Built-in Functions
This section lists the functions built into the Nix language evaluator.
All built-in functions are available through the global builtins
constant.
For convenience, some built-ins can be accessed directly:
derivation attrs
derivation is described in its own section.
-
abort s
-
Abort Nix expression evaluation and print the error message s.
-
add e1 e2
-
Return the sum of the numbers e1 and e2.
-
addDrvOutputDependencies s
-
Create a copy of the given string where a single constant string context element is turned into a "derivation deep" string context element.
The store path that is the constant string context element should point to a valid derivation, and end in
.drv
.The original string context element must not be empty or have multiple elements, and it must not have any other type of element other than a constant or derivation deep element. The latter is supported so this function is idempotent.
This is the opposite of
builtins.unsafeDiscardOutputDependency
. -
all pred list
-
Return
true
if the function pred returnstrue
for all elements of list, andfalse
otherwise. -
any pred list
-
Return
true
if the function pred returnstrue
for at least one element of list, andfalse
otherwise. -
attrNames set
-
Return the names of the attributes in the set set in an alphabetically sorted list. For instance,
builtins.attrNames { y = 1; x = "foo"; }
evaluates to[ "x" "y" ]
. -
attrValues set
-
Return the values of the attributes in the set set in the order corresponding to the sorted attribute names.
-
baseNameOf s
-
Return the base name of the string s, that is, everything following the final slash in the string. This is similar to the GNU
basename
command. -
bitAnd e1 e2
-
Return the bitwise AND of the integers e1 and e2.
-
bitOr e1 e2
-
Return the bitwise OR of the integers e1 and e2.
-
bitXor e1 e2
-
Return the bitwise XOR of the integers e1 and e2.
-
break v
-
In debug mode (enabled using
--debugger
), pause Nix expression evaluation and enter the REPL. Otherwise, return the argumentv
. -
catAttrs attr list
-
Collect each attribute named attr from a list of attribute sets. Attrsets that don't contain the named attribute are ignored. For example,
builtins.catAttrs "a" [{a = 1;} {b = 0;} {a = 2;}]
evaluates to
[1 2]
. -
ceil double
-
Converts an IEEE-754 double-precision floating-point number (double) to the next higher integer.
If the datatype is neither an integer nor a "float", an evaluation error will be thrown.
-
compareVersions s1 s2
-
Compare two strings representing versions and return
-1
if version s1 is older than version s2,0
if they are the same, and1
if s1 is newer than s2. The version comparison algorithm is the same as the one used bynix-env -u
. -
concatLists lists
-
Concatenate a list of lists into a single list.
-
concatMap f list
-
This function is equivalent to
builtins.concatLists (map f list)
but is more efficient. -
concatStringsSep separator list
-
Concatenate a list of strings with a separator between each element, e.g.
concatStringsSep "/" ["usr" "local" "bin"] == "usr/local/bin"
. -
deepSeq e1 e2
-
This is like
seq e1 e2
, except that e1 is evaluated deeply: if it’s a list or set, its elements or attributes are also evaluated recursively. -
dirOf s
-
Return the directory part of the string s, that is, everything before the final slash in the string. This is similar to the GNU
dirname
command. -
div e1 e2
-
Return the quotient of the numbers e1 and e2.
-
elem x xs
-
Return
true
if a value equal to x occurs in the list xs, andfalse
otherwise. -
elemAt xs n
-
Return element n from the list xs. Elements are counted starting from 0. A fatal error occurs if the index is out of bounds.
-
fetchClosure args
-
Fetch a store path closure from a binary cache, and return the store path as a string with context.
This function can be invoked in three ways, that we will discuss in order of preference.
Fetch a content-addressed store path
Example:
builtins.fetchClosure { fromStore = "https://cache.nixos.org"; fromPath = /nix/store/ldbhlwhh39wha58rm61bkiiwm6j7211j-git-2.33.1; }
This is the simplest invocation, and it does not require the user of the expression to configure
trusted-public-keys
to ensure their authenticity.If your store path is input addressed instead of content addressed, consider the other two invocations.
Fetch any store path and rewrite it to a fully content-addressed store path
Example:
builtins.fetchClosure { fromStore = "https://cache.nixos.org"; fromPath = /nix/store/r2jd6ygnmirm2g803mksqqjm4y39yi6i-git-2.33.1; toPath = /nix/store/ldbhlwhh39wha58rm61bkiiwm6j7211j-git-2.33.1; }
This example fetches
/nix/store/r2jd...
from the specified binary cache, and rewrites it into the content-addressed store path/nix/store/ldbh...
.Like the previous example, no extra configuration or privileges are required.
