Writing Nix ExpressionsThis chapter shows you how to write Nix expressions, which are
the things that tell Nix how to build components. It starts with a
simple example (a Nix expression for GNU Hello), and then moves
on to a more in-depth look at the Nix expression language.A simple Nix expressionThis section shows how to add and test the GNU Hello
package to the Nix Packages collection. Hello is a program
that prints out the text Hello, world!.To add a component to the Nix Packages collection, you generally
need to do three things:
Write a Nix expression for the component. This is a
file that describes all the inputs involved in building the
component, such as dependencies (other components required by the
component), sources, and so on.Write a builder. This is a
shell scriptIn fact, it can be written in any
language, but typically it's a bash shell
script. that actually builds the component from
the inputs.Add the component to the file
pkgs/system/all-packages-generic.nix. The Nix
expression written in the first step is a
function; it requires other components in order
to build it. In this step you put it all together, i.e., you call
the function with the right arguments to build the actual
component.The Nix expressionNix expression for GNU Hello
(default.nix)
{stdenv, fetchurl, perl}:
stdenv.mkDerivation {
name = "hello-2.1.1";
builder = ./builder.sh;
src = fetchurl {
url = ftp://ftp.nluug.nl/pub/gnu/hello/hello-2.1.1.tar.gz;
md5 = "70c9ccf9fac07f762c24f2df2290784d";
};
inherit perl;
} shows a Nix expression for GNU
Hello. It's actually already in the Nix Packages collection in
pkgs/applications/misc/hello/ex-1/default.nix.
It is customary to place each package in a separate directory and call
the single Nix expression in that directory
default.nix. The file has the following elements
(referenced from the figure by number):
This states that the expression is a
function that expects to be called with three
arguments: stdenv, fetchurl,
and perl. They are needed to build Hello, but
we don't know how to build them here; that's why they are function
arguments. stdenv is a component that is used
by almost all Nix Packages components; it provides a
standard environment consisting of the things you
would expect in a basic Unix environment: a C/C++ compiler (GCC,
to be precise), the Bash shell, fundamental Unix tools such as
cp, grep,
tar, etc. fetchurl is a
function that downloads files. perl is the
Perl interpreter.Nix functions generally have the form {x, y, ...,
z}: e where x, y,
etc. are the names of the expected arguments, and where
e is the body of the function. So
here, the entire remainder of the file is the body of the
function; when given the required arguments, the body should
describe how to build an instance of the Hello component.So we have to build a component. Building something from
other stuff is called a derivation in Nix (as
opposed to sources, which are built by humans instead of
computers). We perform a derivation by calling
stdenv.mkDerivation.
mkDerivation is a function provided by
stdenv that builds a component from a set of
attributes. An attribute set is just a list
of key/value pairs where each value is an arbitrary Nix
expression. They take the general form
{name1 =
expr1; ...nameN =
exprN;}.The attribute name specifies the symbolic
name and version of the component. Nix doesn't really care about
these things, but they are used by for instance nix-env
-q to show a human-readable name for
components. This attribute is required by
mkDerivation.The attribute builder specifies the
builder. This attribute can sometimes be omitted, in which case
mkDerivation will fill in a default builder
(which does a configure; make; make install, in
essence). Hello is sufficiently simple that the default builder
would suffice, but in this case, we will show an actual builder
for educational purposes. The value
./builder.sh refers to the shell script shown
in , discussed below.The builder has to know what the sources of the component
are. Here, the attribute src is bound to the
result of a call to the fetchurl function.
Given a URL and an MD5 hash of the expected contents of the file
at that URL, this function builds a derivation that downloads the
file and checks its hash. So the sources are a dependency that
like all other dependencies is built before Hello itself is
built.Instead of src any other name could have
been used, and in fact there can be any number of sources (bound
to different attributes). However, src is
customary, and it's also expected by the default builder (which we
don't use in this example).Since the derivation requires Perl, we have to pass the
value of the perl function argument to the
builder. All attributes in the set are actually passed as
environment variables to the builder, so declaring an attribute
perl = perl;
will do the trick: it binds an attribute perl
to the function argument which also happens to be called
perl. However, it looks a bit silly, so there
is a shorter syntax. The inherit keyword
causes the specified attributes to be bound to whatever variables
with the same name happen to be in scope.The builderBuild script for GNU Hello
(builder.sh)
. $stdenv/setup
PATH=$perl/bin:$PATH
tar xvfz $src
cd hello-*
./configure --prefix=$out
make
make install shows the builder referenced
from Hello's Nix expression (stored in
pkgs/applications/misc/hello/ex-1/builder.sh).
