系统环境：CentOS Linux release 7.7.1908 (Core)

 

网上看到，很多说关于yum update和yum upgrade的区别是，

update会升级所有包同时也升级软件和系统内核，而upgrade只升级所有包，不升级软件和系统内核

这种说法是错误的！！！！！！！！！

 

首页通过man yum命令看update解释如下：

update If run without any packages, update will update every currently installed package.  If one or more packages or  package  globs  are
       specified, Yum will only update the listed packages.  While updating packages, yum will ensure that all dependencies are satisfied.
       (See Specifying package names for more information) If the packages or globs specified match to packages which  are  not  currently
       installed  then update will not install them. update operates on groups, files, provides and filelists just like the "install" com‐
       mand.

       If the main obsoletes configure option is true (default) or the --obsoletes flag is present yum will include package  obsoletes  in
       its calculations - this makes it better for distro-version changes, for example: upgrading from somelinux 8.0 to somelinux 9.

       Note  that  "update"  works  on installed packages first, and only if there are no matches does it look for available packages. The
       difference is most noticeable when you do "update foo-1-2" which will act exactly as "update foo" if foo-1-2 is installed. You  can
       use the "update-to" if you'd prefer that nothing happen in the above case.

再看upgrade的说明：

upgrade
       Is the same as the update command with the --obsoletes flag set. See update for more details.

 这里可以看出，yum upgrade 其实就是和yum update --obsoletes的效果是一样的，

而默认情况下/etc/yum.conf文件中就注明了obsoletes=1，再加之笔者也测试过，效果确实也是一样的、而且都会升级系统内核！