If you're looking to get started at packaging free software for Debian, you should start with the excellent New Maintainers' Guide or the Introduction to Debian Packaging on the Debian wiki.

Once you know the basics, or if you prefer to learn by example, you may be interested in the full walkthrough which follows. We will look at the contents of three simple packages.

node-libravatar

This package is a node.js library for the Libravatar service.

Version 2.0.0-3 of that package contains the following files in its debian/ directory:

changelog
compat
control
copyright
docs
node-libravatar.install
rules
source/format
watch

debian/control

Source: node-libravatar
Priority: extra
Maintainer: Francois Marier <francois@debian.org>
Build-Depends: debhelper (>= 9)
Standards-Version: 3.9.4
Section: web
Homepage: https://github.com/fmarier/node-libravatar
Vcs-Git: git://git.debian.org/collab-maint/node-libravatar.git
Vcs-Browser: http://git.debian.org/?p=collab-maint/node-libravatar.git;a=summary

Package: node-libravatar
Architecture: all
Depends: ${shlibs:Depends}, ${misc:Depends}, nodejs
Description: libravatar library for NodeJS
 This library allows web application authors to make use of the free Libravatar
 service (https://www.libravatar.org). This service hosts avatar images for
 users and allows other sites to look them up using email addresses.
 .
 node-libravatar includes full support for federated avatar servers.

This is probably the most important file since it contains the bulk of the metadata about this package.

Maintainer is a required field listing the maintainer of that package, which can be a person or a team. It only contains a single value though, any co-maintainers will be listed under the optional Uploaders field.

Build-Depends lists the packages which are needed to build the package (e.g. a compiler), as opposed to those which are needed to install the binary package (e.g. a library it uses).

Standards-Version refers to the version of the Debian Policy that this package complies with.

The Homepage field refers to the upstream homepage, whereas the Vcs-* fields point to the repository where the packaging is stored. If you take a look at the node-libravatar packaging repository you will see that it contains three branches:

upstream is the source as it was in the tarball downloaded from upstream.
master is the upstream branch along with all of the Debian customizations.
pristine-tar is unrelated to the other two branches and is used by the pristine-tar tool to reconstitute the original upstream tarball as needed.

After these fields comes a new section which starts with a Package field. This is the definition of a binary package, not to be confused with the Source field at the top of this file, which refers to the name of the source package. In this particular example, they are both the same and there is only one of each, however this is not always the case, as we'll see later.

Inside that binary package definition, lives the Architecture field which is normally one of these two:

all for a binary package that will work on all architectures but only needs to be built once
any for a binary package that will work everywhere but that will need to be built separately for each architecture

Finally, the last field worth pointing out is the Depends field which lists all of the runtime dependencies that the binary package has. This is what will be pulled in by apt-get when you apt-get install node-libravatar. The two variables will be substituted later by debhelper.

debian/changelog

node-libravatar (2.0.0-3) unstable; urgency=low

  * debian/watch: poll github directly
  * Bump Standards-Version up to 3.9.4

 -- Francois Marier <francois@debian.org>  Mon, 20 May 2013 12:07:49 +1200

node-libravatar (2.0.0-2) unstable; urgency=low

  * More precise license tag and upstream contact in debian/copyright

 -- Francois Marier <francois@debian.org>  Tue, 29 May 2012 22:51:03 +1200

node-libravatar (2.0.0-1) unstable; urgency=low

  * New upstream release
    - new non-backward-compatible API

 -- Francois Marier <francois@debian.org>  Mon, 07 May 2012 14:54:19 +1200

node-libravatar (1.1.1-1) unstable; urgency=low

  * Initial release (Closes: #661771)

 -- Francois Marier <francois@debian.org>  Fri, 02 Mar 2012 15:29:57 +1300

This may seem at first like a mundane file, but it is very important since it is the canonical source of the package version (2.0.0-3 in this case). This is the only place where you need to bump the package version when uploading a new package to the Debian archive.

The first line also includes the distribution where the package will be uploaded. It is usually one of these values:

unstable for the vast majority of uploads
stable for uploads that have been approved by the release maintainers and fix serious bugs in the stable version of Debian
stable-security for security fixes to the stable version of Debian that cannot wait until the next stable point release and have been approved by the security team

Packages uploaded to unstable will migrate automatically to testing provided that a few conditions are met (e.g. no release-critical bugs were introduced). The length of time before that migration is influenced by the urgency field (low, medium or high) in the changelog entry.

Another thing worth noting is that the first upload normally needs to close an ITP (Intent to Package) bug.

debian/rules

#!/usr/bin/make -f
# -*- makefile -*-

%:
    dh $@ 

override_dh_auto_test:

As can be gathered from the first two lines of this file, this is a Makefile. This is what controls how the package is built.