To find out the correct value for
toPath
given afromPath
, usenix store make-content-addressed
:# nix store make-content-addressed --from https://cache.nixos.org /nix/store/r2jd6ygnmirm2g803mksqqjm4y39yi6i-git-2.33.1 rewrote '/nix/store/r2jd6ygnmirm2g803mksqqjm4y39yi6i-git-2.33.1' to '/nix/store/ldbhlwhh39wha58rm61bkiiwm6j7211j-git-2.33.1'
Alternatively, set
toPath = ""
and find the correcttoPath
in the error message.Fetch an input-addressed store path as is
Example:
builtins.fetchClosure { fromStore = "https://cache.nixos.org"; fromPath = /nix/store/r2jd6ygnmirm2g803mksqqjm4y39yi6i-git-2.33.1; inputAddressed = true; }
It is possible to fetch an input-addressed store path and return it as is. However, this is the least preferred way of invoking
fetchClosure
, because it requires that the input-addressed paths are trusted by the Lix configuration.builtins.storePath
fetchClosure
is similar tobuiltins.storePath
in that it allows you to use a previously built store path in a Nix expression. However,fetchClosure
is more reproducible because it specifies a binary cache from which the path can be fetched. Also, using content-addressed store paths does not require users to configuretrusted-public-keys
to ensure their authenticity.This function is only available if the fetch-closure experimental feature is enabled.
-
fetchGit args
-
Fetch a path from git. args can be a URL, in which case the HEAD of the repo at that URL is fetched. Otherwise, it can be an attribute with the following attributes (all except
url
optional):-
url
The URL of the repo.
-
name
(default: basename of the URL)The name of the directory the repo should be exported to in the store.
-
rev
(default: the tip ofref
)The Git revision to fetch. This is typically a commit hash.
-
ref
(default:HEAD
)The Git reference under which to look for the requested revision. This is often a branch or tag name.
By default, the
ref
value is prefixed withrefs/heads/
. As of 2.3.0, Nix will not prefixrefs/heads/
ifref
starts withrefs/
or ifref
looks like a commit hash for backwards compatibility with CppNix 2.3. -
submodules
(default:false
)A Boolean parameter that specifies whether submodules should be checked out.
-
shallow
(default:false
)A Boolean parameter that specifies whether fetching from a shallow remote repository is allowed. This still performs a full clone of what is available on the remote.
-
allRefs
Whether to fetch all references of the repository. With this argument being true, it's possible to load a
rev
from anyref
(by default onlyrev
s from the specifiedref
are supported).
Here are some examples of how to use
fetchGit
.-
To fetch a private repository over SSH:
builtins.fetchGit { url = "git@github.com:my-secret/repository.git"; ref = "master"; rev = "adab8b916a45068c044658c4158d81878f9ed1c3"; }
-
To fetch an arbitrary reference:
builtins.fetchGit { url = "https://github.com/NixOS/nix.git"; ref = "refs/heads/0.5-release"; }
-
If the revision you're looking for is in the default branch of the git repository you don't strictly need to specify the branch name in the
ref
attribute.However, if the revision you're looking for is in a future branch for the non-default branch you will need to specify the the
ref
attribute as well.builtins.fetchGit { url = "https://github.com/nixos/nix.git"; rev = "841fcbd04755c7a2865c51c1e2d3b045976b7452"; ref = "1.11-maintenance"; }
Note
It is nice to always specify the branch which a revision belongs to. Without the branch being specified, the fetcher might fail if the default branch changes. Additionally, it can be confusing to try a commit from a non-default branch and see the fetch fail. If the branch is specified the fault is much more obvious.
-
If the revision you're looking for is in the default branch of the git repository you may omit the
ref
attribute.builtins.fetchGit { url = "https://github.com/nixos/nix.git"; rev = "841fcbd04755c7a2865c51c1e2d3b045976b7452"; }
-
To fetch a specific tag:
builtins.fetchGit { url = "https://github.com/nixos/nix.git"; ref = "refs/tags/1.9"; }
-
To fetch the latest version of a remote branch:
builtins.fetchGit { url = "ssh://git@github.com/nixos/nix.git"; ref = "master"; }
Nix will refetch the branch according to the
tarball-ttl
setting.This behavior is disabled in pure evaluation mode.
