Linux 命令 alternatives和update-alternatives

参考：

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

1. alternatives

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

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、实例

举个使用例子吧，改变系统bin默认java的指向：

安装
#alternatives --install /usr/bin/java java /home/server/jdk1.6.0_21/bin/java 300
查询
#alternatives --display java
删除
#alternatives --remove java /home/server/jdk1.6.0_21/bin/java

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:

update-alternatives --install gen link alt pri [--slave sgen slink salt] ...

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:

update-alternatives --remove name path

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.