There's not much to see and that's because most of its content is automatically added by debhelper. So let's look at it in action by building the package:

$ git buildpackage -us -uc

and then looking at parts of the build log (../node-libravatar_2.0.0-3_amd64.build):

 fakeroot debian/rules clean
dh clean 
   dh_testdir
   dh_auto_clean
   dh_clean

One of the first things we see is the debian/rules file being run with the clean target. To find out what that does, have a look at the dh_auto_clean which states that it will attempt to delete build residues and run something like make clean using the upstream Makefile.

 debian/rules build
dh build 
   dh_testdir
   dh_auto_configure
   dh_auto_build

Next we see the build target being invoked and looking at dh_auto_configure we see that this will essentially run ./configure and its equivalents.

The dh_auto_build helper script then takes care of running make (or equivalent) on the upstream code.

This should be familiar to anybody who has ever built a piece of free software from scratch and has encountered the usual method for building from source:

./configure
make
make install

Finally, we get to actually build the .deb:

 fakeroot debian/rules binary
dh binary 
   dh_testroot
   dh_prep
   dh_installdirs
   dh_auto_install
   dh_install
...
   dh_md5sums
   dh_builddeb
dpkg-deb: building package `node-libravatar' in `../node-libravatar_2.0.0-3_all.deb'.

Here we see a number of helpers, including dh_auto_install which takes care of running make install.

Going back to the debian/rules, we notice that there is manually defined target at the bottom of the file:

override_dh_auto_test:

which essentially disables dh_auto_test by replacing it with an empty set of commands.

The reason for this becomes clear when we take a look at the test target of the upstream Makefile and the dependencies it has: tap, a node.js library that is not yet available in Debian.

In other words, we can't run the test suite on the build machines so we need to disable it here.

debian/compat

This file simply specifies the version of debhelper that is required by the various helpers used in debian/rules. Version 9 is the latest at the moment.

debian/copyright

Format: http://www.debian.org/doc/packaging-manuals/copyright-format/1.0/
Upstream-Name: node-libravatar
Upstream-Contact: Francois Marier <francois@libravatar.org>
Source: https://github.com/fmarier/node-libravatar

Files: *
Copyright: 2011 Francois Marier <francois@libravatar.org>
License: Expat

Files: debian/*
Copyright: 2012 Francois Marier <francois@debian.org>
License: Expat

License: Expat
 Permission is hereby granted, free of charge, to any person obtaining a copy of this
 software and associated documentation files (the "Software"), to deal in the Software
 without restriction, including without limitation the rights to use, copy, modify,
 merge, publish, distribute, sublicense, and/or sell copies of the Software, and to
 permit persons to whom the Software is furnished to do so, subject to the following
 conditions:
 .
 The above copyright notice and this permission notice shall be included in all copies
 or substantial portions of the Software.
 .
 THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
 INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
 PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
 HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF
 CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE
 OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

This machine-readable file lists all of the different licenses encountered in this package.

It requires that the maintainer audits the upstream code for any copyright statements that might be present in addition to the license of the package as a whole.

debian/docs

README.md

This file contains a list of upstream files that will be copied into the /usr/share/doc/node-libravatar/ directory by dh_installdocs.

debian/node-libravatar.install

lib/*    usr/lib/nodejs/

The install file is used by dh_install to supplement the work done by dh_auto_install which, as we have seen earlier, essentially just runs make install on the upstream Makefile.

Looking at that upstream Makefile, it becomes clear that the files will need to be installed manually by the Debian package since that Makefile doesn't have an install target.

debian/watch

version=3
https://github.com/fmarier/node-libravatar/tags /fmarier/node-libravatar/archive/node-libravatar-([0-9.]+)\.tar\.gz

This is the file that allows Debian tools like the Package Tracking System to automatically detect that a new upstream version is available.

What it does is simply visit the upstream page which contains all of the release tarballs and look for links which have an href matching the above regular expression.

Running uscan --report --verbose will show us all of the tarballs that can be automatically discovered using this watch file:

-- Scanning for watchfiles in .
-- Found watchfile in ./debian
-- In debian/watch, processing watchfile line:
   https://github.com/fmarier/node-libravatar/tags /fmarier/node-libravatar/archive/node-libravatar-([0-9.]+)\.tar\.gz
-- Found the following matching hrefs:
     /fmarier/node-libravatar/archive/node-libravatar-2.0.0.tar.gz
     /fmarier/node-libravatar/archive/node-libravatar-1.1.1.tar.gz
     /fmarier/node-libravatar/archive/node-libravatar-1.1.0.tar.gz
     /fmarier/node-libravatar/archive/node-libravatar-1.0.1.tar.gz
     /fmarier/node-libravatar/archive/node-libravatar-1.0.0.tar.gz
Newest version on remote site is 2.0.0, local version is 2.0.0
 => Package is up to date
-- Scan finished

pylibravatar

This second package is the equivalent Python library for the Libravatar service.