The builder can actually be made a lot shorter by using the
generic builder functions provided by
stdenv, but here we write out the build steps to
elucidate what a builder does. It performs the following
steps:When Nix runs a builder, it initially completely clears the
environment (except for the attributes declared in the
derivation). For instance, the PATH variable is
emptyActually, it's initialised to
/path-not-set to prevent Bash from setting it
to a default value.. This is done to prevent
undeclared inputs from being used in the build process. If for
example the PATH contained
/usr/bin, then you might accidentally use
/usr/bin/gcc.So the first step is to set up the environment. This is
done by calling the setup script of the
standard environment. The environment variable
stdenv points to the location of the standard
environment being used. (It wasn't specified explicitly as an
attribute in , but
mkDerivation adds it automatically.)Since Hello needs Perl, we have to make sure that Perl is in
the PATH. The perl environment
variable points to the location of the Perl component (since it
was passed in as an attribute to the derivation), so
$perl/bin is the
directory containing the Perl interpreter.Now we have to unpack the sources. The
src attribute was bound to the result of
fetching the Hello source tarball from the network, so the
src environment variable points to the location in
the Nix store to which the tarball was downloaded. After
unpacking, we cd to the resulting source
directory.The whole build is performed in a temporary directory
created in /tmp, by the way. This directory is
removed after the builder finishes, so there is no need to clean
up the sources afterwards. Also, the temporary directory is
always newly created, so you don't have to worry about files from
previous builds interfering with the current build.GNU Hello is a typical Autoconf-based package, so we first
have to run its configure script. In Nix
every component is stored in a separate location in the Nix store,
for instance
/nix/store/9a54ba97fb71b65fda531012d0443ce2-hello-2.1.1.
Nix computes this path by cryptographically hashing all attributes
of the derivation. The path is passed to the builder through the
out environment variable. So here we give
configure the parameter
--prefix=$out to cause Hello to be installed in
the expected location.Finally we build Hello (make) and install
it into the location specified by out
(make install).If you are wondering about the absence of error checking on the
result of various commands called in the builder: this is because the
shell script is evaluated with Bash's option,
which causes the script to be aborted if any command fails without an
error check.CompositionComposing GNU Hello
(all-packages-generic.nix)
...
rec {
hello = (import ../applications/misc/hello/ex-1 ) {
inherit fetchurl stdenv perl;
};
perl = (import ../development/interpreters/perl) {
inherit fetchurl stdenv;
};
fetchurl = (import ../build-support/fetchurl) {
inherit stdenv; ...
};
stdenv = ...;
}
The Nix expression in is a
function; it is missing some arguments that have to be filled in
somewhere. In the Nix Packages collection this is done in the file
pkgs/system/all-packages-generic.nix, where all
Nix expressions for components are imported and called with the
appropriate arguments. shows
some fragments of
all-packages-generic.nix.This file defines a set of attributes, all of which are
concrete derivations (i.e., not functions). In fact, we define a
mutually recursive set of attributes. That
is, the attributes can refer to each other. This is precisely
what we want since we want to plug the
various components into each other.Here we import the Nix expression for
GNU Hello. The import operation just loads and returns the
specified Nix expression. In fact, we could just have put the
contents of in
all-packages-generic.nix at this point. That
would be completely equivalent, but it would make the file rather
bulky.Note that we refer to
../applications/misc/hello/ex-1, not
../applications/misc/hello/ex-1/default.nix.
When you try to import a directory, Nix automatically appends
/default.nix to the file name.This is where the actual composition takes place. Here we
call the function imported from
../applications/misc/hello/ex-1 with an
attribute set containing the things that the function expects,
namely fetchurl, stdenv, and
perl. We use inherit again to use the
attributes defined in the surrounding scope (we could also have
written fetchurl = fetchurl;, etc.).The result of this function call is an actual derivation
that can be built by Nix (since when we fill in the arguments of
the function, what we get is its body, which is the call to
stdenv.mkDerivation in ).Likewise, we have to instantiate Perl,
fetchurl, and the standard environment.TestingYou can now try to build Hello. The simplest way to do that is
by using nix-env:
$ nix-env -f pkgs/system/i686-linux.nix -i hello
installing `hello-2.1.1'
building path `/nix/store/632d2b22514dcebe704887c3da15448d-hello-2.1.1'
hello-2.1.1/
hello-2.1.1/intl/
hello-2.1.1/intl/ChangeLog
...