-
To fetch the content of a checked-out work directory:
builtins.fetchGit ./work-dir
If the URL points to a local directory, and no
ref
orrev
is given,fetchGit
will use the current content of the checked-out files, even if they are not committed or added to Git's index. It will only consider files added to the Git repository, as listed bygit ls-files
. -
-
fetchTarball args
-
Download the specified URL, unpack it and return the path of the unpacked tree. The file must be a tape archive (
.tar
) compressed withgzip
,bzip2
orxz
. The top-level path component of the files in the tarball is removed, so it is best if the tarball contains a single directory at top level. The typical use of the function is to obtain external Nix expression dependencies, such as a particular version of Nixpkgs, e.g.with import (fetchTarball "https://github.com/NixOS/nixpkgs/archive/nixos-14.12.tar.gz") {}; stdenv.mkDerivation { … }
The fetched tarball is cached for a certain amount of time (1 hour by default) in
~/.cache/nix/tarballs/
. You can change the cache timeout either on the command line with--tarball-ttl
number-of-seconds or in the Nix configuration file by adding the linetarball-ttl =
number-of-seconds.Note that when obtaining the hash with
nix-prefetch-url
the option--unpack
is required.This function can also verify the contents against a hash. In that case, the function takes a set instead of a URL. The set requires the attribute
url
and the attributesha256
, e.g.with import (fetchTarball { url = "https://github.com/NixOS/nixpkgs/archive/nixos-14.12.tar.gz"; sha256 = "1jppksrfvbk5ypiqdz4cddxdl8z6zyzdb2srq8fcffr327ld5jj2"; }) {}; stdenv.mkDerivation { … }
Not available in restricted evaluation mode.
-
fetchurl url
-
Download the specified URL and return the path of the downloaded file.
Not available in restricted evaluation mode.
-
filter f list
-
Return a list consisting of the elements of list for which the function f returns
true
. -
filterSource e1 e2
-
Warning
filterSource
should not be used to filter store paths. SincefilterSource
uses the name of the input directory while naming the output directory, doing so will produce a directory name in the form of<hash2>-<hash>-<name>
, where<hash>-<name>
is the name of the input directory. Since<hash>
depends on the unfiltered directory, the name of the output directory will indirectly depend on files that are filtered out by the function. This will trigger a rebuild even when a filtered out file is changed. Usebuiltins.path
instead, which allows specifying the name of the output directory.This function allows you to copy sources into the Nix store while filtering certain files. For instance, suppose that you want to use the directory
source-dir
as an input to a Nix expression, e.g.stdenv.mkDerivation { ... src = ./source-dir; }
However, if
source-dir
is a Subversion working copy, then all those annoying.svn
subdirectories will also be copied to the store. Worse, the contents of those directories may change a lot, causing lots of spurious rebuilds. WithfilterSource
you can filter out the.svn
directories:src = builtins.filterSource (path: type: type != "directory" || baseNameOf path != ".svn") ./source-dir;
Thus, the first argument e1 must be a predicate function that is called for each regular file, directory or symlink in the source tree e2. If the function returns
true
, the file is copied to the Nix store, otherwise it is omitted. The function is called with two arguments. The first is the full path of the file. The second is a string that identifies the type of the file, which is either"regular"
,"directory"
,"symlink"
or"unknown"
(for other kinds of files such as device nodes or fifos — but note that those cannot be copied to the Nix store, so if the predicate returnstrue
for them, the copy will fail). If you exclude a directory, the entire corresponding subtree of e2 will be excluded. -
findFile search path lookup path
-
Look up the given path with the given search path.
A search path is represented list of attribute sets with two attributes,
prefix
, andpath
.prefix
is a relative path.path
denotes a file system location; the exact syntax depends on the command line interface.Examples of search path attribute sets:
-
{ prefix = "nixos-config"; path = "/etc/nixos/configuration.nix"; }
-
{ prefix = ""; path = "/nix/var/nix/profiles/per-user/root/channels"; }
The lookup algorithm checks each entry until a match is found, returning a path value of the match.
This is the process for each entry: If the lookup path matches
prefix
, then the remainder of the lookup path (the "suffix") is searched for within the directory denoted bypatch
. Note that thepath
may need to be downloaded at this point to look inside. If the suffix is found inside that directory, then the entry is a match; the combined absolute path of the directory (now downloaded if need be) and the suffix is returned.The syntax
<nixpkgs>
is equivalent to:
builtins.findFile builtins.nixPath "nixpkgs"
-
-
flakeRefToString attrs
-
Convert a flake reference from attribute set format to URL format.
For example:
builtins.flakeRefToString { dir = "lib"; owner = "NixOS"; ref = "23.05"; repo = "nixpkgs"; type = "github"; }
evaluates to
"github:NixOS/nixpkgs/23.05?dir=lib"
This function is only available if the flakes experimental feature is enabled.
-
floor double
-
Converts an IEEE-754 double-precision floating-point number (double) to the next lower integer.
If the datatype is neither an integer nor a "float", an evaluation error will be thrown.
-
foldl' op nul list
-
Reduce a list by applying a binary operator, from left to right, e.g.
foldl' op nul [x0 x1 x2 ...] = op (op (op nul x0) x1) x2) ...
