For any work with APT, the list of available packages needs to be updated; this can be done simply through apt update. Depending on the speed of your connection and configuration, the operation can take a while, since it involves downloading a certain number of (usually compressed) files (Packages, Sources, Translation-language-code), which have gradually become bigger and bigger as Debian has developed (at least 10-16 MB of data for the main section). Of course, installing from a CD-ROM/DVD set does not require any downloading — in this case, the operation is very fast.

$ sudo apt -o "Acquire::PDiffs=false" update

With APT, packages can be added or removed from the system, respectively with apt install package and apt remove package. In both cases, APT will automatically install the necessary dependencies or delete the packages which depend on the package that is being removed. The apt purge package command involves a complete uninstallation by deleting the configuration files as well.

$ dpkg --get-selections >pkg-list

## 更新 dpkg 已知套件的資料
#avail=`mktemp`
# apt-cache dumpavail > "$avail"
# dpkg --merge-avail "$avail"
# rm -f "$avail"
## 更新 dpkg 的選擇
# dpkg --set-selections < pkg-list
## 要求 apt-get 安裝指定的套件
# apt-get dselect-upgrade

# apt install package1 package2-

# apt remove package1+ package2

# apt --reinstall install postfix

# apt --reinstall -o Dpkg::Options::="--force-confask,confmiss" install package

If the file sources.list mentions several distributions, it is possible to give the version of the package to install. A specific version number can be requested with apt install package=version, but indicating its distribution of origin (Stable, Testing or Unstable) — with apt install package/distribution — is usually preferred. With this command, it is possible to go back to an older version of a package (if, for instance, you know that it works well), provided that it is still available in one of the sources referenced by the sources.list file. Otherwise the snapshot.debian.org archive can come to the rescue (see sidebar GOING FURTHER Old package versions: snapshot.debian.org and archive.debian.org).

# apt install spamassassin/unstable

If the package to install has been made available to you under the form of a simple .deb file without any associated package repository, it is still possible to use APT to install it together with its dependencies (provided that the dependencies are available in the configured repositories) with a simple command: apt install ./path-to-the-package.deb. The leading ./ is important to make it clear that we are referring to a filename and not to the name of a package available in one of the repositories.

建議使用正常的升級，因為包括最新的安全升級。升級的指令，有 apt upgrade、apt-get upgrade 或 aptitude safe-upgrade (當然是跟著 apt update 之後)。在不移除任何套件的情況下，此命令搜尋已安裝套件中可以升級的部份。換句話說，其目標是做到侵入性最小的升級。apt-get 比 aptitude 或 apt 有更多的需求，因為拒絕安裝不曾安裝過的套件。

apt 會選擇最新的版本號 (除了來自 實驗版 與 穩定反向植入版，其預設忽略版本號)。若指定 測試版 或 不穩定版 於 sources.list 內，則 apt upgrade 會把 穩定版 系統切換至 測試版 或 不穩定版，這些都不是您要的。

To tell apt to use a specific distribution when searching for upgraded packages, you need to use the -t or --target-release option, followed by the name of the distribution you want (for example, apt -t stable upgrade). To avoid specifying this option every time you use apt, you can add APT::Default-Release "stable"; in the file /etc/apt/apt.conf.d/local.

重要的升級，諸如 Debian 的主要版本升級，需使用 apt full-upgrade。在此指令內， 即使刪除若干老舊的套件或安裝新的相依性也會把 apt 完全升級。也是每日於T Debian 不穩定版 使用的命令以及逐日追蹤其變動。它的運作極簡單不需說明：APT 的名聲來自其功能性。

不同於 apt 與 aptitude，apt-get 不知道 full-upgrade 命令。反而，您應使用 apt-get dist-upgrade (”升級發行版”)，此古老且知名的命令也被 apt 與 aptitude 接受給讀者相當的便利。

The results of these operations are logged into /var/log/apt/history.log and /var/log/apt/term.log, whereas dpkg keeps its log in a file called /var/log/dpkg.log.

