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6.3. The apt-cache 命令

apt-cache 命令可显示 APT 内部数据库里的多种信息。这些信息是从 sources.list 文件内聚集不同来源的缓存。于运行 apt update 运作时产生的。
The apt-cache command can do keyword-based package searches with apt-cache search keyword. It can also display the headers of the package's available versions with apt-cache show package. This command provides the package's description, its dependencies, the name of its maintainer, etc. Note that apt search, apt show, aptitude search, and aptitude show work in the same way.
Some features are more rarely used. For instance, apt-cache dumpavail displays the headers of all available versions of all packages. apt-cache pkgnames displays the list of all the packages which appear at least once in the cache.
One of the features which might come in handy more often is apt-cache policy, described in the following section.

6.3.1. The apt-cache policy Command

The apt-cache policy command displays the pinning priorities and distribution properties of each package source as explained in 第 6.2.5 节 “软件包优先级管理”. It can also show the pinning priorities for all available versions and sources of a package. For the sources.list example used in 例 6.2 “给 Debian 稳定版用户的 /etc/apt/sources.list 文件” and APT::Default-Release set to "bullseye", the output will look like this:
$ apt-cache policy
Package files:
 100 /var/lib/dpkg/status
     release a=now
 100 https://deb.debian.org/debian bullseye-backports/main amd64 Packages
     release o=Debian Backports,a=bullseye-backports,n=bullseye-backports,l=Debian Backports,c=main,b=amd64
     origin deb.debian.org
 990 https://deb.debian.org/debian bullseye/non-free amd64 Packages
     release v=11.0,o=Debian,a=stable,n=bullseye,l=Debian,c=non-free,b=amd64
     origin deb.debian.org
 990 https://deb.debian.org/debian bullseye/contrib amd64 Packages
     release v=11.0,o=Debian,a=stable,n=bullseye,l=Debian,c=contrib,b=amd64
     origin deb.debian.org
 990 https://deb.debian.org/debian bullseye/main amd64 Packages
     release v=11.0,o=Debian,a=stable,n=bullseye,l=Debian,c=main,b=amd64
     origin deb.debian.org
 500 http://security.debian.org bullseye-security/main amd64 Packages
     release v=11,o=Debian,a=stable-security,n=bullseye-security,l=Debian-Security,c=main,b=amd64
     origin security.debian.org
Pinned packages:
apt-cache policy 可以显示指定软件包所有可用版本的固定优先级和源。
$ apt-cache policy limnoria
limnoria:
  Installed: 2021.06.15-1
  Candidate: 2021.06.15-1
  Version table:
     2021.07.21-1~bpo11+1 100
        100 https://deb.debian.org/debian bullseye-backports/main amd64 Packages
 *** 2021.06.15-1 990
        990 https://deb.debian.org/debian bullseye/main amd64 Packages
        100 /var/lib/dpkg/status
Although there is a newer version of limnoria in the bullseye-backports repository, APT will not install it automatically based on the priority. One would have to use apt install limnoria/bullseye-backports or add a higher pinning priority to /etc/apt/preferences.d/limnoria.pref:
Package: limnoria
Pin: release o=Debian Backports, a=bullseye-backports
Pin-Priority: 1001
$ apt-cache policy limnoria
limnoria:
  Installed: 2021.06.15-1
  Candidate: 2021.07.21-1~bpo11+1
  Version table:
     2021.07.21-1~bpo11+1 1001
        100 https://deb.debian.org/debian bullseye-backports/main amd64 Packages
 *** 2021.06.15-1 990
        990 https://deb.debian.org/debian bullseye/main amd64 Packages
        100 /var/lib/dpkg/status