. For example,foldl' (x: y: x + y) 0 [1 2 3]
evaluates to 6. The return value of each application ofop
is evaluated immediately, even for intermediate values. -
fromJSON e
-
Convert a JSON string to a Nix value. For example,
builtins.fromJSON ''{"x": [1, 2, 3], "y": null}''
returns the value
{ x = [ 1 2 3 ]; y = null; }
. -
fromTOML e
-
Convert a TOML string to a Nix value. For example,
builtins.fromTOML '' x=1 s="a" [table] y=2 ''
returns the value
{ s = "a"; table = { y = 2; }; x = 1; }
. -
functionArgs f
-
Return a set containing the names of the formal arguments expected by the function f. The value of each attribute is a Boolean denoting whether the corresponding argument has a default value. For instance,
functionArgs ({ x, y ? 123}: ...) = { x = false; y = true; }
."Formal argument" here refers to the attributes pattern-matched by the function. Plain lambdas are not included, e.g.
functionArgs (x: ...) = { }
. -
genList generator length
-
Generate list of size length, with each element i equal to the value returned by generator
i
. For example,builtins.genList (x: x * x) 5
returns the list
[ 0 1 4 9 16 ]
. -
genericClosure attrset
-
Take an attrset with values named
startSet
andoperator
in order to return a list of attrsets by starting with thestartSet
and recursively applying theoperator
function to eachitem
. The attrsets in thestartSet
and the attrsets produced byoperator
must contain a value namedkey
which is comparable. The result is produced by callingoperator
for eachitem
with a value forkey
that has not been called yet including newly produceditem
s. The function terminates when no newitem
s are produced. The resulting list of attrsets contains only attrsets with a unique key. For example,builtins.genericClosure { startSet = [ {key = 5;} ]; operator = item: [{ key = if (item.key / 2 ) * 2 == item.key then item.key / 2 else 3 * item.key + 1; }]; }
evaluates to
[ { key = 5; } { key = 16; } { key = 8; } { key = 4; } { key = 2; } { key = 1; } ]
-
getAttr s set
-
getAttr
returns the attribute named s from set. Evaluation aborts if the attribute doesn’t exist. This is a dynamic version of the.
operator, since s is an expression rather than an identifier. -
getContext s
-
Return the string context of s.
The string context tracks references to derivations within a string. It is represented as an attribute set of store derivation paths mapping to output names.
Using string interpolation on a derivation will add that derivation to the string context. For example,
builtins.getContext "${derivation { name = "a"; builder = "b"; system = "c"; }}"
evaluates to
{ "/nix/store/arhvjaf6zmlyn8vh8fgn55rpwnxq0n7l-a.drv" = { outputs = [ "out" ]; }; }
-
getEnv s
-
getEnv
returns the value of the environment variable s, or an empty string if the variable doesn't exist. This function should be used with care, as it can introduce all sorts of nasty environment dependencies in your Nix expression.getEnv
is used in nixpkgs for evil impurities such as locating the file~/.config/nixpkgs/config.nix
which contains user-local settings for nixpkgs. (That is, it does agetEnv "HOME"
to locate the user's home directory.)When in pure evaluation mode, this function always returns an empty string.
-
getFlake args
-
Fetch a flake from a flake reference, and return its output attributes and some metadata. For example:
(builtins.getFlake "nix/55bc52401966fbffa525c574c14f67b00bc4fb3a").packages.x86_64-linux.nix
Unless impure evaluation is allowed (
--impure
), the flake reference must be "locked", e.g. contain a Git revision or content hash. An example of an unlocked usage is:(builtins.getFlake "github:edolstra/dwarffs").rev
This function is only available if the flakes experimental feature is enabled.
-
groupBy f list
-
Groups elements of list together by the string returned from the function f called on each element. It returns an attribute set where each attribute value contains the elements of list that are mapped to the same corresponding attribute name returned by f.
For example,
builtins.groupBy (builtins.substring 0 1) ["foo" "bar" "baz"]
evaluates to
{ b = [ "bar" "baz" ]; f = [ "foo" ]; }
-
hasAttr s set
-
hasAttr
returnstrue
if set has an attribute named s, andfalse
otherwise. This is a dynamic version of the?
operator, since s is an expression rather than an identifier. -
hasContext s
-
Return
true
if string s has a non-empty context. The context can be obtained withgetContext
.Example
Many operations require a string context to be empty because they are intended only to work with "regular" strings, and also to help users avoid unintentionally losing track of string context elements.
builtins.hasContext
can help create better domain-specific errors in those case.name: meta: if builtins.hasContext name then throw "package name cannot contain string context" else { ${name} = meta; }
-
hashFile type p
-
Return a base-16 representation of the cryptographic hash of the file at path p. The hash algorithm specified by type must be one of
"md5"
,"sha1"
,"sha256"
or"sha512"
. -
hashString type s
-
Return a base-16 representation of the cryptographic hash of string s. The hash algorithm specified by type must be one of
"md5"
,"sha1"
,"sha256"
or"sha512"
. -
head list
-
Return the first element of a list; abort evaluation if the argument isn’t a list or is an empty list. You can test whether a list is empty by comparing it with
[]
. -
import path
-
Load, parse and return the Nix expression in the file path.
