You can then edit it, build updated binary packages (.debs) and install and run them. You can also share your work with others.
This tutorial gives some recipes and hints for this. It assumes you have basic familiarity with git. It does not assume any initial familiarity with Debian's packaging processes.
If you are a package maintainer within Debian; a DM or DD; and/or a sponsee: this tutorial is not for you. Try dgit-nmu-simple(7), dgit-maint-*(7), or dgit(1) and dgit(7).
% dgit clone glibc jessie,-security % cd glibc % curl 'https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=28250;mbox=yes;msg=89' | patch -p1 -u % git commit -a -m 'Fix libc lost output bug' % gbp dch -S --since=dgit/dgit/sid --ignore-branch --commit % mk-build-deps --root-cmd=sudo --install % dpkg-buildpackage -uc -b % sudo dpkg -i ../libc6_*.deb
% git clean -xdf % git reset --hard
% cd glibc % dgit pull jessie,-security % gbp dch -S --since=dgit/dgit/sid --ignore-branch --commit % dpkg-buildpackage -uc -b % sudo dpkg -i ../libc6_*.deb
% dgit clone glibc jessie,-security % cd glibc
dgit clone needs to be told the source package name (which might be different to the binary package name, which was the name you passed to ``apt-get install'') and the codename or alias of the Debian release (this is called the ``suite'').
% dpkg -S /lib/i386-linux-gnu/libc.so.6 libc6:i386: /lib/i386-linux-gnu/libc.so.6 % dpkg -s libc6:i386 Package: libc6 Status: install ok installed ... Source: glibc
(In this example, libc6 is a ``multi-arch: allowed'' package, which means that it exists in several different builds for different architectures. That's where ":i386" comes from.)
If you don't know what you're running, try this:
% grep '^deb' /etc/apt/sources.list deb http://the.earth.li/debian/ jessie main non-free contrib ... %
For Debian, you should add ",-security" to the end of the suite name, unless you're on unstable or testing. Hence, in our example "jessie" becomes "jessie,-security". (Yes, with a comma.)
For each release (like "jessie") there is a tracking branch for the contents of the archive, called "remotes/dgit/dgit/jessie" (and similarly for other suites). This can be updated with "dgit fetch jessie". This, the remote suite branch, is synthesized by your local copy of dgit. It is fast forwarding.
Debian separates out the security updates, into "*-security". Telling dgit "jessie,-security" means that it should include any updates available in "jessie-security". The comma notation is a request to dgit to track jessie, or jessie-security if there is an update for the package there.
(You can also dgit fetch in a tree that wasn't made by dgit clone. If there's no "debian/changelog" you'll have to supply a "-p"package option to dgit fetch.)
The package's Debian metadata and the scripts for building binary packages are under "debian/". "debian/control", "debian/changelog" and "debian/rules" are the starting points. The Debian Policy Manual has most of the in-depth technical details.
For many Debian packages, there will also be some things in "debian/patches/". It is best to ignore these. Insofar as they are relevant the changes there will have been applied to the actual files, probably by means of actual comments in the git history. The contents of debian/patches are ignored when building binaries from dgitish git branches.
(For Debian afficionados: the git trees that come out of dgit are ``patches-applied packaging branches without a .pc directory''.)
But for many packages the real git history does not exist, or has not been published in a dgitish form. So you may find that the history is a rather short history invented by dgit.
dgit histories often contain automatically-generated commits, including commits which make no changes but just serve to make a rebasing branch fast-forward. This is particularly true of combining branches like "jessie,-security".
If the package maintainer is using git then after dgit clone you may find that there is a useful "vcs-git" remote referring to the Debian package maintainer's repository for the package. You can see what's there with "git fetch vcs-git". But use what you find there with care: Debian maintainers' git repositories often have contents which are very confusing and idiosyncratic. In particular, you may need to manually apply the patches that are in debian/patches before you do anything else!
