6.3. The apt-cache 命令

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。

术语缓存

缓存是一种暂存系统，当常用访问方法昂贵 (性能方面) 时，用于加快常用数据的取用速度。这个概念应用范围极广，从微处理器核心至高端保存系统。

In the case of APT, the reference Packages files are those located on Debian mirrors. That said, it would be very ineffective to go through the network for every search that we might want to do in the database of available packages. That is why APT stores a copy of those files (in /var/lib/apt/lists/) and searches are done within those local files. Similarly, /var/cache/apt/archives/ contains a cache of already downloaded packages (see sidebar 扩展 .deb 文件的缓存) to avoid downloading them again if you need to reinstall them after a removal.

另一方面，必须运行 apt update 更新缓存。否则您的搜索结果会错过Debian镜像站发行版的最新更新。

apt-cache 命令可以通过 apt-cache search keyword做基于关键词的软件包搜索。也能使用 apt-cache show 软件包名称 显示可用版本软件包的标头。这个命令提供软件包说明、依赖性、维护者名称等。请注意，apt search、apt show、aptitude search 和 aptitude show 都以同样方式运作。

其他 axi-cache

apt-cache search 是基础的工具，使用 grep 搜索软件包描述。输入过多键词可能返回大量结果或完全无数据。

axi-cache search trem，从另一方面来说提供更好的结果，以相关性排序。使用 Xapian 搜索引擎且它是 apt-xapian-index 软件包的一部分索引所有软件包的信息 (进一步来说，就像是来自所有 Debian 软件包的 .desktop 档)。显示标签 (见专栏 进阶阅读Tag 字段) 并在毫秒之间送回结果。

$ axi-cache search package use::searching
axi-cache search package use::searching
13 results found.
Results 1-13:
100% packagesearch - GUI for searching packages and viewing package information
87% debtags - Debian Package Tags support tools
86% whohas - query multiple distributions' package archives
84% recoll - Personal full text search package
82% apt-forktracer - utility for tracking non-official package versions
[..]
66% apt - commandline package manager
More terms: debian dpkg search debtags whohas forktracer dctrl
More tags: role::program admin::package-management suite::debian works-with::software:package interface::commandline scope::utility protocol::http

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