「更新」所有已安裝的程式、套件：
$ yum update只更新特定程式，套件：
$ yum update httpd
$ yum check-update
$ yum upgrade$ yum upgrade = yum --obsoletes update
man page 的說明
#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-ver‐
sion 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.
--obsoletes
This option only has affect for an update, it enables yum´s obsoletes processing logic.
For more information see the update command above.
Configuration Option: obsoletes
upgrade 會清除舊版/過時的 package ， update 不會
若某些程式有相依舊版 packeage 建議不要用 upgrade 命令
#
update / upgrade 都會更新作業系統版本 (是指 子版本更新)
例如
目前是 CentOS 7.3
更新完 就變最新 CentOS 7.5
清除 /var/cache/yum 底下的暫存檔案：
* 另 /etc/yum.conf
$ yum clean*有可能清出 3+GB 的空間
相關
$ yum clean packages
$ yum clean headers
查詢套件資訊：
$ yum info [套件名稱] 列出已安裝的套件：
$ yum list installed list 是一行行列出
像這樣：
:: :::
yum-metadata-parser.x86_64 1.1.4-10.el7 @anaconda
yum-plugin-fastestmirror.noarch 1.1.31-45.el7 @anaconda
yum-utils.noarch 1.1.31-45.el7 @anaconda
zenity.x86_64 3.22.0-1.el7 @anaconda
zip.x86_64 3.0-11.el7 @anaconda
zlib.x86_64 1.2.7-17.el7 @anaconda
改用 info 的話，會顯示套件詳細資料：
$ yum info installed :: :::
Name : nss-sysinit
Arch : x86_64
Version : 3.34.0
Release : 4.el7
Size : 14 k
Repo : installed
From repo : anaconda
Summary : System NSS Initialization
URL : http://www.mozilla.org/projects/security/pki/nss/
Licence : MPLv2.0
Description : Default Operating System module that manages applications loading
: NSS globally on the system. This module loads the system defined
: PKCS #11 modules for NSS and chains with other NSS modules to load
: any system or user configured modules.
列出可更新的套件：
yum list updates
yum info updates
搜尋
$ yum search [名稱/關鍵字]ref:
https://access.redhat.com/documentation/zh-tw/red_hat_network_satellite/5.5/html/reference_guide/sect-reference_guide-package_updater-updating_packages_from_the_command_line_with_yum