This will build Hello and install it into your profile. The file
i686-linux is just a simple Nix expression that
imports all-packages-generic.nix and instantiates
it for Linux on the x86 platform.Note that the hello argument here refers to
the symbolic name given to the Hello derivation (the
name attribute in ),
not the hello attribute in
all-packages-generic.nix.
nix-env simply walks through all derivations
defined in the latter file, looking for one with a
name attribute matching the command-line
argument.You can test whether it works:
$ hello
Hello, world!Generally, however, using nix-env is not the
best way to test components, since you may not want to install them
into your profile right away (they might not work properly, after
all). A better way is to write a short file containing the
following:
(import pkgs/system/i686-linux.nix).hello
Call it test.nix. You can then build it without
installing it using the command nix-build:
$ nix-build ./test.nix
...
/nix/store/632d2b22514dcebe704887c3da15448d-hello-2.1.1nix-build will build the derivation and print the
output path. It also creates a symlink to the output path called
result in the current directory. This is
convenient for testing the component:
$ ./result/bin/hello
Hello, world!Nix has a transactional semantics. Once a build finishes
successfully, Nix makes a note of this in its database: it registers
that the path denoted by out is now
valid. If you try to build the derivation again, Nix
will see that the path is already valid and finish immediately. If a
build fails, either because it returns a non-zero exit code, because
Nix or the builder are killed, or because the machine crashes, then
the output path will not be registered as valid. If you try to build
the derivation again, Nix will remove the output path if it exists
(e.g., because the builder died half-way through make
install) and try again. Note that there is no
negative caching: Nix doesn't remember that a build
failed, and so a failed build can always be repeated. This is because
Nix cannot distinguish between permanent failures (e.g., a compiler
error due to a syntax error in the source) and transient failures
(e.g., a disk full condition).Nix also performs locking. If you run multiple Nix builds
simultaneously, and they try to build the same derivation, the first
Nix instance that gets there will perform the build, while the others
block (or perform other derivations if available) until the build
finishes. So it is always safe to run multiple instances of Nix in
parallel (contrary to, say, make).If you have a system with multiple CPUs, you may want to have
Nix build different derivations in parallel (insofar as possible).
Just pass the option ,
where N is the maximum number of jobs to be
run in parallel. Typically this should be the number of CPUs.The generic builderRecall from that the builder
looked something like this:
PATH=$perl/bin:$PATH
tar xvfz $src
cd hello-*
./configure --prefix=$out
make
make install
The builders for almost all Unix packages look like this — set up some
environment variables, unpack the sources, configure, build, and
install. For this reason the standard environment provides some Bash
functions that automate the build process. A builder using the
generic build facilities in shown in .Build script using the generic
build functions
buildInputs="$perl"
. $stdenv/setup
genericBuild The buildInputs variable tells
setup to use the indicated components as
inputs. This means that if a component provides a
bin subdirectory, it's added to
PATH; if it has a include
subdirectory, it's added to GCC's header search path; and so
on.The function genericBuild is defined in
the file $stdenv/setup.The final step calls the shell function
genericBuild, which performs the steps that
were done explicitly in . The
generic builder is smart enough to figure out whether to unpack
the sources using gzip,
bzip2, etc. It can be customised in many ways;
see .Discerning readers will note that the
buildInputs could just as well have been set in the Nix
expression, like this:
buildInputs = [perl];
The perl attribute can then be removed, and the
builder becomes even shorter:
. $stdenv/setup
genericBuild
In fact, mkDerivation provides a default builder
that looks exactly like that, so it is actually possible to omit the
builder for Hello entirely.The Nix expression languageThe Nix expression language is a pure, lazy, functional
language. Purity means that operations in the language don't have
side-effects (for instance, there is no variable assignment).
Laziness means that arguments to functions are evaluated only when
they are needed. Functional means that functions are
normal values that can be passed around and manipulated
in interesting ways. The language is not a full-featured, general
purpose language. It's main job is to describe components,
compositions of components, and the variability within
components.This section presents the various features of the
language.Simple valuesNix has the following basic datatypes:
Strings, enclosed between
double quotes, e.g., "foo bar".Integers, e.g.,
123.URIs as defined in appendix B
of RFC
2396, e.g.,
https://svn.cs.uu.nl:12443/dist/trace/trace-nix-trunk.tar.bz2.Paths, e.g.,
/bin/sh or ./builder.sh.
