centos 7的升级

　　centos 7早已不被维护，大多数的源都失效了。找到一个：

wget http://mirrors.aliyun.com/repo/Centos-altarch-7.repo -O /etc/yum.repos.d/CentOS-Base.repo

　　执行yum update -y

 附：目前可用的：CentOS-Base.repo

# CentOS-Base.repo
#
# The mirror system uses the connecting IP address of the client and the
# update status of each mirror to pick mirrors that are updated to and
# geographically close to the client.  You should use this for CentOS updates
# unless you are manually picking other mirrors.
#
# If the mirrorlist= does not work for you, as a fall back you can try the 
# remarked out baseurl= line instead.
#
#

[base]
name=CentOS-$releasever - Base
baseurl=http://mirrors.aliyun.com/centos-altarch/$releasever/os/$basearch/
gpgcheck=0
gpgkey=http://mirrors.aliyun.com/centos-altarch/7/os/$basearch/RPM-GPG-KEY-CentOS-7

#released updates 
[updates]
name=CentOS-$releasever - Updates
baseurl=http://mirrors.aliyun.com/centos-altarch/$releasever/updates/$basearch/
gpgcheck=0
gpgkey=http://mirrors.aliyun.com/centos-altarch/7/os/$basearch/RPM-GPG-KEY-CentOS-7

#additional packages that may be useful
[extras]
name=CentOS-$releasever - Extras
baseurl=http://mirrors.aliyun.com/centos-altarch/$releasever/extras/$basearch/
gpgcheck=0
gpgkey=http://mirrors.aliyun.com/centos-altarch/7/os/$basearch/RPM-GPG-KEY-CentOS-7
enabled=1

#additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-$releasever - Plus
baseurl=http://mirrors.aliyun.com/centos-altarch/$releasever/centosplus/$basearch/
gpgcheck=0
enabled=0
gpgkey=http://mirrors.aliyun.com/centos-altarch/7/os/$basearch/RPM-GPG-KEY-CentOS-7