hard link 是直接连接源文件的 inode 的. 而 soft link 不是
如果是创建的 hard link , 修改了源文件的内容, link 里面的内容也会跟着一起修改

注意, 在使用 uniq 的时候,前面要先 sort . 即 file | grep xxx | sort | uniq

sed 's/^$/ /g'

^表示一行的开头, $表示一行的结尾 /^$/表示这一行为空

sed 's/\t/ /g'

\t 表示 tap 键

sed '8!s/var1/var2/'

8!表示出了第八行,其他的都替换

dmesg 表示 error 或者 fail , warning 的 message; 即 display message
netstat -rnv 查 gateway 等网络信息
free physical mem and swap virtual mem

1. 安装 httpd yum install httpd

[root@localhost ~]# yum install httpd
Last metadata expiration check: 1 day, 23:17:30 ago on Mon 20 Sep 2021 05:52:58 PM EDT.
Package httpd-2.4.37-40.module_el8.5.0+852+0aafc63b.x86_64 is already installed.
Dependencies resolved.
Nothing to do.
Complete!

2. 查询 rpm rpm -qa | grep httpd

[root@localhost ~]# rpm -qa | grep httpd
centos-logos-httpd-85.8-1.el8.noarch
httpd-filesystem-2.4.37-40.module_el8.5.0+852+0aafc63b.noarch
httpd-tools-2.4.37-40.module_el8.5.0+852+0aafc63b.x86_64
httpd-2.4.37-40.module_el8.5.0+852+0aafc63b.x86_64

3. 查看历史记录 yum history

[root@localhost ~]# yum history
ID     | Command line              | Date and time    | Action(s)      | Altered
--------------------------------------------------------------------------------
    11 | -y install sysstat        | 2021-09-12 20:45 | Install        |    2   
    10 | -y install git            | 2021-09-10 19:19 | Install        |    6   
     9 | -y install telnet         | 2021-09-10 11:12 | Install        |    1   
     8 | install httpd             | 2021-09-07 15:40 | Install        |    9   
     7 | -y install lrzsz          | 2021-08-21 08:17 | Install        

4. 选择你需要 downgrade 的 yum history undo 8

[root@localhost ~]# yum history undo 8
Last metadata expiration check: 1 day, 23:58:58 ago on Mon 20 Sep 2021 05:52:58 PM EDT.
Dependencies resolved.
================================================================================
 Package          Arch   Version                               Repository  Size
================================================================================                  

Complete!

5. 再查询安装包 rpm -qa | grep httpd 已经没有了

[root@localhost ~]# rpm -qa | grep httpd
[root@localhost ~]#

[root@localhost ~]# yum update
CentOS Stream 8 - AppStream                     7.1 kB/s | 4.4 kB     00:00    
CentOS Stream 8 - BaseOS                        7.5 kB/s | 3.9 kB     00:00    
CentOS Stream 8 - Extras                        7.3 kB/s | 3.0 kB     00:00    

[root@localhost ~]# rpm -qa | grep sql
sqlite-3.26.0-13.el8.x86_64
sqlite-libs-3.26.0-13.el8.x86_64
msodbcsql17-17.8.1.1-1.x86_64
mssql-server-15.0.4153.1-6.x86_64
mssql-tools-17.8.1.1-1.x86_64


[root@localhost ~]# rpm -e mssql-tools-17.8.1.1-1.x86_64

################### 验证 ############################
[root@localhost ~]# rpm -qa | grep sql
sqlite-3.26.0-13.el8.x86_64
sqlite-libs-3.26.0-13.el8.x86_64
msodbcsql17-17.8.1.1-1.x86_64
mssql-server-15.0.4153.1-6.x86_64