CentOS 升级至指定版本
CentOS系统下用yum upgrade命令只能升级到最新版本，CentOS仓库并不维护历史版本，所以只能使用 vault.centos.org 历史版本快照进行更新。
本文以将7.2.1511升级到7.5.1804为例。
- 备份久的版本仓库文件
mkdir -p /etc/yum.repos.d/backup
mv /etc/yum.repos.d/*.repo /etc/yum.repos.d/backup/
- 创建要升级到版本
(7.5.1804)的Vault版本仓库文件：
VERSION=7.5.1804
cat <<'EOF'> /etc/yum.repos.d/CentOS-Vault.repo
# CentOS Vault contains rpms from older releases in the CentOS-7
# tree.
# CentOS-VERSION
[CentOS-VERSION-base]
name=CentOS-VERSION - Base
baseurl=http://vault.centos.org/VERSION/os/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
enabled=1
[CentOS-VERSION-updates]
name=CentOS-VERSION - Updates
baseurl=http://vault.centos.org/VERSION/updates/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
enabled=1
[CentOS-VERSION-extras]
name=CentOS-VERSION - Extras
baseurl=http://vault.centos.org/VERSION/extras/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
enabled=1
[CentOS-VERSION-centosplus]
name=CentOS-VERSION - CentOSPlus
baseurl=http://vault.centos.org/VERSION/centosplus/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
enabled=1
[CentOS-VERSION-fasttrack]
name=CentOS-VERSION - Fasttrack
baseurl=http://vault.centos.org/VERSION/fasttrack/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
enabled=1
EOF
sed -i "s/VERSION/$VERSION/g" /etc/yum.repos.d/CentOS-Vault.repo
- 查看现在可用仓库列表：
yum repolist
- 查看现在可用仓库版本：
yum list updates | grep "centos-release"
- 清空本地缓存：
yum clean all
rm -rf /var/cache/yum
- 版本升级：
yum -y update
- 备份过渡的Vault仓库文件，启用新版的仓库文件：
mv /etc/yum.repos.d/CentOS-Vault.repo /etc/yum.repos.d/backup/
mv /etc/yum.repos.d/CentOS-Vault.repo.rpmnew /etc/yum.repos.d/CentOS-Vault.repo
- 清空本地缓存，更新版本库文件：
yum clean all
yum makecache
