6.3. The apt-cache 命令

6.3. The `apt-cache` 命令

apt-cache 命令可顯示 APT 內部資料庫裡的多種資訊。這些資訊是從 sources.list 檔案內聚集不同來源的快取。於執行 apt update 運作時產生的。

術語快取

快取是一種暫存系統，近用方法昂貴 (講究性能) 時，用於加快常用資料的取用速度。這個概念應用範圍極廣，從微處理器核心至高階儲存系統。

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 下一步 快取 .deb 檔) to avoid downloading them again if you need to reinstall them after a removal.

On the other hand, it is mandatory to run apt update regularly to update the cache. Otherwise your package search results will always miss the latest updates distributed by the Debian mirrors.

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.

其他 axi-cache

apt-cache search 是基礎的工具，應用在 grep 套件描述。輸入過多鍵詞可能送回大量結果或完全無資料。

axi-cache search term, on the other hand, provides better results, sorted by relevancy. It uses the Xapian search engine and is part of the apt-xapian-index package which indexes all package information (and more, like the .desktop files from all Debian packages). It knows about tags (see sidebar 進一步 Tag 欄位) and returns results in a matter of milliseconds.

$ 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 節 6.2.5, “管理套件優先性”. It can also show the pinning priorities for all available versions and sources of a package. For the sources.list example used in 範例 6.2, “供 Debian 穩定版使用者的 /etc/apt/sources.list” 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 can also show the pinning priorities for all available versions and sources of a given package.

$ 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