Linux常用命令-alternatives

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命令

alternatives

描述

maintain symbolic links determining default commands
更改默认命令链接

用法

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alternatives --install <link> <name> <path> <priority>
                    [--initscript <service>]
                    [--family <family>]
                    [--slave <link> <name> <path>]*
alternatives --remove <name> <path>
alternatives --auto <name>
alternatives --config <name>
alternatives --display <name>
alternatives --set <name> <path>
alternatives --list

选项

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Options:
COMMON OPTIONS
  --verbose  显示详情
  --quiet    静默模式,只显示错误信息,功能尚未实现
  --test     测试执行,功能尚未实现
  --keep-missing  如果切换到新版本时出错,则重新切换回上一个版本
  --altdir   指定软链接目录,默认目录/etc/alternatives
  --admindir 指定保存软链接信息的管理目录,默认目录/var/lib/alternatives
ACTIONS
  --install <link> <name> <path> <priority> [--slave slink sname spath] [--initscript service]...
            创建新的链接组,link是软链接路径,name是链接组名称,path是命令路径,priority是优先级
            NOTE: --initscript is a Red Hat Linux specific option.
            NOTE: --family is a Red Hat Linux specific option.
  --remove <name> <path>  从链接组中删除一个路径
  --set <name> <path> 命令模式切换链接
  --config <name>     交互模式切换链接
  --auto <name>       切换链接组为自动模式,即自动选择优先级最高的命令路径作为命令链接
  --display <name>    显示指定链接组的信息
  --list  显示所有链接组信息

注意

不要更改系统本身的软链接路径,否则可能导致系统部分组件无法运行,如更改/usr/bin/python的链接路径后可能导致yum命令无法使用
Ubuntu系统中的命令为update-alternatives

示例

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$ alternatives --version
alternatives version 1.7.6

# 同时安装java8和java11
$ yum install java-1.8.0-openjdk java-11-openjdk
# 查看默认的java版本为java8,模式为auto,因为java8的优先级更高priority 1800312
$ alternatives --display java |head
java - status is auto.
 link currently points to /usr/lib/jvm/java-1.8.0-openjdk-1.8.0.312.b07-1.el7_9.x86_64/jre/bin/java
/usr/lib/jvm/java-1.8.0-openjdk-1.8.0.312.b07-1.el7_9.x86_64/jre/bin/java - family java-1.8.0-openjdk.x86_64 priority 1800312
 slave alt-java: /usr/lib/jvm/java-1.8.0-openjdk-1.8.0.312.b07-1.el7_9.x86_64/jre/bin/alt-java
 slave jjs: /usr/lib/jvm/java-1.8.0-openjdk-1.8.0.312.b07-1.el7_9.x86_64/jre/bin/jjs
 slave keytool: /usr/lib/jvm/java-1.8.0-openjdk-1.8.0.312.b07-1.el7_9.x86_64/jre/bin/keytool
$ alternatives --display java |grep family
/usr/lib/jvm/java-1.8.0-openjdk-1.8.0.312.b07-1.el7_9.x86_64/jre/bin/java - family java-1.8.0-openjdk.x86_64 priority 1800312
/usr/lib/jvm/java-11-openjdk-11.0.13.0.8-1.el7_9.x86_64/bin/java - family java-11-openjdk.x86_64 priority 1
$ java -version
openjdk version "1.8.0_312"
OpenJDK Runtime Environment (build 1.8.0_312-b07)
OpenJDK 64-Bit Server VM (build 25.312-b07, mixed mode)
# 手动修改java版本
$ alternatives --config java
There are 2 programs which provide 'java'.
  Selection    Command
-----------------------------------------------
*+ 1           java-1.8.0-openjdk.x86_64 (/usr/lib/jvm/java-1.8.0-openjdk-1.8.0.312.b07-1.el7_9.x86_64/jre/bin/java)
   2           java-11-openjdk.x86_64 (/usr/lib/jvm/java-11-openjdk-11.0.13.0.8-1.el7_9.x86_64/bin/java)
Enter to keep the current selection[+], or type selection number: 2
# 查看java链接组,模式切换为了手动manual,当前链接指向为java11
$ update-alternatives --display java |head
java - status is manual.
 link currently points to /usr/lib/jvm/java-11-openjdk-11.0.13.0.8-1.el7_9.x86_64/bin/java
$ java -version
openjdk version "11.0.13" 2021-10-19 LTS
OpenJDK Runtime Environment 18.9 (build 11.0.13+8-LTS)
OpenJDK 64-Bit Server VM 18.9 (build 11.0.13+8-LTS, mixed mode, sharing)
# 还原模式为auto,版本自动切换为java8
$ alternatives --auto java
$ java -version
openjdk version "1.8.0_312"
OpenJDK Runtime Environment (build 1.8.0_312-b07)
OpenJDK 64-Bit Server VM (build 25.312-b07, mixed mode)
# 由于java默认配置的是链接组名family,所以切换的时候也可以指定组名
$ grep @ /var/lib/alternatives/java
@java-1.8.0-openjdk.x86_64@1800312
@java-11-openjdk.x86_64@1
# 非交互式修改java版本为11,注意必须和配置中的名称或路径一致,也可以参考--config显示的Command字段
$ alternatives --set java java-11-openjdk.x86_64

