→

Rebuilding a Debian package starts with getting its source code. The easiest way is to use the command. This command requires a deb-src line in the /etc/apt/sources.list file, and up-to-date index files (i.e. apt-get update). These conditions should already be met if you followed the instructions from the chapter dealing with APT configuration (see 第 6.1 节 “写入sources.list文件”). Note however, that you will be downloading the source packages from the Debian version mentioned in the deb-src line. If you need another version, you may need to download it manually from a Debian mirror or from the web site. This involves fetching two or three files (with extensions *.dsc — for Debian Source Control — *.tar.*comp*, and sometimes *.diff.gz or *.debian.tar.*comp* — comp taking one value among gz, bz2 or xz depending on the compression tool in use), then run the dpkg-source -x *file.dsc* command. If the *.dsc file is directly accessible at a given URL, there is an even simpler way to fetch it all, with the dget *URL* command. This command (which can be found in the devscripts package) fetches the *.dsc file at the given address, then analyzes its contents, and automatically fetches the file or files referenced within. Once everything has been downloaded, it extracts the source package (unless the -d or --download-only option is used).

现在包的代码在以源包及其版本命名的目录中（例如，samba-4.1.17+dfsg);这是我们将处理本地更改的地方。

The first thing to do is to change the package version number, so that the rebuilt packages can be distinguished from the original packages provided by Debian. Assuming the current version is 2:4.1.17+dfsg-2, we can create version 2:4.1.17+dfsg-2falcot1, which clearly indicates the origin of the package. This makes the package version number higher than the one provided by Debian, so that the package will easily install as an update to the original package. Such a change is best effected with the command (Debian CHangelog) from the devscripts package, with an command such as dch --local falcot. This invokes a text editor (sensible-editor — this should be your favorite editor if it is mentioned in the VISUAL or EDITOR environment variables, and the default editor otherwise) to allow documenting the differences brought by this rebuild. This editor shows us that dch really did change the debian/changelog file.

When a change in build options is required, the changes need to be made in debian/rules, which drives the steps in the package build process. In the simplest cases, the lines concerning the initial configuration (./configure …) or the actual build ($(MAKE) … or make …) are easy to spot. If these commands are not explicitly called, they are probably a side effect of another explicit command, in which case please refer to their documentation to learn more about how to change the default behavior. With packages using dh, you might need to add an override for the dh_auto_configure or dh_auto_build commands (see their respective manual pages for explanations on how to achieve this).

If you know for sure that these build-dependencies are too strict, you should feel free to relax them locally. Reading the files which document the standard way of building the software — these files are often called INSTALL — will help you figure out the appropriate dependencies. Ideally, all dependencies should be satisfiable from the distribution used for the rebuild; if they are not, a recursive process starts, whereby the packages mentioned in the Build-Depends field must be backported before the target package can be. Some packages may not need backporting, and can be installed as-is during the build process (a notable example is debhelper). Note that the backporting process can quickly become complex if you are not careful. Therefore, backports should be kept to a strict minimum when possible.

技巧 安装构建依赖

apt-get allows installing all packages mentioned in the Build-Depends fields of a source package available in a distribution mentioned in a line of the /etc/apt/sources.list file. This is a simple matter of running the apt-get build-dep *source-package* command.

当所有要做的更改都应用到源代码后，我们就可以开始生成实际的二进制包（.deb）。整个过程由dpkg-buildpackage管理。

例 15.1. 重构建一个包

In essence, the package creation process is a simple matter of gathering in an archive a set of existing (or built) files; most of the files will end up being owned by root in the archive. However, building the whole package under this user would imply increased risks; fortunately, this can be avoided with the fakeroot command. This tool can be used to run a program and give it the impression that it runs as root and creates files with arbitrary ownership and permissions. When the program creates the archive that will become the Debian package, it is tricked into creating an archive containing files marked as belonging to arbitrary owners, including root. This setup is so convenient that dpkg-buildpackage uses fakeroot by default when building packages.

Note that the program is only tricked into “believing” that it operates as a privileged account, and the process actually runs as the user running fakeroot *program* (and the files are actually created with that user’s permissions). At no time does it actually get root privileges that it could abuse.

The previous command can fail if the Build-Depends fields have not been updated, or if the related packages are not installed. In such a case, it is possible to overrule this check by passing the -d option to dpkg-buildpackage. However, explicitly ignoring these dependencies runs the risk of the build process failing at a later stage. Worse, the package may seem to build correctly but fail to run properly: some programs automatically disable some of their features when a required library is not available at build time.

More often than not, Debian developers use a higher-level program such as debuild; this runs dpkg-buildpackage as usual, but it also adds an invocation of a program that runs many checks to validate the generated package against the Debian policy. This script also cleans up the environment so that local environment variables do not “pollute” the package build. The debuild command is one of the tools in the devscripts suite, which share some consistency and configuration to make the maintainers’ task easier.

QUICK LOOK pbuilder

This tool allows the build process to happen in an environment that is not altered by users’ manipulations. This also allows for quick detection of the missing build-dependencies (since the build will fail unless the appropriate dependencies are documented). Finally, it allows building a package for a Debian version that is not the one used by the system as a whole: the machine can be using Stable for its normal workload, and a pbuilder running on the same machine can be using Unstable for package builds.