Build Phases

stdenv provides many different phases, with default behavior of what to run if no script for that phase is provided.

Many of the phases follow the build steps introduced by Autotools. When we are building non-C/C++ projects, only a few phases are essential. Still, it is useful to take a quick look at what phases are there, and what they offers.

The nixpkgs manual has the full list of phases.

The phases Attribute

We can force stdenv to run only specific phases by specifying them in the phases attribute.

nix-repl> nixpkgs.stdenv.mkDerivation {
            name = "hello";
            phases = [ "installPhase" ];
            installPhase = "echo 'Hello World!' > $out";
«derivation /nix/store/m09hj2xs3yc45y3d4rdm8wks7cay00ak-hello.drv»
$ nix-build /nix/store/m09hj2xs3yc45y3d4rdm8wks7cay00ak-hello.drv
these derivations will be built:
building '/nix/store/m09hj2xs3yc45y3d4rdm8wks7cay00ak-hello.drv'...

$ cat /nix/store/hpj3y6as9s07444qi6nap0f5dp5k84b6-hello
Hello World!

You may notice that the build log for this version of hello is much shorter than the usual output of standard derivations. Messages such as post-installation fixup are not shown here.

This is because those are implicit steps performed in phases such as fixupPhase. We will go through later why those phases are there. But as you can see, these phases can be disabled by explicitly specifying the phases to run.

There is also no need to explicitly skip required phases like unpackPhase, which we previously set to true.

Unpack Phase

The unpack phase is used to unpack source code into temporary directories to be used for compilation. By default, unpackPhase unpacks the source code specified in $src, and if none is provided, it will abort with an error.

nix-repl> nixpkgs.stdenv.mkDerivation {
            name = "hello";
            installPhase = "echo 'Hello World!' > $out";
«derivation /nix/store/cx3p30jp0y0l8ixl426drsp81vcqagpr-hello.drv»
$ nix-build /nix/store/cx3p30jp0y0l8ixl426drsp81vcqagpr-hello.drv
these derivations will be built:
building '/nix/store/cx3p30jp0y0l8ixl426drsp81vcqagpr-hello.drv'...
unpacking sources
variable $src or $srcs should point to the source
builder for '/nix/store/cx3p30jp0y0l8ixl426drsp81vcqagpr-hello.drv' failed with exit code 1
error: build of '/nix/store/cx3p30jp0y0l8ixl426drsp81vcqagpr-hello.drv' failed

This is why when we don't have any source code, we have to explicitly skip the unpackPhase by telling it to run true instead.

Let's try to see what is done in unpackPhase when we give it some source directories.

nix-repl> nixpkgs.stdenv.mkDerivation {
            name = "fibonacci-src";
            src = ./04-derivations/03-fibonacci;
            installPhase = ''
              set -x

              ls -la .
              cp -r ./ $out/

              set +x
«derivation /nix/store/8l0s01nk0fc1zicb9qkdmpwsw01qr5p8-fibonacci.drv»
$ nix-build /nix/store/8l0s01nk0fc1zicb9qkdmpwsw01qr5p8-fibonacci.drv
these derivations will be built:
building '/nix/store/8l0s01nk0fc1zicb9qkdmpwsw01qr5p8-fibonacci.drv'...
unpacking sources
unpacking source archive /nix/store/a5f73yy0a8dn0p12pfriqbqyag0ksfkq-03-fibonacci
source root is 03-fibonacci
patching sources
no configure script, doing nothing
no Makefile, doing nothing
++ pwd
++ ls -la .
total 16
drwxr-xr-x 2 nixbld nixbld 4096 Jan  1  1970 .
drwx------ 3 nixbld nixbld 4096 Dec  1 09:19 ..
-rw-r--r-- 1 nixbld nixbld 1772 Jan  1  1970 fib-serialized.nix
-rw-r--r-- 1 nixbld nixbld 1602 Jan  1  1970 fib.nix
++ cp -r ./ /nix/store/qqd4msyqya0xhqxcyra0lf7v09z2q522-fibonacci/
++ set +x
post-installation fixup
shrinking RPATHs of ELF executables and libraries in /nix/store/qqd4msyqya0xhqxcyra0lf7v09z2q522-fibonacci
strip is /nix/store/bnjps68g8ax6abzvys2xpx12imrx8949-binutils-2.31.1/bin/strip
patching script interpreter paths in /nix/store/qqd4msyqya0xhqxcyra0lf7v09z2q522-fibonacci
checking for references to /build/ in /nix/store/qqd4msyqya0xhqxcyra0lf7v09z2q522-fibonacci...

$ ls -la /nix/store/qqd4msyqya0xhqxcyra0lf7v09z2q522-fibonacci
total 5728
dr-xr-xr-x    2 user user    4096 Jan  1  1970 .
drwxr-xr-x 5403 user user 5849088 Dez  1 10:19 ..
-r--r--r--    1 user user    1602 Jan  1  1970 fib.nix
-r--r--r--    1 user user    1772 Jan  1  1970 fib-serialized.nix

As we can see, unpackPhase copies the content of the source code specified in $src into the temporary build directory. It also modifies the chmod permissions to allow write permission to the files and directories.

Patch Phase

Configure Phase

Build Phase

Check Phase

Install Phase

Fixup Phase