A path must contain at least one slash to be recognised as such; for
instance, builder.sh is not a
pathIt's parsed as an expression that selects the
attribute sh from the variable
builder.. If the filename is
relative, i.e., if it does not begin with a slash, it is made
absolute at parse time relative to the directory of the Nix
expression that contained it. For instance, if a Nix expression in
/foo/bar/bla.nix refers to
../xyzzy/fnord.nix, the absolutised path is
/foo/xyzzy/fnord.nix.Booleans with values
true and
false.ListsLists are formed by enclosing a whitespace-separated list of
values between square bracktes. For example,
[ 123 ./foo.nix "abc" (f {x=y;}) ]
defines a list of four elements, the last being the result of a call
to the function f. Note that function calls have
to be enclosed in parentheses. If they had been omitted, e.g.,
[ 123 ./foo.nix "abc" f {x=y;} ]
the result would be a list of five elements, the fourth one being a
function and the fifth being an attribute set.Attribute setsAttribute sets are really the core of the language, since
ultimately it's all about creating derivations, which are really just
sets of attributes to be passed to build scripts.Attribute sets are just a list of name/value pairs enclosed in
curly brackets, where each value is an arbitrary expression terminated
by a semicolon. For example:
{ x = 123;
text = "Hello";
y = f { bla = 456; };
}
This defines an attribute set with attributes named
x, test, y.
The order of the attributes is irrelevant. An attribute name may only
occur once.Attributes can be selected from an attribute set using the
. operator. For instance,
{ a = "Foo"; b = "Bar"; }.a
evaluates to "Foo".Recursive attribute setsRecursive attribute sets are just normal attribute sets, but the
attributes can refer to each other. For example,
rec {
x = y;
y = 123;
}.x
evaluates to 123. Note that without
rec the binding x = y; would
refer to the variable y in the surrounding scope,
if one exists, and would be invalid if no such variable exists. That
is, in a normal (non-recursive) attribute set, attributes are not
added to the lexical scope; in a recursive set, they are.Recursive attribute sets of course introduce the danger of
infinite recursion. For example,
rec {
x = y;
y = x;
}.x
does not terminateActually, Nix detects infinite
recursion in this case and aborts (infinite recursion
encountered)..Let expressionsA let expression is a simple short-hand for a
rec expression followed by an attribute selection:
let { attrs } translates
to rec { attrs
}.body.For instance,
let {
x = "foo";
y = "bar";
body = x + y;
}
is equivalent to
rec {
x = "foo";
y = "bar";
body = x + y;
}.body
and evaluates to "foobar".
Inheriting attributesWhen defining an attribute set it is often convenient to copy
variables from the surrounding lexical scope (e.g., when you want to
propagate attributes). This can be shortened using the
inherit keyword. For instance,
let {
x = 123;
body = {
inherit x;
y = 456;
};
}
evaluates to {x = 123; y = 456;}. (Note that this
works because x is added to the lexical scope by
the let construct.) It is also possible to inherit
attributes from another attribute set. For instance, in this fragment
from all-packages-generic.nix,
graphviz = (import ../tools/graphics/graphviz) {
inherit fetchurl stdenv libpng libjpeg expat x11 yacc;
inherit (xlibs) libXaw;
};
xlibs = {
libX11 = ...;
libXaw = ...;
...
}
libpng = ...;
libjpg = ...;
...
the attribute set used in the function call to the function defined in
../tools/graphics/graphviz inherits a number of
variables from the surrounding scope (fetchurl
... yacc), but also inherits
libXaw (the X Athena Widgets) from the
xlibs (X11 client-side libraries) attribute
set.FunctionsFunctions have the following form:
{params}: body
This defines a function that must be called with an attribute set
containing the attributes listed in params,
which is a comma-separated list of attribute names. Optionally, for
each parameter a default value may be specified
by writing param ?
