Chương 5. Hệ thống gói: Công cụ và Nguyên tắc cơ bản

Như một người quản trị hệ thống Debian, bạn sẽ thường xuyên làm việc với các gói .deb, vì chúng chứa các đơn vị chức năng (ứng dụng, tài liệu, etc.) trong việc cài đặt và bảo trì. Vì vậy, là một ý tưởng tốt để biết nó là gì và làm sao đế sử dụng nó.

This chapter describes the structure and contents of “binary” and “source” packages. The former are files directly usable by dpkg, while the latter contain the source code, as well as instructions for building binary packages.

5.1. Cấu trúc của một Gói nhị phân

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 Các công cụ dpkg, APT and 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 Phần 9.10, “Backup”).

Các công cụ dpkg, APT and 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, provided by the binutils package, 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 (/):

# ar x dpkg_1.20.9_amd64.deb
# tar -C / -p -xJf data.tar.xz

Trở về cơ bản Man page notation

Có thể nhầm lần cho người mới bắt đầu để tham chiếu tới “ar(1)” trong các tài liệu. Đây là thường là một phương tiện tiện lợi để tham chiếu tới trang tài liệu ar trong nội dung 1.

Đôi khi những ký hiệu này cũng được sử dụng để loại bỏ những mơ hồ, ví dụ phân biệt giữa lệnh printf lệnh này có thể cũng được chỉ ra bởi printf(1) và hàm printf trong ngôn ngữ lập trình C, có thể được tham chiếu bằng printf(3).

Chương 7, Giải quyết các vấn đề và tìm thông tin liên quan thảo luận về trang hương dẫn chi tiết hơn (see Phần 7.1.1, “Manual Pages”).

Cái nhìn về nội dung trong tệp tin .deb

$ ar t dpkg_1.20.9_amd64.deb
debian-binary
control.tar.gz
data.tar.xz
$ ar x dpkg_1.20.9_amd64.deb
$ ls
control.tar.gz  data.tar.xz  debian-binary  dpkg_1.20.9_amd64.deb
$ tar tJf data.tar.xz | head -n 16
./
./etc/
./etc/alternatives/
./etc/alternatives/README
./etc/cron.daily/
./etc/cron.daily/dpkg
./etc/dpkg/
./etc/dpkg/dpkg.cfg
./etc/dpkg/dpkg.cfg.d/
./etc/logrotate.d/
./etc/logrotate.d/alternatives
./etc/logrotate.d/dpkg
./sbin/
./sbin/start-stop-daemon
./usr/
./usr/bin/
$ tar tJf control.tar.xz
./
./conffiles
./control
./md5sums
./postrm
$ cat debian-binary
2.0

Như bạn có thể thấy, với góc nhìn qua tệp nén từ lệnh ar của một gói Debian gồm có ba tập tin:

debian-binary: This is a text file which simply indicates the version of the .deb file package format version. In Debian Bullseye 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.