The value path can be a path, a string, or an attribute set with an
__toString
attribute or aoutPath
attribute (as derivations or flake inputs typically have).If path is a directory, the file
default.nix
in that directory is loaded.Evaluation aborts if the file doesn’t exist or contains an incorrect Nix expression.
import
implements Nix’s module system: you can put any Nix expression (such as a set or a function) in a separate file, and use it from Nix expressions in other files.Note
Unlike some languages,
import
is a regular function in Nix. Paths using the angle bracket syntax (e.g.,import
<foo>) are normal path values.A Nix expression loaded by
import
must not contain any free variables (identifiers that are not defined in the Nix expression itself and are not built-in). Therefore, it cannot refer to variables that are in scope at the call site. For instance, if you have a calling expressionrec { x = 123; y = import ./foo.nix; }
then the following
foo.nix
will give an error:x + 456
since
x
is not in scope infoo.nix
. If you wantx
to be available infoo.nix
, you should pass it as a function argument:rec { x = 123; y = import ./foo.nix x; }
and
x: x + 456
(The function argument doesn’t have to be called
x
infoo.nix
; any name would work.) -
intersectAttrs e1 e2
-
Return a set consisting of the attributes in the set e2 which have the same name as some attribute in e1.
Performs in O(n log m) where n is the size of the smaller set and m the larger set's size.
-
isAttrs e
-
Return
true
if e evaluates to a set, andfalse
otherwise. -
isBool e
-
Return
true
if e evaluates to a bool, andfalse
otherwise. -
isFloat e
-
Return
true
if e evaluates to a float, andfalse
otherwise. -
isFunction e
-
Return
true
if e evaluates to a function, andfalse
otherwise. -
isInt e
-
Return
true
if e evaluates to an integer, andfalse
otherwise. -
isList e
-
Return
true
if e evaluates to a list, andfalse
otherwise. -
isNull e
-
Return
true
if e evaluates tonull
, andfalse
otherwise.This is equivalent to
e == null
. -
isPath e
-
Return
true
if e evaluates to a path, andfalse
otherwise. -
isString e
-
Return
true
if e evaluates to a string, andfalse
otherwise. -
length e
-
Return the length of the list e.
-
lessThan e1 e2
-
Return
true
if the number e1 is less than the number e2, andfalse
otherwise. Evaluation aborts if either e1 or e2 does not evaluate to a number. -
listToAttrs e
-
Construct a set from a list specifying the names and values of each attribute. Each element of the list should be a set consisting of a string-valued attribute
name
specifying the name of the attribute, and an attributevalue
specifying its value.In case of duplicate occurrences of the same name, the first takes precedence.
Example:
builtins.listToAttrs [ { name = "foo"; value = 123; } { name = "bar"; value = 456; } { name = "bar"; value = 420; } ]
evaluates to
{ foo = 123; bar = 456; }
-
map f list
-
Apply the function f to each element in the list list. For example,
map (x: "foo" + x) [ "bar" "bla" "abc" ]
evaluates to
[ "foobar" "foobla" "fooabc" ]
. -
mapAttrs f attrset
-
Apply function f to every element of attrset. For example,
builtins.mapAttrs (name: value: value * 10) { a = 1; b = 2; }
evaluates to
{ a = 10; b = 20; }
. -
match regex str
-
Returns a list if the extended POSIX regular expression regex matches str precisely, otherwise returns
null
. Each item in the list is a regex group.builtins.match "ab" "abc"
Evaluates to
null
.builtins.match "abc" "abc"
Evaluates to
[ ]
.builtins.match "a(b)(c)" "abc"
Evaluates to
[ "b" "c" ]
.builtins.match "[[:space:]]+([[:upper:]]+)[[:space:]]+" " FOO "
Evaluates to
[ "FOO" ]
. -
mul e1 e2
-
Return the product of the numbers e1 and e2.
-
outputOf derivation-reference output-name
-
Return the output path of a derivation, literally or using a placeholder if needed.
If the derivation has a statically-known output path (i.e. the derivation output is input-addressed, or fixed content-addresed), the output path will just be returned. But if the derivation is content-addressed or if the derivation is itself not-statically produced (i.e. is the output of another derivation), a placeholder will be returned instead.
derivation reference
must be a string that may contain a regular store path to a derivation, or may be a placeholder reference. If the derivation is produced by a derivation, you must explicitly selectdrv.outPath
. This primop can be chained arbitrarily deeply. For instance,builtins.outputOf (builtins.outputOf myDrv "out) "out"
will return a placeholder for the output of the output of
myDrv
.This primop corresponds to the
^
sigil for derivable paths, e.g. as part of installable syntax on the command line.This function is only available if the dynamic-derivations experimental feature is enabled.