e, where
e is an arbitrary expression. If a
parameter has a default, the corresponding attribute may be omitted in
function calls.Note that functions do not have names. If you want to give them
a name, you can bind them to an attribute, e.g.,
let {
concat = {x, y}: x + y;
body = concat {x = "foo"; y = "bar";};
}It is also possible to define a function that takes a single
argument and that does not need to be called with an attribute set as
argument. The syntax is
var: body
where var is the name of the argument. It
is not possible to define a default. Example:
let {
negate = x: !x;
concat = x: y: x + y;
body = if negate true then concat "foo" "bar" else "";
}
Note that concat is a function that takes one
arguments and returns a function that takes another argument. This
allows partial parameterisation (i.e., only filling some of the
arguments of a function); e.g.,
map (concat "foo") ["bar", "bla", "abc"]
evaluates to ["foobar" "foobla" "fooabc"].ConditionalsConditionals look like this:
if e1 then e2 else e3
where e1 is an expression that should
evaluate to a boolean value (true or
false).AssertionsAssertions are generally used to check that certain requirements
on or between features and dependencies hold. They look like this:
assert e1; e2
where e1 is an expression that should
evaluate to a boolean value. If it evaluates to
true, e2 is returned;
otherwise expression evaluation is aborted and a backtrace is printed.Nix expression for Subversion
{ localServer ? false
, httpServer ? false
, sslSupport ? false
, pythonBindings ? false
, javaSwigBindings ? false
, javahlBindings ? false
, stdenv, fetchurl
, openssl ? null, httpd ? null, db4 ? null, expat, swig ? null, j2sdk ? null
}:
assert localServer -> db4 != null;
assert httpServer -> httpd != null && httpd.expat == expat;
assert sslSupport -> openssl != null && (httpServer -> httpd.openssl == openssl);
assert pythonBindings -> swig != null && swig.pythonSupport;
assert javaSwigBindings -> swig != null && swig.javaSupport;
assert javahlBindings -> j2sdk != null;
stdenv.mkDerivation {
name = "subversion-1.1.1";
...
openssl = if sslSupport then openssl else null;
...
} show how assertions are
used in the Nix expression for Subversion.This assertion states that if Subversion is to have support
for local repositories, then Berkeley DB is needed. So if the
Subversion function is called with the
localServer argument set to
true but the db4 argument
set to null, then the evaluation fails.This is a more subtle condition: if Subversion is built with
Apache (httpServer) support, then the Expat
library (an XML library) used by Subversion should be same as the
one used by Apache. This is because in this configuration
Subversion code ends up being linked with Apache code, and if the
Expat libraries do not match, a build- or runtime link error or
incompatibility might occur.This assertion says that in order for Subversion to have SSL
support (so that it can access https URLs), an
OpenSSL library must be passed. Additionally, it says that
if Apache support is enabled, then Apache's
OpenSSL should match Subversion's. (Note that if Apache support
is not enabled, we don't care about Apache's OpenSSL.)The conditional here is not really related to assertions,
but is worth pointing out: it ensures that if SSL support is
disabled, then the Subversion derivation is not dependent on
OpenSSL, even if a non-null value was passed.
This prevents an unnecessary rebuild of Subversion if OpenSSL
changes.With expressionsA with expression,
with e1; e2
introduces the attribute set e1 into the
lexical scope of the expression e2. For
instance,
let {
as = {x = "foo"; y = "bar";};
body = with as; x + y;
}
evaluates to "foobar" since the
with adds the x and
y attributes of as to the
lexical scope in the expression x + y. The most
common use of with is in conjunction with the
import function. E.g.,
with (import ./definitions.nix); ...
makes all attributes defined in the file
definitions.nix available as if they were defined
locally in a rec-expression.Operators lists the operators in the
Nix expression language, in order of precedence (from strongest to
weakest binding).
OperatorsSyntaxAssociativityDescriptione1 ~ e2noneConstruct a reference to a subpath of a derivation.
E.g., hello ~ "/bin/sh" refers to the
/bin/sh path within the Hello derivation.
Useful in specifying derivation attributes.e ?
idnoneTest whether attribute set e
contains an attribute named
id.e1 + e2leftString or path concatenation.! eleftBoolean negation.e1 //
e2rightReturn an attribute set consisting of the attributes in
e1 and
e2 (with the latter taking
precedence over the former in case of equally named attributes).e1 ==
e2noneEquality.e1 !=
e2noneInequality.e1 &&
e2leftLogical AND.e1 ||
e2leftLogical OR.e1 ->
e2noneLogical implication (equivalent to
!e1 ||
e2).
