CentOS中的 yum upgrade 和 yum update 的區別

　　經過 man yum 的幫助信息瞭解 yum update 和 yum upgrade：

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" command.
    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.

　　從幫助信息咱們能夠看到，upgrade 與 update --obsoletes 是同樣的。

　　然而，update 即便不附帶 --obsoletes 選項時，在默認的配置中已經將其設置爲true(開啓)，在 /etc/yum.conf 文件中能夠查看到默認的配置信息：

[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=19&ref=http://bugs.centos.org/bug_report_page.php?category=yum
distroverpkg=centos-release

　　就是說默認狀況下 update 和 upgrade 沒有區別，都是將系統包更新到最新的版本而且強制刪除過期的老版本，這種更新操做是危險的，由於可能有些運行中的服務仍舊依賴着舊版本。

　　所以咱們能夠修改默認配置，sed -i 's/obsoletes=1/obsoletes=0/' /etc/yum.conf，從而使用 update 來進行系統包的更新。

　　參考：

　　https://unix.stackexchange.com/questions/55777/in-centos-what-is-the-difference-between-yum-update-and-yum-upgrade

　　https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/system_administrators_guide/sec-configuring_yum_and_yum_repositories