-
parseDrvName s
-
Split the string s into a package name and version. The package name is everything up to but not including the first dash not followed by a letter, and the version is everything following that dash. The result is returned in a set
{ name, version }
. Thus,builtins.parseDrvName "nix-0.12pre12876"
returns{ name = "nix"; version = "0.12pre12876"; }
. -
parseFlakeRef flake-ref
-
Parse a flake reference, and return its exploded form.
For example:
builtins.parseFlakeRef "github:NixOS/nixpkgs/23.05?dir=lib"
evaluates to:
{ dir = "lib"; owner = "NixOS"; ref = "23.05"; repo = "nixpkgs"; type = "github"; }
This function is only available if the flakes experimental feature is enabled.
-
partition pred list
-
Given a predicate function pred, this function returns an attrset containing a list named
right
, containing the elements in list for which pred returnedtrue
, and a list namedwrong
, containing the elements for which it returnedfalse
. For example,builtins.partition (x: x > 10) [1 23 9 3 42]
evaluates to
{ right = [ 23 42 ]; wrong = [ 1 9 3 ]; }
-
path args
-
An enrichment of the built-in path type, based on the attributes present in args. All are optional except
path
:-
path
The underlying path. -
name
The name of the path when added to the store. This can used to reference paths that have nix-illegal characters in their names, like@
. -
filter
A function of the type expected bybuiltins.filterSource
, with the same semantics. -
recursive
Whenfalse
, whenpath
is added to the store it is with a flat hash, rather than a hash of the NAR serialization of the file. Thus,path
must refer to a regular file, not a directory. This allows similar behavior tofetchurl
. Defaults totrue
. -
sha256
When provided, this is the expected hash of the file at the path. Evaluation will fail if the hash is incorrect, and providing a hash allowsbuiltins.path
to be used even when thepure-eval
nix config option is on.
-
-
pathExists path
-
Return
true
if the path path exists at evaluation time, andfalse
otherwise. -
placeholder output
-
Return a placeholder string for the specified output that will be substituted by the corresponding output path at build time. Typical outputs would be
"out"
,"bin"
or"dev"
. -
readDir path
-
Return the contents of the directory path as a set mapping directory entries to the corresponding file type. For instance, if directory
A
contains a regular fileB
and another directoryC
, thenbuiltins.readDir ./A
will return the set{ B = "regular"; C = "directory"; }
The possible values for the file type are
"regular"
,"directory"
,"symlink"
and"unknown"
. -
readFile path
-
Return the contents of the file path as a string.
-
readFileType p
-
Determine the directory entry type of a filesystem node, being one of "directory", "regular", "symlink", or "unknown".
-
removeAttrs set list
-
Remove the attributes listed in list from set. The attributes don’t have to exist in set. For instance,
removeAttrs { x = 1; y = 2; z = 3; } [ "a" "x" "z" ]
evaluates to
{ y = 2; }
. -
replaceStrings from to s
-
Given string s, replace every occurrence of the strings in from with the corresponding string in to.
The argument to is lazy, that is, it is only evaluated when its corresponding pattern in from is matched in the string s
Example:
builtins.replaceStrings ["oo" "a"] ["a" "i"] "foobar"
evaluates to
"fabir"
. -
seq e1 e2
-
Evaluate e1, then evaluate and return e2. This ensures that a computation is strict in the value of e1.
-
sort comparator list
-
Return list in sorted order. It repeatedly calls the function comparator with two elements. The comparator should return
true
if the first element is less than the second, andfalse
otherwise. For example,builtins.sort builtins.lessThan [ 483 249 526 147 42 77 ]
produces the list
[ 42 77 147 249 483 526 ]
.This is a stable sort: it preserves the relative order of elements deemed equal by the comparator.
-
split regex str
-
Returns a list composed of non matched strings interleaved with the lists of the extended POSIX regular expression regex matches of str. Each item in the lists of matched sequences is a regex group.
builtins.split "(a)b" "abc"
Evaluates to
[ "" [ "a" ] "c" ]
.builtins.split "([ac])" "abc"
Evaluates to
[ "" [ "a" ] "b" [ "c" ] "" ]
.builtins.split "(a)|(c)" "abc"
Evaluates to
[ "" [ "a" null ] "b" [ null "c" ] "" ]
.builtins.split "([[:upper:]]+)" " FOO "
Evaluates to
[ " " [ "FOO" ] " " ]
. -
splitVersion s
-
Split a string representing a version into its components, by the same version splitting logic underlying the version comparison in
nix-env -u
. -
storePath path
-
This function allows you to define a dependency on an already existing store path. For example, the derivation attribute
src = builtins.storePath /nix/store/f1d18v1y…-source
causes the derivation to depend on the specified path, which must exist or be substitutable. Note that this differs from a plain path (e.g.src = /nix/store/f1d18v1y…-source
) in that the latter causes the path to be copied again to the Nix store, resulting in a new path (e.g./nix/store/ld01dnzc…-source-source
).Not available in pure evaluation mode. Lix may change this, tracking issue: https://git.lix.systems/lix-project/lix/issues/402
See also
builtins.fetchClosure
. -
stringLength e
-
Return the length of the string e. If e is not a string, evaluation is aborted.
