
file referenced. This is done so that the system administrator's
changes can be confined within the /etc directory: the FHS (q.v.) gives
reasons why this is a Good Thing. When each package providing a file with a particular functionality is
installed, changed or removed, update-alternatives is called to update
information about that file in the alternatives system.
update-alternatives is usually called from the postinst (configure) or
prerm (install) scripts in Debian packages. It is often useful for a number of alternatives to be synchronised, so
that they are changed as a group; for example, when several versions of
the vi(1) editor are installed, the man page referenced by
/usr/share/man/man1/vi.1 should correspond to the executable referenced
by /usr/bin/vi. update-alternatives handles this by means of master
and slave links; when the master is changed, any associated slaves are
changed too. A master link and its associated slaves make up a link
group. Each link group is, at any given time, in one of two modes: automatic
or manual. When a group is in automatic mode, the alternatives system
will automatically decide, as packages are installed and removed,
whether and how to update the links. In manual mode, the alternatives
system will retain the choice of the administrator and avoid changing
the links (except when something is broken). Link groups are in automatic mode when they are first introduced to the
system. If the system administrator makes changes to the system's
automatic settings, this will be noticed the next time
update-alternatives is run on the changed link's group, and the group
will automatically be switched to manual mode. Each alternative has a priority associated with it. When a link group
is in automatic mode, the alternatives pointed to by members of the
group will be those which have the highest priority. When using the --config option, update-alternatives will list all of
the choices for the link group of which given name is the master
alternative name. The current choice is marked with a '*'. You will
then be prompted for your choice regarding this link group. Depending
on the choice made, the link group might no longer be in auto mode. You
will need to use the --auto option in order to return to the automatic
mode (or you can rerun --config and select the entry marked as
automatic). If you want to configure non-interactively you can use the --set option
instead (see below). Different packages providing the same file need to do so cooperatively.
In other words, the usage of update-alternatives is mandatory for all
involved packages in such case. It is not possible to override some
file in a package that does not employ the update-alternatives
mechanism.

TERMINOLOGY

Since the activities of update-alternatives are quite involved, some
specific terms will help to explain its operation. generic name (or alternative link)
A name, like /usr/bin/editor, which refers, via the alternatives
system, to one of a number of files of similar function. alternative name
The name of a symbolic link in the alternatives directory. alternative (or alternative path)
The name of a specific file in the filesystem, which may be made
accessible via a generic name using the alternatives system. alternatives directory
A directory, by default /etc/alternatives, containing the
symlinks. administrative directory
A directory, by default /var/lib/dpkg/alternatives, containing
update-alternatives' state information. link group
A set of related symlinks, intended to be updated as a group. master link
The alternative link in a link group which determines how the
other links in the group are configured. slave link
An alternative link in a link group which is controlled by the
setting of the master link. automatic mode
When a link group is in automatic mode, the alternatives system
ensures that the links in the group point to the highest
priority alternative appropriate for the group. manual mode
When a link group is in manual mode, the alternatives system
will not make any changes to the system administrator's
settings.

COMMANDS

--install link name path priority [--slave link name path]...
Add a group of alternatives to the system. link is the generic
name for the master link, name is the name of its symlink in the
alternatives directory, and path is the alternative being
introduced for the master link. The arguments after --slave are
the generic name, symlink name in the alternatives directory and
the alternative path for a slave link. Zero or more --slave
options, each followed by three arguments, may be specified.
Note that the master alternative must exist or the call will
fail. However if a slave alternative doesn't exist, the
corresponding slave alternative link will simply not be
installed (a warning will still be displayed). If some real file
is installed where an alternative link has to be installed, it
is kept unless --force is used. If the alternative name specified exists already in the
alternatives system's records, the information supplied will be
added as a new set of alternatives for the group. Otherwise, a
new group, set to automatic mode, will be added with this
information. If the group is in automatic mode, and the newly
added alternatives' priority is higher than any other installed
alternatives for this group, the symlinks will be updated to
point to the newly added alternatives. --set name path
Set the program path as alternative for name. This is
equivalent to --config but is non-interactive and thus
scriptable. --remove name path
Remove an alternative and all of its associated slave links.
name is a name in the alternatives directory, and path is an
absolute filename to which name could be linked. If name is
indeed linked to path, name will be updated to point to another
appropriate alternative (and the group is put back in automatic
mode), or removed if there is no such alternative left.
Associated slave links will be updated or removed,
correspondingly. If the link is not currently pointing to path,
no links are changed; only the information about the alternative
is removed. --remove-all name
Remove all alternatives and all of their associated slave links.
name is a name in the alternatives directory. --all Call --config on all alternatives. It can be usefully combined
with --skip-auto to review and configure all alternatives which
are not configured in automatic mode. Broken alternatives are
also displayed. Thus a simple way to fix all broken
alternatives is to call yes '' | update-alternatives --force
--all. --auto name
Switch the link group behind the alternative for name to
automatic mode. In the process, the master symlink and its
slaves are updated to point to the highest priority installed
alternatives. --display name
Display information about the link group. Information displayed
includes the group's mode (auto or manual), which alternative
the master link currently points to, what other alternatives are
available (and their corresponding slave alternatives), and the
highest priority alternative currently installed. --get-selections
List all master alternative names (those controlling a link
group) and their status. Each line contains up to 3 fields
(separated by one or more spaces). The first field is the
alternative name, the second one is the status (either "auto" or
"manual"), and the last one contains the current choice in the
alternative (beware: it's a filename and thus might contain
spaces). --set-selections
Read configuration of alternatives on standard input in the
format generated by update-alternatives --get-selections and
reconfigure them accordingly. --query name
Display information about the link group like --display does,
but in a machine parseable way (see section QUERY FORMAT below). --list name
Display all targets of the link group. --config name
Show available alternatives for a link group and allow the user
to interactively select which one to use. The link group is
updated. --help Show the usage message and exit. --version
Show the version and exit.

