參考：
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
Linux代碼 
- alternatives version 1.3.13.5.EL4 - Copyright (C) 2001 Red Hat, Inc.
- This may be freely redistributed under the terms of the GNU Public License.
- usage: alternatives --install <link> <name> <path> <priority>
- [--initscript <service>]
- [--slave <link> <name> <path>]*
- alternatives --remove <name> <path>
- alternatives --auto <name>
- alternatives --config <name>
- alternatives --display <name>
- alternatives --set <name> <path>
- common options: --verbose --test --help --usage --version
- --altdir <directory> --admindir <directory>
2. update-alternatives
Linux代碼
- alternatives version 1.3.13.5.EL4 - Copyright (C) 2001 Red Hat, Inc.
- This may be freely redistributed under the terms of the GNU Public License.
- usage: alternatives --install <link> <name> <path> <priority>
- [--initscript <service>]
- [--slave <link> <name> <path>]*
- alternatives --remove <name> <path>
- alternatives --auto <name>
- alternatives --config <name>
- alternatives --display <name>
- alternatives --set <name> <path>
- common options: --verbose --test --help --usage --version
- --altdir <directory> --admindir <directory>
3、實例
舉個使用例子吧，改變系統bin默認java的指向：
Linux代碼
- 安裝
- #alternatives --install /usr/bin/java java /home/server/jdk1.6.0_21/bin/java 300
- 查詢
- #alternatives --display java
- 刪除
- #alternatives --remove java /home/server/jdk1.6.0_21/bin/java
4、總結
1、alternatives和update-alternatives其實一個東東，都指向alternatives。
2、網上找了一個說明（也不知道對不對）
用於存放系統的一些默認打開程序的信息和配置, 比如默認的編輯器、默認的網絡瀏覽器、 默認的圖形登陸器、默認的鼠標指針 等。
=====================================================================
Linux 發展到今天，可用的軟件已經非常多了。這樣自然會有一些軟件的功能大致上相同。例如，同樣是編輯器，就有 nvi、vim、emacs、nano，而且我說的這些還只是一部分。大多數情況下，這樣的功能相似的軟件都是同時安裝在系統里的，可以用它們的名稱來執行。例如，要執行 vim，只要在終端下輸入 vim 並按回車就可以了。不過，有些情況下我們需要用一個相對固定的命令調用這些程序中的一個。例如，當我們寫一個腳本程序時，只要寫下 editor，而不希望要為“編輯器是哪個”而操心。Debian 提供了一種機制來解決這個問題，而 update-alternatives 就是用來實現這種機制的。
首先要介紹的參數是 --display。它使我們可以看到一個命令的所有可選命令。執行：
update-alternatives --display editor
可以看到我的機器上的所有可以用來被 editor 鏈接的命令。
--config。這個選項使我們可以選擇其中一個命令程序來作為editor,執行：
update-alternatives --config editor
首先，update-alternatives 在一般情況下是由postinst 和 prerm 這樣的安裝腳本自動調用的，所以一個 alternative 的狀態有兩種：自動和手動。每個 alternative 的初始狀態都是自動。如果系統發現管理員手動修改了一個 alternative，它的狀態就從自動變成了手動，這樣安裝腳本就不會更新它了。如果你希望將一個 alternative 變回自動，只要執行代碼:
update-alternatives --auto editor
general name -- 這是指一系列功能相似的程序的“公用”名字（包括絕對路徑），比如 /usr/bin/editor。
link -- 這是指一個 alternative 在 /etc/alternative 中的名字，比如 editor。
alternative -- 顧名思義，這是指一個可選的程序所在的路徑（包括絕對路徑），比如 /usr/bin/vim。
-auto，--display 和 --config 跟的都是 link。我們要說的第三個概念是優先級。這個 比較簡單，當然優先級越高的程序越好啦（在大多數情況下，我不想爭論）最后一個概 念是主和從的 alternative。想想看，你將 /usr/bin/editor 鏈接到了 vim，可是當你執 行 man editor 時看到的卻是 emacs 的 manpage，你會做何感想呢？這就引出了主和從 alternative 的概念了：當更新主的 alternative 時，從的 alternative 也會被更新。
另外兩個選項：
第一個是 --install。它的格式是： 代碼:
update-alternatives --install gen link alt pri [--slave sgen slink salt] ...
gen，link，alt，pri 分別是我們上面說過的。如果需要從的 alternative，你可以用--slave 加在后面。如果你在向一個已經存在的 alternative 組中添加新的 alternatives，該命令會把這些 alternatives 加入到這個已經存在的 alternative 組的 列表中，並用新的可選命令作為新的命令；否則，將會建立一個新的自動的 alternative組。
嗚呼！我加入了一個錯誤的 alternative。我不想要這個 alternative 了。在這種情況
下，可以執行下面的命令：代碼:
update-alternatives --remove name path
name 是一個在 /etc/alternatives 中的名字，也就是上面的 link，而 path 是希望刪除 的可選程序名的絕對路徑名（放心，這樣只是從列表中刪除了這個程序，並不會真的從硬盤 上刪除程序的可執行文件）。如果從一個 alternative 組中刪除了一個正在被鏈接的程序並且這個組仍然沒有變成空的，update-alternatives 會自動用一個具有其他優先級的可選程序代替原來的程序。如果這個組變成空的了，那么連這個 alternative 組都會被移 除。如果刪除的程序沒有被鏈接，則只有有關這個程序的信息會被移除。
=====================================================================
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 gccNow, 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-4.5Here, 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.