-
sub e1 e2
-
Return the difference between the numbers e1 and e2.
-
substring start len s
-
Return the substring of s from character position start (zero-based) up to but not including start + len. If start is greater than the length of the string, an empty string is returned, and if start + len lies beyond the end of the string, only the substring up to the end of the string is returned. start must be non-negative. For example,
builtins.substring 0 3 "nixos"
evaluates to
"nix"
. -
tail list
-
Return the second to last elements of a list; abort evaluation if the argument isn’t a list or is an empty list.
Warning
This function should generally be avoided since it's inefficient: unlike Haskell's
tail
, it takes O(n) time, so recursing over a list by repeatedly callingtail
takes O(n^2) time. -
throw s
-
Throw an error message s. This usually aborts Nix expression evaluation, but in
nix-env -qa
and other commands that try to evaluate a set of derivations to get information about those derivations, a derivation that throws an error is silently skipped (which is not the case forabort
). -
toFile name s
-
Store the string s in a file in the Nix store and return its path. The file has suffix name. This file can be used as an input to derivations. One application is to write builders “inline”. For instance, the following Nix expression combines the Nix expression for GNU Hello and its build script into one file:
{ stdenv, fetchurl, perl }: stdenv.mkDerivation { name = "hello-2.1.1"; builder = builtins.toFile "builder.sh" " source $stdenv/setup PATH=$perl/bin:$PATH tar xvfz $src cd hello-* ./configure --prefix=$out make make install "; src = fetchurl { url = "http://ftp.nluug.nl/pub/gnu/hello/hello-2.1.1.tar.gz"; sha256 = "1md7jsfd8pa45z73bz1kszpp01yw6x5ljkjk2hx7wl800any6465"; }; inherit perl; }
It is even possible for one file to refer to another, e.g.,
builder = let configFile = builtins.toFile "foo.conf" " # This is some dummy configuration file. ... "; in builtins.toFile "builder.sh" " source $stdenv/setup ... cp ${configFile} $out/etc/foo.conf ";
Note that
${configFile}
is a string interpolation, so the result of the expressionconfigFile
(i.e., a path like/nix/store/m7p7jfny445k...-foo.conf
) will be spliced into the resulting string.It is however not allowed to have files mutually referring to each other, like so:
let foo = builtins.toFile "foo" "...${bar}..."; bar = builtins.toFile "bar" "...${foo}..."; in foo
This is not allowed because it would cause a cyclic dependency in the computation of the cryptographic hashes for
foo
andbar
.It is also not possible to reference the result of a derivation. If you are using Nixpkgs, the
writeTextFile
function is able to do that. -
toJSON e
-
Return a string containing a JSON representation of e. Strings, integers, floats, booleans, nulls and lists are mapped to their JSON equivalents. Sets (except derivations) are represented as objects. Derivations are translated to a JSON string containing the derivation’s output path. Paths are copied to the store and represented as a JSON string of the resulting store path.
-
toPath s
-
DEPRECATED. Use
/. + "/path"
to convert a string into an absolute path. For relative paths, use./. + "/path"
. -
toString e
-
Convert the expression e to a string. e can be:
-
A string (in which case the string is returned unmodified).
-
A path (e.g.,
toString /foo/bar
yields"/foo/bar"
. -
A set containing
{ __toString = self: ...; }
or{ outPath = ...; }
. -
An integer.
-
A list, in which case the string representations of its elements are joined with spaces.
-
A Boolean (
false
yields""
,true
yields"1"
). -
null
, which yields the empty string.