# 注意以下命令仅用于演示创建和链接组,不推荐更改系统默认的/usr/bin/python命令链接
# 安装python3
$ yum install python36
# 创建python命令链接组
$ alternatives --install /usr/bin/python python /usr/bin/python2.7 20
$ alternatives --install /usr/bin/python python /usr/bin/python3.6 10
# 默认模式为auto,默认版本为python2.7,优先级为20更高
$ alternatives --display python
python - status is auto.
 link currently points to /usr/bin/python2.7
/usr/bin/python2.7 - priority 20
/usr/bin/python3.6 - priority 10
Current 'best' version is /usr/bin/python2.7.
$ python -V
Python 2.7.5
# 更改python版本为3.6,即默认的系统链接/usr/bin/python指向了python3.6,会导致yum命令无法使用
# 使用交互式命令--config修改版本
$ alternatives --config python
There are 2 programs which provide 'python'.
  Selection    Command
-----------------------------------------------
*+ 1           /usr/bin/python2.7
   2           /usr/bin/python3.6
Enter to keep the current selection[+], or type selection number: 2
$ ll /usr/bin/python
lrwxrwxrwx 1 root root 24 Jan 18 18:57 /usr/bin/python -> /etc/alternatives/python
$ ll /etc/alternatives/python
lrwxrwxrwx 1 root root 18 Jan 18 18:57 /etc/alternatives/python -> /usr/bin/python3.6
$ ll /usr/bin/python3.6
-rwxr-xr-x 2 root root 11328 Nov 17  2020 /usr/bin/python3.6
$ yum repolist
  File "/bin/yum", line 30
    except KeyboardInterrupt, e:
                            ^
SyntaxError: invalid syntax
# 更改python版本为默认的2.7
# 使用非交互式命令--set直接修改版本,必须和配置的路径或名称对应
$ alternatives --set python /usr/bin/python2.7
$ python -V
Python 2.7.5
# 查看python命令链接组的配置信息
$  more /var/lib/alternatives/python
manual
/usr/bin/python
/usr/bin/python2.7
20
/usr/bin/python3.6
10
# 删除链接组
$ alternatives --remove python /usr/bin/python2.7
$ alternatives --remove python /usr/bin/python3.6
$ alternatives --display python
# 注意如果更改了系统默认的命令链接,--remove后会删除链接文件,所以需要重新手动创建命令链接
$ ll /usr/bin/python
ls: cannot access /usr/bin/python: No such file or directory
$ ln -s /usr/bin/python2.7 /usr/bin/python
$ python -V
Python 2.7.5