linux下centos無法使用yum問題解決（已解決）

前言
剛安裝的centos系統，發現無法使用yum指令
出現以下錯誤：


One of the configured repositories failed (Unknown),
and yum doesn’t have enough cached data to continue. At this point the only
safe thing yum can do is fail. There are a few ways to work “fix” this:

 1. Contact the upstream for the repository and get them to fix the problem.

 2. Reconfigure the baseurl/etc. for the repository, to point to a working
    upstream. This is most often useful if you are using a newer
    distribution release than is supported by the repository (and the
    packages for the previous distribution release still work).

 3. Disable the repository, so yum won't use it by default. Yum will then
    just ignore the repository until you permanently enable it again or use
    --enablerepo for temporary usage:

        yum-config-manager --disable <repoid>

 4. Configure the failing repository to be skipped, if it is unavailable.
    Note that yum will try to contact the repo. when it runs most commands,
    so will have to try and fail each time (and thus. yum will be be much
    slower). If it is a very temporary problem though, this is often a nice
    compromise:

        yum-config-manager --save --setopt=<repoid>.skip_if_unavailable=true

24

1
One of the configured repositories failed (Unknown),
2
and yum doesn’t have enough cached data to continue. At this point the only
3
safe thing yum can do is fail. There are a few ways to work “fix” this:
4
5
1. Contact the upstream for the repository and get them to fix the problem.
6
7
2. Reconfigure the baseurl/etc. for the repository, to point to a working
8
upstream. This is most often useful if you are using a newer
9
distribution release than is supported by the repository (and the
10
packages for the previous distribution release still work).
11
12
3. Disable the repository, so yum won’t use it by default. Yum will then
13
just ignore the repository until you permanently enable it again or use
14
–enablerepo for temporary usage:
15
16
yum-config-manager --disable
17
18
4. Configure the failing repository to be skipped, if it is unavailable.
19
Note that yum will try to contact the repo. when it runs most commands,
20
so will have to try and fail each time (and thus. yum will be be much
21
slower). If it is a very temporary problem though, this is often a nice
22
compromise:
23
24
yum-config-manager --save --setopt=.skip_if_unavailable=true

問題分析：
無法使用yum的原因在於剛安裝的Linux系統並沒有自動啓動網卡開機自動開啓服務
因此我們需要手動開啓網卡開機自啓動
解決方法：
1、進入/etc/sysconfig/network-scripts 目錄。即輸入命令 "cd /etc/sysconfig/network-scripts"

2、修改ifcfg-ens33的網卡配置文件，輸入命令 “vi ifcfg-ens33” 進入vi編輯器，按下"i"或者"insert"鍵進入編輯模式。

3、將 “ONBOOT” 的值修改爲 “yes” ，之後按esc退出編輯模式，輸入 “:wq” 保存退出

但是在此筆者發現一個問題：
即在修改文件時並沒有權限，出現保存文件時，提示“readonly option is set”
解決方法：
1.按Esc鍵
2.輸入：set noreadonly
3.輸入：:wq (保存並退出)
如果依舊報錯“is read-only (add ! to override)”：
1、首先強制退出vi模式：q!
2、退出後執行sudo ！！
3、再次編輯後輸入： :wq!（強制保存並退出）

參考：
https://www.cnblogs.com/crowsong/p/9371216.html
https://blog.csdn.net/longerandlonger/article/details/7266055
https://www.cnblogs.com/mabc/p/13784398.html