pacman -Syu 和 pacman -Syyu 的区别是什么
-y, --refresh Download a fresh copy of the master package database from the server(s) defined in pacman.conf(5). This should typically be used each time you use --sysupgrade or -u. Passing two --refresh or -y flags will force a refresh of all package databases, even if they appear to be up-to-date.
区别在于，两个 y 将会是强制更新所有的软件包数据库。一般不建议加上 u，因为实在太浪费时间了。
-S, --sync Synchronize packages. Packages are installed directly from the remote repositories, including all dependencies required to run the packages. For example, pacman -S qt will download and install qt and all the packages it depends on. If a package name exists in more than one repository, the repository can be explicitly specified to clarify the package to install: pacman -S testing/qt. You can also specify version requirements: pacman -S "bash>=3.2". Quotes are needed, otherwise the shell interprets ">" as redirection to a file. In addition to packages, groups can be specified as well. For example, if gnome is a defined package group, then pacman -S gnome will provide a prompt allowing you to select which packages to install from a numbered list. The package selection is specified using a space- and/or comma-separated list of package numbers. Sequential packages may be selected by specifying the first and last package numbers separated by a hyphen (-). Excluding packages is achieved by prefixing a number or range of numbers with a caret (^). Packages that provide other packages are also handled. For example, pacman -S foo will first look for a foo package. If foo is not found, packages that provide the same functionality as foo will be searched for. If any package is found, it will be installed. A selection prompt is provided if multiple packages providing foo are found. You can also use pacman -Su to upgrade all packages that are out-of-date. See Sync Options below. When upgrading, pacman performs version comparison to determine which packages need upgrading. This behavior operates as follows: Alphanumeric: 1.0a < 1.0b < 1.0beta < 1.0p < 1.0pre < 1.0rc < 1.0 < 1.0.a < 1.0.1 Numeric: 1 < 1.0 < 1.1 < 1.1.1 < 1.2 < 2.0 < 3.0.0 Additionally, version strings can have an epoch value defined that will overrule any version comparison, unless the epoch values are equal. This is specified in an epoch:version-rel format. For example, 2:1.0-1 is always greater than 1:3.6-1.
SYNC OPTIONS (APPLY TO -S)
-u, --sysupgrade Upgrades all packages that are out-of-date. Each currently-installed package will be examined and upgraded if a newer package exists. A report of all packages to upgrade will be presented, and the operation will not proceed without user confirmation. Dependencies are automatically resolved at this level and will be installed/upgraded if necessary. Pass this option twice to enable package downgrades; in this case, pacman will select sync packages whose versions do not match with the local versions. This can be useful when the user switches from a testing repository to a stable one. Additional targets can also be specified manually, so that -Su foo will do a system upgrade and install/upgrade the "foo" package in the same operation.