DerivationsThe most important built-in function is
derivation, which is used to describe a
single derivation (a build action). It takes as input an attribute
set, the attributes of which specify the inputs of the build.There must be an attribute named
system whose value must be a string specifying a
Nix platform identifier, such as "i686-linux" or
"powerpc-darwin"To figure out
your platform identifier, look at the line Checking for the
canonical Nix system name in the output of Nix's
configure script. The build
can only be performed on a machine and operating system matching the
platform identifier. (Nix can automatically forward builds for
other platforms by forwarding them to other machines; see .)There must be an attribute named
name whose value must be a string. This is used
as a symbolic name for the component by nix-env,
and it is appended to the hash in the output path of the
derivation.There must be an attribute named
builder that identifies the program that is
executed to perform the build. It can be either a derivation or a
source (a local file reference, e.g.,
./builder.sh).Every attribute is passed as an environment variable
to the builder. Attribute values are translated to environment
variables as follows:
Strings, URIs, and integers are just passed
verbatim.A path (e.g.,
../foo/sources.tar) causes the referenced
file to be copied to the store; its location in the store is put
in the environment variable. The idea is that all sources
should reside in the Nix store, since all inputs to a derivation
should reside in the Nix store.A derivation causes that
derivation to be built prior to the present derivation; the
output path is put in the environment
variable.Lists of the previous types are also allowed.
They are simply concatenated, separated by
spaces.The optional argument args
specifies command-line arguments to be passed to the builder. It
should be a list.(Note that mkDerivation in the standard
environment is a wrapper around derivation that
adds a default value for system and always uses
Bash as the builder, to which the supplied builder is passed as a
command-line argument. See .)The builder is executed as follows:
A temporary directory is created under the directory
specified by TMPDIR (default
/tmp) where the build will take place. The
current directory is changed to this directory.The environment is cleared and set to the derivation
attributes, as specified above.In addition, the following variables are set:
NIX_BUILD_TOP contains the path of
the temporary directory for this build.Also, TMPDIR,
TEMPDIR, TMP, TEMP
are set to point to the temporary directory. This is to prevent
the builder from accidentally writing temporary files anywhere
else. Doing so might cause interference by other
processes.PATH is set to
/path-not-set to prevent shells from
initialising it to their built-in default value.HOME is set to
/homeless-shelter to prevent programs from
using /etc/passwd or the like to find the
user's home directory, which could cause impurity. Usually, when
HOME is set, it is used as the location of the home
directory, even if it points to a non-existent
path.NIX_STORE is set to the path of the
top-level Nix store directory (typically,
/nix/store).out is set to point to the output
path of the derivation, which is a subdirectory of the Nix store.
The output path is a concatenation of the cryptographic hash of
all build inputs, and the name
attribute.If the output path already exists, it is removed.
Also, locks are acquired to prevent multiple Nix instances from
performing the same build at the same time.A log of the combined standard output and error is
written to /nix/var/log/nix.The builder is executed with the arguments specified
by the attribute args. If it exits with exit
code 0, it is considered to have succeeded.The temporary directory is removed (unless the
option was specified).If the build was successful, Nix scans the output
for references to the paths of the inputs. These so-called
retained dependencies could be used when the
output of the derivation is used (e.g., when it's executed or used
as input to another derivation), so if we deploy the derivation, we
should copy the retained dependencies as well. The scan is
performed by looking for the hash parts of file names of the
inputs.After the build, Nix sets the last-modified
timestamp on all files in the build result to 0 (00:00:00 1/1/1970
UTC), sets the group to the default group, and sets the mode of the
file to 0444 or 0555 (i.e., read-only, with execute permission
enabled if the file was originally executable). Note that possible
setuid and setgid bits are
cleared. Setuid and setgid programs are not currently supported by
Nix. This is because the Nix archives used in deployment have no
concept of ownership information, and because it makes the build
result dependent on the user performing the build.Other built-in functionsTODOCommentsComments can be single-line, started with a #
character, or inline/multi-line, enclosed within /*
... */.The standard environmentThe standard build environment in the Nix Packages collection
provides a basic environment for building Unix packages. It consists
of the following components:
The GNU C Compiler, configured with C and C++
support. On Linux, the compiler has been patched to provide greater
purity assurance. For instance, the compiler doesn't
search in locations such as /usr/include. In
fact, attempts to add such directories through the
flag are filtered out. Likewise, the linker
(from GNU binutils) doesn't search in standard locations such as
/usr/lib. Programs built on Linux are linked
against a GNU C Library that likewise doesn't search in the default
system locations.GNU coreutils (contains a few dozen standard Unix
commands).GNU findutils (contains
find).GNU diffutils (contains diff,
cmp).GNU sed.GNU grep.GNU awk.GNU tar.gzip and
bzip2.GNU Make. It has been patched to provide
nested output that can be fed into the
log2xml command and log2html
stylesheet to create a structured, readable output of the build
steps performed by Make.Bash. This is the shell used for all builders in
the Nix Packages collection. Not using /bin/sh
removes a large source of portability problems.Patch.The standard environment is used by passing it as an input
called stdenv to the derivation, and then doing
. $stdenv/setup
at the top of the builder.Apart from adding the aforementioned commands to the
PATH, setup also does the
following:
All input components specified in the
buildInputs environment variable have their
/bin subdirectory added to PATH,
their /include subdirectory added to the C/C++
header file search path, and their /lib
subdirectory added to the linker search path. This can be extended.
