I want to read the source code of some package in Debian; how can I do that?
Downloading source code from Debian repositories is as simple as running
apt-get source <PACKAGE>. It will put 3 files in your current directory,
.dsc that describes package,
.orig.tar.gz that contains unmodified sources, and
.diff.gz that contains Debian-specific changes. But if you get error
E: Unable to find a source package for <PACKAGE>, then you might not have the source code repository specified, so go to
/etc/apt/sources.list (or some file in
/etc/apt/sources.list.d/ that contains your Debian repository), find a string like:
deb ftp://ftp.debian.org/debian wheezy main contrib non-free
deb-src ftp://ftp.debian.org/debian wheezy main contrib non-free
Update package index files with
sudo apt-get update and try again. See also:
A second possibility is to download the source code from upstream directly instead of downloading it from Debian repositories. This has the advantage that if you not only want to read the source code but might like to change something you can directly commit and submit it to upstream (assuming it is not a Debian patch).
You can usually find out the upstream source code repository URL in the file
$ head /usr/share/doc/git/copyright Format: http://www.debian.org/doc/packaging-manuals/copyright-format/1.0/ Upstream-Contact: firstname.lastname@example.org Source: https://www.kernel.org/pub/software/scm/git/ Files: * Copyright: © 2005-2014, Linus Torvalds and others. License: GPL-2 Files: xdiff/* Copyright: © 2003-2009, Davide Libenzi, Johannes E. Schindelin
This file is also referenced from packages.debian.org (search for "Copyright File").