Version 1.6-2 of that package contains similar files in its debian/ directory, but let's look at two in particular:

control
upstream/signing-key.asc

debian/control

Source: pylibravatar
Section: python
Priority: optional
Maintainer: Francois Marier <francois@debian.org>
Build-Depends: debhelper (>= 9), python-all, python3-all
Standards-Version: 3.9.5
Homepage: https://launchpad.net/pyLibravatar
...

Package: python-libravatar
Architecture: all
Depends: ${misc:Depends}, ${python:Depends}, python-dns, python
Description: Libravatar module for Python 2
 Module to make use of the federated Libravatar.org avatar hosting service
 from within Python applications.
...

Package: python3-libravatar
Architecture: all
Depends: ${misc:Depends}, ${python3:Depends}, python3-dns, python3
Description: Libravatar module for Python 3
 Module to make use of the federated Libravatar.org avatar hosting service
 from within Python applications.
...

Here is an example of a source package (pylibravatar) which builds two separate binary packages: python-libravatar and python3-libravatar.

This highlights the fact that a given upstream source can be split into several binary packages in the archive when it makes sense. In this case, there is no point in Python 2 applications pulling in the Python 3 files, so the two separate packages make sense.

Another common example is the use of a -doc package to separate the documentation from the rest of a package so that it doesn't need to be installed on production servers for example.

debian/upstream/signing-key.asc

This is simply the OpenPGP key that the upstream developer uses to sign release tarballs.

Since PGP signatures are available on the upstream download page, it's possible to instruct uscan to check signatures before downloading tarballs.

The way to do that is to use the pgpsigurlmange option in debian/watch:

version=3
opts=pgpsigurlmangle=s/$/.asc/ https://pypi.python.org/pypi/pyLibravatar https://pypi.python.org/packages/source/p/pyLibravatar/pyLibravatar-(.*)\.tar\.gz

which is simply a regular expression replacement string which takes the tarball URL and converts it to the URL of the matching PGP signature.

fcheck

The last package we will look at is a file integrity checker. It essentially goes through all of the files in /usr/bin/ and /usr/lib/ and stores a hash of them in its database. When one of these files changes, you get an email.

In particular, we will look at the following files in the debian/ directory of version 2.7.59-18:

dirs
fcheck.cron.d
fcheck.postrm
fcheck.postinst
patches/
README.Debian
rules
source/format

debian/patches

This directory contains ten patches as well as a file called series which lists the patches that should be applied to the upstream source and in which order. Should you need to temporarily disable a patch, simply remove it from this file and it will no longer be applied at build time.

Let's have a look at patches/04_cfg_sha256.patch:

Description: Switch to sha256 hash algorithm
Forwarded: not needed
Author: Francois Marier <francois@debian.org>
Last-Update: 2009-03-15

--- a/fcheck.cfg
+++ b/fcheck.cfg
@@ -149,8 +149,7 @@ TimeZone        = EST5EDT
 #$Signature      = /usr/bin/sum
 #$Signature      = /usr/bin/cksum
 #$Signature      = /usr/bin/md5sum
-$Signature      = /bin/cksum
-
+$Signature      = /usr/bin/sha256sum


 # Include an optional configuration file.

This is a very simple patch which changes the default configuration of fcheck to promote the use of a stronger hash function. At the top of the file is a bunch of metadata in the DEP-3 format.

Why does this package contain so many customizations to the upstream code when Debian's policy is to push fixes upstream and work towards reduce the delta between upstream and Debian's code? The answer can be found in debian/control:

Homepage: http://web.archive.org/web/20050415074059/www.geocities.com/fcheck2000/

This package no longer has an upstream maintainer and its original source is gone. In other words, the Debian package is where all of the new bug fixes get done.

debian/source/format

3.0 (quilt)

This file contains what is called the source package format. What it basically says is that the patches found in debian/patches/ will be applied to the upstream source using the quilt tool at build time.

