Use 2 spaces of indentation per indentation level in Nix expressions, 4 spaces in shell scripts. Do not use tab characters, i.e. configure your editor to use soft tabs. For instance, use (setq-default indent-tabs-mode nil) in Emacs. Everybody has different tab settings so it’s asking for trouble. Use lowerCamelCase for variable names, not UpperCamelCase. Note, this rule does not apply to package attribute names, which instead follow the rules in . Function calls with attribute set arguments are written as foo { arg = ...; } not foo { arg = ...; } Also fine is foo { arg = ...; } if it's a short call. In attribute sets or lists that span multiple lines, the attribute names or list elements should be aligned: # A long list. list = [ elem1 elem2 elem3 ]; # A long attribute set. attrs = { attr1 = short_expr; attr2 = if true then big_expr else big_expr; }; # Alternatively: attrs = { attr1 = short_expr; attr2 = if true then big_expr else big_expr; }; Short lists or attribute sets can be written on one line: # A short list. list = [ elem1 elem2 elem3 ]; # A short set. attrs = { x = 1280; y = 1024; }; Breaking in the middle of a function argument can give hard-to-read code, like someFunction { x = 1280; y = 1024; } otherArg yetAnotherArg (especially if the argument is very large, spanning multiple lines). Better: someFunction { x = 1280; y = 1024; } otherArg yetAnotherArg or let res = { x = 1280; y = 1024; }; in someFunction res otherArg yetAnotherArg The bodies of functions, asserts, and withs are not indented to prevent a lot of superfluous indentation levels, i.e. { arg1, arg2 }: assert system == "i686-linux"; stdenv.mkDerivation { ... not { arg1, arg2 }: assert system == "i686-linux"; stdenv.mkDerivation { ... Function formal arguments are written as: { arg1, arg2, arg3 }: but if they don't fit on one line they're written as: { arg1, arg2, arg3 , arg4, ... , # Some comment... argN }: Functions should list their expected arguments as precisely as possible. That is, write { stdenv, fetchurl, perl }: ... instead of args: with args; ... or { stdenv, fetchurl, perl, ... }: ... For functions that are truly generic in the number of arguments (such as wrappers around mkDerivation) that have some required arguments, you should write them using an @-pattern: { stdenv, doCoverageAnalysis ? false, ... } @ args: stdenv.mkDerivation (args // { ... if doCoverageAnalysis then "bla" else "" ... }) instead of args: args.stdenv.mkDerivation (args // { ... if args ? doCoverageAnalysis && args.doCoverageAnalysis then "bla" else "" ... })

The key words must, must not, required, shall, shall not, should, should not, recommended, may, and optional in this section are to be interpreted as described in RFC 2119. Only emphasized words are to be interpreted in this way. In Nixpkgs, there are generally three different names associated with a package: The name attribute of the derivation (excluding the version part). This is what most users see, in particular when using nix-env. The variable name used for the instantiated package in all-packages.nix, and when passing it as a dependency to other functions. Typically this is called the package attribute name. This is what Nix expression authors see. It can also be used when installing using nix-env -iA. The filename for (the directory containing) the Nix expression. Most of the time, these are the same. For instance, the package e2fsprogs has a name attribute "e2fsprogs-version", is bound to the variable name e2fsprogs in all-packages.nix, and the Nix expression is in pkgs/os-specific/linux/e2fsprogs/default.nix. There are a few naming guidelines: The name attribute should be identical to the upstream package name. The name attribute must not contain uppercase letters — e.g., "mplayer-1.0rc2" instead of "MPlayer-1.0rc2". The version part of the name attribute must start with a digit (following a dash) — e.g., "hello-0.3.1rc2". If a package is not a release but a commit from a repository, then the version part of the name must be the date of that (fetched) commit. The date must be in "YYYY-MM-DD" format. Also append "unstable" to the name - e.g., "pkgname-unstable-2014-09-23". Dashes in the package name should be preserved in new variable names, rather than converted to underscores or camel cased — e.g., http-parser instead of http_parser or httpParser. The hyphenated style is preferred in all three package names. If there are multiple versions of a package, this should be reflected in the variable names in all-packages.nix, e.g. json-c-0-9 and json-c-0-11. If there is an obvious “default” version, make an attribute like json-c = json-c-0-9;. See also

Names of files and directories should be in lowercase, with dashes between words — not in camel case. For instance, it should be all-packages.nix, not allPackages.nix or AllPackages.nix.