Besides the configuration elements already mentioned, it is possible to configure certain aspects of APT by adding directives in a file of the /etc/apt/apt.conf.d/ directory or /etc/apt/apt.conf itself. Remember, for instance, that it is possible for APT to tell dpkg to ignore file conflict errors by specifying DPkg::options { "--force-overwrite"; }.

If the Web can only be accessed through a proxy, add a line like Acquire::http::proxy "http://yourproxy:3128". For an FTP proxy, write Acquire::ftp::proxy "ftp://yourproxy". To discover more configuration options, read the apt.conf(5) manual page (for details on manual pages, see 節 7.1.1, “手冊頁面”).

組態 APT 最重要的層面之一是管理每個套件來源的優先性。例如，從 測試版、不穩定 或 實驗版 中選取一個或多個套件。可以指令每個套件的優先性 (視其版本或發行版，同個套件可以有多個優先性)。這些優先性將影響 APT 的行為：每個套件總是選擇最優先的版本 (除非舊於已安裝的版本或優先性小於 1000)。

APT 設定若干預設的優先性。已安裝套件版本的優先性是 100。未安裝版本預設優先性為 500，若是另個釋出的目標則可跳至 990 (以 -t 命令列選項或 APT::Default-Release 組態指令調整)。

You can modify the priorities by adding entries in a file in /etc/apt/preferences.d/ or the /etc/apt/preferences file with the names of the affected packages, their version, their origin and their new priority.

APT will never install an older version of a package (that is, a package whose version number is lower than the one of the currently installed package) except if its priority is higher than 1000 (or it is explicitly requested by the user, see 節 6.2.2, “安裝與移除”). APT will always install the highest priority package which follows this constraint. If two packages have the same priority, APT installs the newest one (whose version number is the highest). If two packages of same version have the same priority but differ in their content, APT installs the version that is not installed (this rule has been created to cover the case of a package update without the increment of the revision number, which is usually required).

In more concrete terms, a package whose priority is

When APT checks /etc/apt/preferences and /etc/apt/preferences.d/, it first takes into account the most specific entries (often those specifying the concerned package), then the more generic ones (including, for example, all the packages of a distribution). If several generic entries exist, the first match is used. The available selection criteria include the package's name and the source providing it. Every package source is identified by the information contained in a Release file that APT downloads together with the Packages files. It specifies the origin (usually “Debian” for the packages of official mirrors, but it can also be a person's or an organization's name for third-party repositories). It also gives the name of the distribution (usually Stable, Testing, Unstable or Experimental for the standard distributions provided by Debian) together with its version (for example, 11 for Debian Bullseye). Let's have a look at its syntax through some realistic case studies of this mechanism.

Package: *
Pin: release a=experimental
Pin-Priority: 500

暫時假設您祗需用到 Debian 穩定版的套件。除非特別指明，不會安裝其他版本的套件。可以在 /etc/apt/preferences 檔案內寫入以上的款目：

Package: *
Pin: release a=stable
Pin-Priority: 900

Package: *
Pin: release o=Debian
Pin-Priority: -10

a=stable 設定發行版的名稱。o=Debian 限制來自 “Debian” 的套件。

Let's now assume that you have a server with several local programs depending on the version 5.28 of Perl and that you want to ensure that upgrades will not install another version of it. You could use this entry:

Package: perl
Pin: version 5.28*
Pin-Priority: 1001

To gain a better understanding of the mechanisms of priority and distribution or repository properties to pin do not hesitate to execute apt-cache policy to display the default priority associated with each package source, or apt-cache policy package to display the default priority for each available version and source of a package as explained in 節 6.3.1, “The apt-cache policy Command”.

The reference documentation for the files /etc/apt/preferences and /etc/apt/preferences.d/ is available in the manual page apt_preferences(5), which you can display with man apt_preferences.

