5.1. 二进制包的结构

The Debian package format is designed so that its content may be extracted on any Unix system that has the classic commands ar, tar, and xz or sometimes gzip or bzip2. This seemingly trivial property is important for portability and disaster recovery.

Imagine, for example, that you mistakenly deleted the dpkg program, and that you could thus no longer install Debian packages. dpkg being a Debian package itself, it would seem your system would be done for… Fortunately, you know the format of a package and can therefore download the .deb file of the dpkg package and install it manually (see sidebar 工具 dpkg、APT 和 ar). If by some misfortune one or more of the programs ar, tar or gzip/xz/bzip2 have disappeared, you will only need to copy the missing program from another system (since each of these operates in a completely autonomous manner, without dependencies, a simple copy will suffice). If your system suffered some even more outrageous fortune, and even these don’t work (maybe the deepest system libraries are missing?), you should try the static version of busybox (provided in the busybox-static package), which is even more self-contained, and provides subcommands such as busybox ar, busybox tar and busybox xz.

In case of a misfortune you better also have a backup of your system (see 第 9.10 节 “备份”).

工具 dpkg、APT 和 ar

dpkg is the program that handles .deb files (binary packages), notably extracting, analyzing, and unpacking them.

APT (the abbreviation of “Advanced Packaging Tool”) is a group of programs that allows the execution of higher-level modifications to the system: installing or removing a package (while keeping dependencies satisfied), updating and upgrading the system, listing the available packages, etc.

As for the ar program, it allows handling files of the same name: ar t *archive* displays the list of files contained in such an archive, ar x *archive* extracts the files from the archive into the current working directory, ar d *archive* *file* deletes a file from the archive, etc. Its man page (ar(1)) documents all its other features. ar is a very rudimentary tool that a Unix administrator would only use on rare occasions, but admins routinely use tar, a more evolved archive and file management program. This is why it is easy to restore dpkg in the event of an erroneous deletion. You would only have to download the Debian package and extract the content from the data.tar.xz archive in the system’s root (/):

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基础知识 Man 手册注记

初学者可能会在看到“ar(1)”这样的文字注记时感到疑惑。这通常是个简写，它表示指向 man 手册第一类中标题是 ar 的页面。

有时这种注记也用来避免歧义，比如要区分清楚是 printf 命令，可以用 printf(1) 来指明，如果是 C 语言里的 printf 函数，就可以用 printf(3) 来引用。

第 7 章 问题的解决与相关信息的检索 更详细地讨论手册页 (参见 第 7.1.1 节 “手册页面”)。

来看看 .deb 文件的内容：

$

如你所见， Debian 包的 ar 存档格式由三个文件组成：

debian-binary

This is a text file which simply indicates the version of the .deb file package format version. In Debian Buster it is still version 2.0.

control.tar.xz

This archive file contains all of the available meta-information, like the name and version of the package as well as some scripts to run before, during or after (un-)installation of it. Some of the meta-information allows package management tools to determine if it is possible to install or uninstall it, for example according to the list of packages already on the machine, and if files shipped have been modified locally.

data.tar.xz, data.tar.bz2, data.tar.gz

This archive contains all of the files to be extracted from the package; this is where the executable files, libraries, documentation, etc., are all stored. Packages may use different compression formats, in which case the file will be named differently for xz, bzip2 or gzip.