For instance, when the pkgconfig component is
used, the subdirectory /lib/pkgconfig of each
input is added to the PKG_CONFIG_PATH environment
variable.The environment variable
NIX_CFLAGS_STRIP is set so that the compiler strips
debug information from object files. This can be disabled by
setting NIX_STRIP_DEBUG to
0.The setup script also exports a function
called genericBuild that knows how to build
typical Autoconf-style components. It can be customised to perform
builds for any type of component. It is advisable to use
genericBuild since it provides facilities that
are almost always useful such as unpacking of sources, patching of
sources, nested logging, etc.The operation of the generic builder can be modified in many
places by setting certain variables. These hook
variables are typically set to the name of some shell
function defined by you. For instance, to perform some additional
steps after make install you would set the
postInstall variable:
postInstall=myPostInstall
myPostInstall() {
mkdir $out/share/extra
cp extrafiles/* $out/share/extra
}The generic builder has a number of phases,
each of which can be override in its entirety by setting the indicated
variable. The phases are:
unpackPhase unpacks the source files
listed in the src environment variable to the
current directory. It supports tar files,
optionally compressed with gzip or
bzip2; Zip files (but note that the
unzip command is not a part of the standard
environment; you should add it as a build input yourself); and
unpacked source trees (i.e., directories; they are copied
verbatim). You can add support for other file types by setting
the findUnpacker hook. This hook should set
the variable unpackCmd to contain the command
to be executed to unpack the file.After unpacking all source files,
unpackPhase changes the current directory to
the directory created by unpacking the sources. If there are
multiple source directories, you should set
sourceRoot to the name of the intended
directory.It also calls the hook postUnpack after
unpacking.patchPhase calls the
patch command with the
option for each patch file listed in the patches
variable.configurePhase runs the script called
configure in the current directory with a
set to the output path. You can add
additional flags through the configureFlags
variable. If configure does not exist,
nothing happens.Before and after running configure, the
hooks preConfigure and
postConfigure are called, respectively.buildPhase calls
make. You can set flags for
make through the makeFlags
variable.Before and after running make, the hooks
preBuild and postBuild are
called, respectively.checkPhase calls make
check, but only if the doCheck variable
is set to 1. Additional flags can be set through
the checkFlags variable.installPhase calls make
install. Additional flags can be set through the
installFlags variable. It also strips any
static libraries in the output path of debug information unless
dontStrip is set to
1.Before and after running make install,
the hooks preInstall and
postInstall are called, respectively.distPhase calls make
dist, but only if the doDist variable
is set to 1. Additional flags can be set
through the distFlags variable. The resulting
tarball is copied to the /tarballs
subdirectory of the output path.Before and after running make dist, the
hooks preDist and postDist
are called, respectively.You can change the order in which phases are executed, or add
new phases, by setting the phases variable. The
default is patchPhase configurePhase buildPhase checkPhase
installPhase distPhase.At the beginning of each phase, the set of all shell variables
is written to the file env-vars at the top-level
build directory. This is useful for debugging: it allows you to
recreate the environment in which a build was performed. For
instance, if a build fails, then assuming you used the
flag, you can go to the output directory and
switch to the environment of the builder:
$ nix-build -K ./foo.nix
... fails, keeping build directory `/tmp/nix-1234-0'
$ cd /tmp/nix-1234-0
$ source env-vars
(edit some files...)
$ make
(execution continues with the same GCC, make, etc.)The definitive, up-to-date documentation of the generic builder
is the source itself, which resides in
pkgs/stdenv/generic/setup.sh.