Explanation: The package xserver-xorg-video-intel provided
Explanation: in experimental can be used safely
Package: xserver-xorg-video-intel
Pin: release a=experimental
Pin-Priority: 500

apt 是很奇妙的工具，可從其他發行版提取套件。例如，安裝 穩定版 系統之後，或許想要在不偏離系統原來狀態下，試試 測試版 或 不穩定版 的套件。

混用不同版本套件出問題時，Even if you will occasionally encounter problems while mixing packages from different distributions, apt 可以處理得極為恰當把風險降到最低。最好的方法是把所有的發行版置於 /etc/apt/sources.list 檔案內 (某些人總是置入三個發行版，不過還是要記得 不穩定版 是有經驗使用者的專屬。) 且在 APT::Default-Release 參數中設定偏好的發行版 (見 節 6.2.3, “系統升級”) 。

假設 穩定版 是您的參考版本，但是 測試版 與 不穩定版 也列在 sources.list 檔案內。在這個情況下，您可使用 apt install 套件名稱/testing 安裝來自 測試版 的套件。若因相依性未滿足而安裝失敗，可以在 測試版 內加入 -t testing 參數。同樣的方式也適用於 不穩定版。

In this situation, upgrades (upgrade and full-upgrade) are done within Stable except for packages already upgraded to another distribution: those will follow updates available in the other distributions. We will explain this behavior with the help of the default priorities set by APT below. Do not hesitate to use apt-cache policy (節 6.3.1, “The apt-cache policy Command”) to verify the given priorities.

每件事都圍繞在 APT 祗處理高於或等於已安裝版本號套件的前提下 (假設 /etc/apt/preferences 還未強迫優先性高於 1000 的某些套件)。

假設從Let's assume that you have installed version 1 of a first package from 穩定版 安裝第一個套件的第一版且從and that version 2 and 3 are available respectively in 測試版 與 不穩定版 安裝同套件的第二版與第三版水。已安裝的版本優先性為 100 但在but the version available in 穩定版 (相同的) 優先性為 990 (因為它是目標釋出版的一部份)。在 測試版 與 不穩定版 的套件其優先性為 500 (預設不安裝版本的優先性)。獲選者為優先性 990 的版本1。套件 “留在 穩定版 內”。

Let's take the example of another package whose version 2 has been installed from Testing. Version 1 is available in Stable and version 3 in Unstable. Version 1 (of priority 990 — thus lower than 1000) is discarded because it is lower than the installed version. This only leaves version 2 and 3, both of priority 500. Faced with this alternative, APT selects the newest version, the one from Unstable. If you don't want a package installed from Testing to migrate to the version in Unstable, you have to assign a priority lower than 500 (490 for example) to packages coming from Unstable. You can modify /etc/apt/preferences to this effect:

Package: *
Pin: release a=unstable
Pin-Priority: 490

One of the essential functionalities of apt is the tracking of packages installed only through dependencies. These packages are called “automatic”, and often include libraries.

With this information, when packages are removed, the package managers can compute a list of automatic packages that are no longer needed (because there is no “manually installed” packages depending on them). apt-get autoremove or apt autoremove will get rid of those packages. aptitude does not have this command because it removes them automatically as soon as they are identified. In all cases, the tools display a clear message listing the affected packages.

把套件標記為自動是個好習慣，不需要時就被自動移除。apt-mark auto 套件名稱 會標記該套件為自動而 apt-mark manual 套件名稱 則不標記。aptitude markauto 與 aptitude unmarkauto 作用相同祗是功能較多 (見 節 6.5.1, “aptitude”)。命令互動介面的 aptitude 也可檢視多個套件的 “自動旗標”。

自動安裝套件出現在系統內。係由於從命令列取得該等資訊，可使用 aptitude why 套件名稱 (apt 與 apt-get 沒有類似的功能)：

$ aptitude why python3-debian
i   aptitude         Suggests apt-xapian-index
p   apt-xapian-index Depends  python3-debian (>= 0.1.14)

Patterns allow you to specify complex search queries to select the packages you want to install or show. They were first implemented for aptitude (see 節 6.5, “前置：aptitude、synaptic”), but now APT can also make use of them.

For example, we can use apt list ?automatic to list all the packages automatically installed. To find packages automatically installed no longer depended upon by manually installed packages the pattern ?garbage can be used.

Logic patterns can be combined with other packages to form more complex expressions. For instance, we could use a pattern like ?and(PATTERN, PATTERN). See apt-patterns(7) and glob(7) for all the patterns you can use and the complex expressions you can create with them.