OPTIONS

--altdir directory
Specifies the alternatives directory, when this is to be
different from the default. --admindir directory
Specifies the administrative directory, when this is to be
different from the default. --log file
Specifies the log file, when this is to be different from the
default (/var/log/alternatives.log). --force
Let update-alternatives replace any real file that is installed
where an alternative link has to be installed. --skip-auto
Skip configuration prompt for alternatives which are properly
configured in automatic mode. This option is only relevant with
--config or --all. --verbose
Generate more comments about what update-alternatives is doing. --quiet
Don't generate any comments unless errors occur.

ENVIRONMENT

DPKG_ADMINDIR
If set and the --admindir option has not been specified, it will
be used as the base administrative directory.

FILES

/etc/alternatives/
The default alternatives directory. Can be overridden by the
--altdir option. /var/lib/dpkg/alternatives/
The default administration directory. Can be overridden by the
--admindir option.

EXIT STATUS

0 The requested action was successfully performed. 2 Problems were encountered whilst parsing the command line or
performing the action.

QUERY FORMAT

The update-alternatives --query format is using an RFC822-like flat
format. It's made of n + 1 blocks where n is the number of alternatives
available in the queried link group. The first block contains the
following fields: Link: <link>
The generic name of the alternative. Status: <status>
The status of the alternative (auto or manual). Best: <best choice>
The path of the best alternative for this link group. Not
present if there is no alternatives available. Value: <currently selected alternative>
The path of the currently selected alternative. It can also take
the magic value none. It is used if the link doesn't exist. The other blocks describe the available alternatives in the
queried link group: Alternative: <path of this alternative>
Path to this block's alternative. Priority: <priority value>
Value of the priority of this alternative. Slaves: <list of slaves>
When this header is present, the next lines hold all
slave alternatives associated to the master link of the
alternative. There is one slave per line. Each line
contains one space, the generic name of the slave
alternative, another space, and the path to the slave
alternative. Example
$ update-alternatives --query editor
Link: editor
Status: auto
Best: /usr/bin/vim.gtk
Value: /usr/bin/vim.gtk Alternative: /bin/ed
Priority: -100
Slaves:
editor.1.gz /usr/share/man/man1/ed.1.gz Alternative: /usr/bin/vim.gtk
Priority: 50
Slaves:
editor.1.gz /usr/share/man/man1/vim.1.gz
editor.ru.1.gz /usr/share/man/ru/man1/vim.1.gz
editor.pl.ISO8859-2.1.gz /usr/share/man/pl.ISO8859-2/man1/vim.1.gz
editor.it.ISO8859-1.1.gz /usr/share/man/it.ISO8859-1/man1/vim.1.gz
editor.pl.UTF-8.1.gz /usr/share/man/pl.UTF-8/man1/vim.1.gz
editor.it.1.gz /usr/share/man/it/man1/vim.1.gz
editor.fr.UTF-8.1.gz /usr/share/man/fr.UTF-8/man1/vim.1.gz
editor.fr.1.gz /usr/share/man/fr/man1/vim.1.gz
editor.it.UTF-8.1.gz /usr/share/man/it.UTF-8/man1/vim.1.gz
editor.pl.1.gz /usr/share/man/pl/man1/vim.1.gz
editor.fr.ISO8859-1.1.gz /usr/share/man/fr.ISO8859-1/man1/vim.1.gz

DIAGNOSTICS

With --verbose update-alternatives chatters incessantly about
its activities on its standard output channel. If problems
occur, update-alternatives outputs error messages on its
standard error channel and returns an exit status of 2. These
diagnostics should be self-explanatory; if you do not find them
so, please report this as a bug.

EXAMPLES

There are several packages which provide a text editor
compatible with vi, for example nvi and vim. Which one is used
is controlled by the link group vi, which includes links for the
program itself and the associated manpage. To display the available packages which provide vi and the
current setting for it, use the --display action: update-alternatives --display vi To choose a particular vi implementation, use this command as
root and then select a number from the list: update-alternatives --config vi To go back to having the vi implementation chosen automatically,
do this as root: update-alternatives --auto vi

BUGS

If you find a bug, please report it using the Debian bug-
tracking system. If you find any discrepancy between the operation of
update-alternatives and this manual page, it is a bug, either in
the implementation or the documentation; please report it.

AUTHORS

Copyright (C) 1995 Ian Jackson
Copyright (C) 2009 Raphael Hertzog This is free software; see the GNU General Public Licence
version 2 or later for copying conditions. There is NO WARRANTY. This manual page is copyright 1997,1998 Charles Briscoe-Smith
and others. This is free documentation; see the GNU General Public Licence
version 2 or later for copying conditions. There is NO WARRANTY.

SEE ALSO

ln(1), FHS, the Filesystem Hierarchy Standard.