% wget 'https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=28250;mbox=yes;msg=89' | patch -p1 -u % git commit -a -m 'Fix libc lost output bug'
Debian package builds are often quite messy: they may modify files which are also committed to git, or leave outputs and temporary files not covered by ".gitignore".
If you always commit, you can use
% git clean -xdf % git reset --hard
to tidy up after a build. (If you forgot to commit, don't use those commands; instead, you may find that you can use "git add -p" to help commit what you actually wanted to keep.)
These are destructive commands which delete all new files (so you must remember to say "git add") and throw away edits to every file (so you must remember to commit).
% gbp dch -S --since=dgit/dgit/sid --ignore-branch --commit
The binaries you build will have a version number which ultimately comes from the "debian/changelog". You want to be able to tell your binaries apart from your distro's.
So you should update "debian/changelog" to add a new stanza at the top, for your build.
This rune provides an easy way to do this. It adds a new changelog entry with an uninformative message and a plausible version number (containing a bit of your git commit id).
If you want to be more sophisticated, the package "dpkg-dev-el" has a good Emacs mode for editing changelogs. Alternatively, you could edit the changelog with another text editor, or run "dch" or "gbp dch" with different options. Choosing a good version number is slightly tricky and a complete treatment is beyond the scope of this tutorial.
% mk-build-deps --root-cmd=sudo --install % dpkg-buildpackage -uc -b
dpkg-buildpackage is the primary tool for building a Debian source package. "-uc" means not to pgp-sign the results. "-b" means build all binary packages, but not to build a source package.
% git clean -xdf % sbuild -c jessie -A --no-clean-source \ --dpkg-source-opts='-Zgzip -z1 --format=1.0 -sn'
Note that this will seem to leave a ``source package'' (.dsc and .tar.gz) in the parent directory, but that source package should not be used. It is likely to be broken. For more information see Debian bug #868527.
% sudo dpkg -i ../libc6_*.deb
You can use "dpkg -i" to install the .debs that came out of your package.
If the dependencies aren't installed, you will get an error, which can usually be fixed with "apt-get -f install".
% sudo apt install ../libc6_*.deb
dpkg: error processing package libpcre3-dev:amd64 (--configure): package libpcre3-dev:amd64 2:8.39-3~3.gbp8f25f5 cannot be configured because libpcre3-dev:i386 is at a different version (2:8.39-2)
The multiarch system used by Debian requires each package which is present for multiple architectures to be exactly the same across all the architectures for which it is installed.
The proper solution is to build the package for all the architectures you have enabled. You'll need a chroot for each of the secondary architectures. This is somewhat tiresome, even though Debian has excellent tools for managing chroots. "sbuild-debian-developer-setup" from the package of the same name and "sbuild-createchroot" from the "sbuild" package are good starting points.
Otherwise you could deinstall the packages of interest for those other architectures with something like "dpkg --remove libpcre3:i386".
If neither of those are an option, your desperate last resort is to try using the same version number as the official package for your own package. (The version is controlled by "debian/changelog" - see above.) This is not ideal because it makes it hard to tell what is installed, and because it will mislead and confuse apt.
With the ``same number'' approach you may still get errors like
but passing "--force-overwrite" to dpkg will help - assuming you know what you're doing.
Anyone who gets that git branch from you will be able to build binary packages (.deb) just as you did.
If you want to contribute your changes back to Debian, you should probably send them as attachments to an email to the Debian Bug System <https://bugs.debian.org/> (either a followup to an existing bug, or a new bug). Patches in "git-format-patch" format are usually very welcome.
If a git branch is not enough, and you need to provide a source package but don't care about its format/layout (for example because some software you have consumes source packages, not git histories) you can use this recipe to generate a "3.0 (native)" source package, which is just a tarball with accompanying .dsc metadata file:
% echo '3.0 (native)' >debian/source/format % git commit -m 'switch to native source format' debian/source/format % dgit -wgf build-source
If you need to provide a good-looking source package, be prepared for a lot more work. You will need to read much more, perhaps starting with dgit-nmu-simple(7), dgit-sponsorship(7) or dgit-maint-*(7)