-
-
toXML e
-
Return a string containing an XML representation of e. The main application for
toXML
is to communicate information with the builder in a more structured format than plain environment variables.Here is an example where this is the case:
{ stdenv, fetchurl, libxslt, jira, uberwiki }: stdenv.mkDerivation (rec { name = "web-server"; buildInputs = [ libxslt ]; builder = builtins.toFile "builder.sh" " source $stdenv/setup mkdir $out echo "$servlets" | xsltproc ${stylesheet} - > $out/server-conf.xml ① "; stylesheet = builtins.toFile "stylesheet.xsl" ② "<?xml version='1.0' encoding='UTF-8'?> <xsl:stylesheet xmlns:xsl='http://www.w3.org/1999/XSL/Transform' version='1.0'> <xsl:template match='/'> <Configure> <xsl:for-each select='/expr/list/attrs'> <Call name='addWebApplication'> <Arg><xsl:value-of select=\"attr[@name = 'path']/string/@value\" /></Arg> <Arg><xsl:value-of select=\"attr[@name = 'war']/path/@value\" /></Arg> </Call> </xsl:for-each> </Configure> </xsl:template> </xsl:stylesheet> "; servlets = builtins.toXML [ ③ { path = "/bugtracker"; war = jira + "/lib/atlassian-jira.war"; } { path = "/wiki"; war = uberwiki + "/uberwiki.war"; } ]; })
The builder is supposed to generate the configuration file for a Jetty servlet container. A servlet container contains a number of servlets (
*.war
files) each exported under a specific URI prefix. So the servlet configuration is a list of sets containing thepath
andwar
of the servlet (①). This kind of information is difficult to communicate with the normal method of passing information through an environment variable, which just concatenates everything together into a string (which might just work in this case, but wouldn’t work if fields are optional or contain lists themselves). Instead the Nix expression is converted to an XML representation withtoXML
, which is unambiguous and can easily be processed with the appropriate tools. For instance, in the example an XSLT stylesheet (at point ②) is applied to it (at point ①) to generate the XML configuration file for the Jetty server. The XML representation produced at point ③ bytoXML
is as follows:<?xml version='1.0' encoding='utf-8'?> <expr> <list> <attrs> <attr name="path"> <string value="/bugtracker" /> </attr> <attr name="war"> <path value="/nix/store/d1jh9pasa7k2...-jira/lib/atlassian-jira.war" /> </attr> </attrs> <attrs> <attr name="path"> <string value="/wiki" /> </attr> <attr name="war"> <path value="/nix/store/y6423b1yi4sx...-uberwiki/uberwiki.war" /> </attr> </attrs> </list> </expr>
Note that we used the
toFile
built-in to write the builder and the stylesheet “inline” in the Nix expression. The path of the stylesheet is spliced into the builder using the syntaxxsltproc ${stylesheet}
. -
trace e1 e2
-
Evaluate e1 and print its abstract syntax representation on standard error. Then return e2. This function is useful for debugging.
If the
debugger-on-trace
option is set totrue
and the--debugger
flag is given, the interactive debugger will be started whentrace
is called (likebreak
). -
traceVerbose e1 e2
-
Evaluate e1 and print its abstract syntax representation on standard error if
--trace-verbose
is enabled. Then return e2. This function is useful for debugging. -
tryEval e
-
Try to shallowly evaluate e. Return a set containing the attributes
success
(true
if e evaluated successfully,false
if an error was thrown) andvalue
, equalling e if successful andfalse
otherwise.tryEval
will only prevent errors created bythrow
orassert
from being thrown. ErrorstryEval
will not catch are for example those created byabort
and type errors generated by builtins. Also note that this doesn't evaluate e deeply, solet e = { x = throw ""; }; in (builtins.tryEval e).success
will betrue
. Usingbuiltins.deepSeq
one can get the expected result:let e = { x = throw ""; }; in (builtins.tryEval (builtins.deepSeq e e)).success
will befalse
. -
typeOf e
-
Return a string representing the type of the value e, namely
"int"
,"bool"
,"string"
,"path"
,"null"
,"set"
,"list"
,"lambda"
or"float"
. -
unsafeDiscardOutputDependency s
-
Create a copy of the given string where every "derivation deep" string context element is turned into a constant string context element.
This is the opposite of
builtins.addDrvOutputDependencies
.This is unsafe because it allows us to "forget" store objects we would have otherwise refered to with the string context, whereas Nix normally tracks all dependencies consistently. Safe operations "grow" but never "shrink" string contexts.
builtins.addDrvOutputDependencies
in contrast is safe because "derivation deep" string context element always refers to the underlying derivation (among many more things). Replacing a constant string context element with a "derivation deep" element is a safe operation that just enlargens the string context without forgetting anything. -
zipAttrsWith f list
-
Transpose a list of attribute sets into an attribute set of lists, then apply
mapAttrs
.f
receives two arguments: the attribute name and a non-empty list of all values encountered for that attribute name.The result is an attribute set where the attribute names are the union of the attribute names in each element of
list
. The attribute values are the return values off
.builtins.zipAttrsWith (name: values: { inherit name values; }) [ { a = "x"; } { a = "y"; b = "z"; } ]
evaluates to
{ a = { name = "a"; values = [ "x" "y" ]; }; b = { name = "b"; values = [ "z" ]; }; }