Each package should be stored in its own directory somewhere in the pkgs/ tree, i.e. in pkgs/category/subcategory/.../pkgname. Below are some rules for picking the right category for a package. Many packages fall under several categories; what matters is the primary purpose of a package. For example, the libxml2 package builds both a library and some tools; but it’s a library foremost, so it goes under pkgs/development/libraries. When in doubt, consider refactoring the pkgs/ tree, e.g. creating new categories or splitting up an existing category. If it’s used to support software development: If it’s a library used by other packages: development/libraries (e.g. libxml2) If it’s a compiler: development/compilers (e.g. gcc) If it’s an interpreter: development/interpreters (e.g. guile) If it’s a (set of) development tool(s): If it’s a parser generator (including lexers): development/tools/parsing (e.g. bison, flex) If it’s a build manager: development/tools/build-managers (e.g. gnumake) Else: development/tools/misc (e.g. binutils) Else: development/misc If it’s a (set of) tool(s): (A tool is a relatively small program, especially one intended to be used non-interactively.) If it’s for networking: tools/networking (e.g. wget) If it’s for text processing: tools/text (e.g. diffutils) If it’s a system utility, i.e., something related or essential to the operation of a system: tools/system (e.g. cron) If it’s an archiver (which may include a compression function): tools/archivers (e.g. zip, tar) If it’s a compression program: tools/compression (e.g. gzip, bzip2) If it’s a security-related program: tools/security (e.g. nmap, gnupg) Else: tools/misc If it’s a shell: shells (e.g. bash) If it’s a server: If it’s a web server: servers/http (e.g. apache-httpd) If it’s an implementation of the X Windowing System: servers/x11 (e.g. xorg — this includes the client libraries and programs) Else: servers/misc If it’s a desktop environment: desktops (e.g. kde, gnome, enlightenment) If it’s a window manager: applications/window-managers (e.g. awesome, stumpwm) If it’s an application: A (typically large) program with a distinct user interface, primarily used interactively. If it’s a version management system: applications/version-management (e.g. subversion) If it’s for video playback / editing: applications/video (e.g. vlc) If it’s for graphics viewing / editing: applications/graphics (e.g. gimp) If it’s for networking: If it’s a mailreader: applications/networking/mailreaders (e.g. thunderbird) If it’s a newsreader: applications/networking/newsreaders (e.g. pan) If it’s a web browser: applications/networking/browsers (e.g. firefox) Else: applications/networking/misc Else: applications/misc If it’s data (i.e., does not have a straight-forward executable semantics): If it’s a font: data/fonts If it’s related to SGML/XML processing: If it’s an XML DTD: data/sgml+xml/schemas/xml-dtd (e.g. docbook) If it’s an XSLT stylesheet: (Okay, these are executable...) data/sgml+xml/stylesheets/xslt (e.g. docbook-xsl) If it’s a game: games Else: misc

Because every version of a package in Nixpkgs creates a potential maintenance burden, old versions of a package should not be kept unless there is a good reason to do so. For instance, Nixpkgs contains several versions of GCC because other packages don’t build with the latest version of GCC. Other examples are having both the latest stable and latest pre-release version of a package, or to keep several major releases of an application that differ significantly in functionality. If there is only one version of a package, its Nix expression should be named e2fsprogs/default.nix. If there are multiple versions, this should be reflected in the filename, e.g. e2fsprogs/1.41.8.nix and e2fsprogs/1.41.9.nix. The version in the filename should leave out unnecessary detail. For instance, if we keep the latest Firefox 2.0.x and 3.5.x versions in Nixpkgs, they should be named firefox/2.0.nix and firefox/3.5.nix, respectively (which, at a given point, might contain versions 2.0.0.20 and 3.5.4). If a version requires many auxiliary files, you can use a subdirectory for each version, e.g. firefox/2.0/default.nix and firefox/3.5/default.nix. All versions of a package must be included in all-packages.nix to make sure that they evaluate correctly.

There are multiple ways to fetch a package source in nixpkgs. The general guideline is that you should package sources with a high degree of availability. Right now there is only one fetcher which has mirroring support and that is fetchurl. Note that you should also prefer protocols which have a corresponding proxy environment variable. You can find many source fetch helpers in pkgs/build-support/fetch*. In the file pkgs/top-level/all-packages.nix you can find fetch helpers, these have names on the form fetchFrom*. The intention of these are to provide snapshot fetches but using the same api as some of the version controlled fetchers from pkgs/build-support/. As an example going from bad to good: Bad: Uses git:// which won't be proxied. src = fetchgit { url = "git://github.com/NixOS/nix.git"; rev = "1f795f9f44607cc5bec70d1300150bfefcef2aae"; sha256 = "1cw5fszffl5pkpa6s6wjnkiv6lm5k618s32sp60kvmvpy7a2v9kg"; } Better: This is ok, but an archive fetch will still be faster. src = fetchgit { url = "https://github.com/NixOS/nix.git"; rev = "1f795f9f44607cc5bec70d1300150bfefcef2aae"; sha256 = "1cw5fszffl5pkpa6s6wjnkiv6lm5k618s32sp60kvmvpy7a2v9kg"; } Best: Fetches a snapshot archive and you get the rev you want. src = fetchFromGitHub { owner = "NixOS"; repo = "nix"; rev = "1f795f9f44607cc5bec70d1300150bfefcef2aae"; sha256 = "1i2yxndxb6yc9l6c99pypbd92lfq5aac4klq7y2v93c9qvx2cgpc"; } Find the value to put as sha256 by running nix run -f '<nixpkgs>' nix-prefetch-github -c nix-prefetch-github --rev 1f795f9f44607cc5bec70d1300150bfefcef2aae NixOS nix or nix-prefetch-url --unpack https://github.com/NixOS/nix/archive/1f795f9f44607cc5bec70d1300150bfefcef2aae.tar.gz.

Patches available online should be retrieved using fetchpatch. patches = [ (fetchpatch { name = "fix-check-for-using-shared-freetype-lib.patch"; url = "http://git.ghostscript.com/?p=ghostpdl.git;a=patch;h=8f5d285"; sha256 = "1f0k043rng7f0rfl9hhb89qzvvksqmkrikmm38p61yfx51l325xr"; }) ]; Otherwise, you can add a .patch file to the nixpkgs repository. In the interest of keeping our maintenance burden to a minimum, only patches that are unique to nixpkgs should be added in this way. patches = [ ./0001-changes.patch ]; If you do need to do create this sort of patch file, one way to do so is with git: Move to the root directory of the source code you're patching. $ cd the/program/source If a git repository is not already present, create one and stage all of the source files. $ git init $ git add . Edit some files to make whatever changes need to be included in the patch. Use git to create a diff, and pipe the output to a patch file: $ git diff > nixpkgs/pkgs/the/package/0001-changes.patch