debian/fcheck.postrm

#!/bin/sh
# postrm script for fcheck
#
# see: dh_installdeb(1)

set -e

# summary of how this script can be called:
#        * <postrm> `remove'
#        * <postrm> `purge'
#        * <old-postrm> `upgrade' <new-version>
#        * <new-postrm> `failed-upgrade' <old-version>
#        * <new-postrm> `abort-install'
#        * <new-postrm> `abort-install' <old-version>
#        * <new-postrm> `abort-upgrade' <old-version>
#        * <disappearer's-postrm> `disappear' <overwriter>
#          <overwriter-version>
# for details, see http://www.debian.org/doc/debian-policy/ or
# the debian-policy package


case "$1" in
    remove|upgrade|failed-upgrade|abort-install|abort-upgrade|disappear)
    ;;

    purge)
      if [ -e /var/lib/fcheck/fcheck.dbf ]; then
        echo "Purging old database file ..."
        rm -f /var/lib/fcheck/fcheck.dbf
      fi
      rm -rf /var/lib/fcheck
      rm -rf /var/log/fcheck
      rm -rf /etc/fcheck
    ;;

    *)
        echo "postrm called with unknown argument \`$1'" >&2
        exit 1
    ;;
esac

# dh_installdeb will replace this with shell code automatically
# generated by other debhelper scripts.

#DEBHELPER#

exit 0

This script is one of the many possible maintainer scripts that a package can provide if needed.

This particular one, as the name suggests, will be run after the package is removed (apt-get remove fcheck) or purged (apt-get remove --purge fcheck). Looking at the case statement above, it doesn't do anything extra in the remove case, but it deletes a few files and directories when called with the purge argument.

debian/README.Debian

This optional README file contains Debian-specific instructions that might be useful to users. It supplements the upstream README which is often more generic and cannot assume a particular system configuration.

debian/rules

#!/usr/bin/make -f
# -*- makefile -*-
# Sample debian/rules that uses debhelper.
# This file was originally written by Joey Hess and Craig Small.
# As a special exception, when this file is copied by dh-make into a
# dh-make output file, you may use that output file without restriction.
# This special exception was added by Craig Small in version 0.37 of dh-make.

# Uncomment this to turn on verbose mode.
#export DH_VERBOSE=1

build-arch:
build-indep:
build: build-stamp

build-stamp:
    dh_testdir
    pod2man --section=8 $(CURDIR)/debian/fcheck.pod > $(CURDIR)/fcheck.8
    touch build-stamp

clean:
    dh_testdir
    dh_testroot
    rm -f build-stamp 
    rm -f $(CURDIR)/fcheck.8
    dh_clean

install: build
    dh_testdir
    dh_testroot
    dh_prep
    dh_installdirs
    cp $(CURDIR)/fcheck $(CURDIR)/debian/fcheck/usr/sbin/fcheck
    cp $(CURDIR)/fcheck.cfg $(CURDIR)/debian/fcheck/etc/fcheck/fcheck.cfg

# Build architecture-independent files here.
binary-arch: build install

# Build architecture-independent files here.
binary-indep: build install
    dh_testdir
    dh_testroot
    dh_installdocs
    dh_installcron
    dh_installman fcheck.8
    dh_installchangelogs
    dh_installexamples
    dh_installlogcheck
    dh_link
    dh_strip
    dh_compress
    dh_fixperms
    dh_installdeb
    dh_shlibdeps
    dh_gencontrol
    dh_md5sums
    dh_builddeb

binary: binary-indep binary-arch
.PHONY: build clean binary-indep binary-arch binary install

This is an example of a old-style debian/rules file which you still encounter in packages which haven't yet upgraded to the latest version of debhelper 9, as can be shown by the contents of debian/compat:

It does essentially the same thing that what we've seen in the build log, but in a more verbose way.

debian/dirs

usr/sbin
etc/fcheck

This file contains a list of directories that dh_installdirs will create in the build directory.

The reason why these directories need to be created is that files are copied into these directories in the install target of the debian/rules file.

Note that this is different from directories which are created at the time of installation of the package. In that case, the directory (e.g. /var/log/fcheck/) must be created in the postinst script and removed in the postrm script.

debian/fcheck.cron.d

#
# Regular cron job for the fcheck package
#
30 */2  * * *   root    test -x /usr/sbin/fcheck && if ! nice ionice -c3 /usr/sbin/fcheck -asxrf /etc/fcheck/fcheck.cfg >/var/run/fcheck.out 2>&1; then mailx -s "ALERT: [fcheck] `hostname --fqdn`" root </var/run/fcheck.out ; /usr/sbin/fcheck -cadsxlf /etc/fcheck/fcheck.cfg ; fi ; rm -f /var/run/fcheck.out

This file is the cronjob which drives the checks performed by this package. It will be copied to /etc/cron.d/fcheck by dh_installcron.

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