关于yum update和yum upgrade的区别

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.

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