查看/etc/yum.repos.d/CentOS-Base.repo
[base]
name=CentOS-$releasever - Base
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os&infra=$infra
#baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6
其实在日常使用之中，我们在很多地方，特别是脚本内看到了调用$releasever和$basearch
- $releasever 的值，这个表示当前系统的发行版本
[root@ultraera /]# rpm -qi centos-release
Name : centos-release Relocations: (not relocatable)
Version : 6 Vendor: CentOS
Release : 7.el6.centos.12.3 Build Date: Tue 04 Aug 2015 12:13:46 AM CST
Install Date: Sat 21 May 2016 03:31:27 AM CST Build Host: c6b9.bsys.dev.centos.org
Group : System Environment/Base Source RPM: centos-release-6-7.el6.centos.12.3.src.rpm
Size : 35634 License: GPLv2
Signature : RSA/SHA1, Tue 04 Aug 2015 12:15:08 AM CST, Key ID 0946fca2c105b9de
Packager : CentOS BuildSystem [http://bugs.centos.org](http://bugs.centos.org)
Summary : CentOS release file
Description :
CentOS release files
# 其中Version 6 就是我们的版本号
$basearch 是我们的系统硬件架构 (CPU 指令集),就是我们常说的 i386\i486\i586\i686\
[root@ultraera /]# arch
i386
但是，当我在命令行查看是却发现$releasever和$basearch是空的。
Google 一下之后，说明 Yum 变量的说明可以在这里（5.3.3. Using Yum Variables）找到，说明如下：
> 5.3.3. Using Yum Variables
You can use and reference the following built-in variables in yum commands and in all Yum configuration files (that is, /etc/yum.conf and all .repo files in the /etc/yum.repos.d/ directory):
$releasever
You can use this variable to reference the release version of Red Hat Enterprise Linux. Yum obtains the value of $releasever from the distroverpkg=value line in the /etc/yum.conf configuration file. If there is no such line in /etc/yum.conf, then yum infers the correct value by deriving the version number from the redhat-release package.
$arch
You can use this variable to refer to the system’s CPU architecture as returned when calling Python’s os.uname() function. Valid values for $arch include: i586, i686 and x86_64.
$basearch
You can use $basearch to reference the base architecture of the system. For example, i686 and i586 machines both have a base architecture of i386, and AMD64 and Intel64 machines have a base architecture of x86_64.
$YUM0-9
These ten variables are each replaced with the value of any shell environment variables with the same name. If one of these variables is referenced (in /etc/yum.conf for example) and a shell environment variable with the same name does not exist, then the configuration file variable is not replaced.
To define a custom variable or to override the value of an existing one, create a file with the same name as the variable (without the "$" sign) in the /etc/yum/vars/ directory, and add the desired value on its first line.
For example, repository descriptions often include the operating system name. To define a new variable called $osname, create a new file with "Red Hat Enterprise Linux" on the first line and save it as /etc/yum/vars/osname:
`echo "Red Hat Enterprise Linux" > /etc/yum/vars/osname`
Instead of "Red Hat Enterprise Linux 6", you can now use the following in the .repo files: name=$osname $releasever
文中说到$releasever 的定义可以查看 /etc/yum.conf 文件的事 distroverpkg=value 行，打开 /etc/yum.conf 看一下，默认文件（我的是 CentOS 6）内容如下：
[main]
cachedir=/var/cache/yum/$basearch/$releasever
keepcache=0
debuglevel=2
logfile=/var/log/yum.log
exactarch=1
obsoletes=1
gpgcheck=1
plugins=1
installonly_limit=5
bugtracker_url=http://bugs.centos.org/set_project.php?project_id=16&ref=http://bugs.centos.org/bug_report_page.php?category=yum
distroverpkg=centos-release
distroverpkg=centos-release 代表什么？去哪找 centos-release。找到这里我再一次表示困惑。 不过另一篇文章进行了我的视野：yum 的$releasever 真是太反动了，关键内空如下： 在 /etc/yum.repos.d/ 目录下的软件库定义文件中，常常会在 baseurl 的路径中提到 $releasever 这个变量，表示当前发行版的大版本号，但大家知道这个变量是在哪设置的吗？我 grep 了整个 etc 目录都没找到，还是看了 yum.conf 才知道的，是在 yum.conf 文件里 distroverpkg 选项定义的。但这个选项就很有问题： distroverpkg 和 releasever 名字不同，且看不出什么联系 distroverpkg 的值，并不是明文，而是“redhat-release”。不知道大家看到这个会有什么想法，反正我是首先想到了 /etc/redhat-release 文件，但我错了。实际上指的是 redhat-release 这个 RPM 包。所谓“distroverpkg=redhat-release”的意思，其实是将 $releasever 设置为 redhat-release 这个 RPM 包的版本号。
小结
yum 中的$releasever 变量是取 redhat-release-server rpm 包的属性值 ( $version)。 [root@ldap01 ~]# rpm -q --qf $version redhat-release-server;echo 6Server