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34
doc/manual/src/SUMMARY.md.in
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@ -26,21 +26,14 @@
- [Copying Closures via SSH](package-management/copy-closure.md)
- [Serving a Nix store via SSH](package-management/ssh-substituter.md)
- [Serving a Nix store via S3](package-management/s3-substituter.md)
- [Writing Nix Expressions](expressions/writing-nix-expressions.md)
- [A Simple Nix Expression](expressions/simple-expression.md)
- [Expression Syntax](expressions/expression-syntax.md)
- [Build Script](expressions/build-script.md)
- [Arguments and Variables](expressions/arguments-variables.md)
- [Building and Testing](expressions/simple-building-testing.md)
- [Generic Builder Syntax](expressions/generic-builder.md)
- [Writing Nix Expressions](expressions/expression-language.md)
- [Values](expressions/language-values.md)
- [Language Constructs](expressions/language-constructs.md)
- [Operators](expressions/language-operators.md)
- [Derivations](expressions/derivations.md)
- [Advanced Attributes](expressions/advanced-attributes.md)
- [Built-in Constants](expressions/builtin-constants.md)
- [Built-in Functions](expressions/builtins.md)
- [Nix Language](language/index.md)
- [Data Types](language/values.md)
- [Language Constructs](language/constructs.md)
- [Operators](language/operators.md)
- [Derivations](language/derivations.md)
- [Advanced Attributes](language/advanced-attributes.md)
- [Built-in Constants](language/builtin-constants.md)
- [Built-in Functions](language/builtins.md)
- [Advanced Topics](advanced-topics/advanced-topics.md)
- [Remote Builds](advanced-topics/distributed-builds.md)
- [Tuning Cores and Jobs](advanced-topics/cores-vs-jobs.md)
@ -66,12 +59,23 @@
@manpages@
- [Files](command-ref/files.md)
- [nix.conf](command-ref/conf-file.md)
<!--
- [Architecture](architecture/architecture.md)
- [Store](architecture/store/store.md)
- [Closure](architecture/store/store/closure.md)
- [Build system terminology](architecture/store/store/build-system-terminology.md)
- [Store Path](architecture/store/path.md)
- [File System Object](architecture/store/fso.md)
-->
- [Glossary](glossary.md)
- [Contributing](contributing/contributing.md)
- [Hacking](contributing/hacking.md)
- [CLI guideline](contributing/cli-guideline.md)
- [Release Notes](release-notes/release-notes.md)
- [Release X.Y (202?-??-??)](release-notes/rl-next.md)
- [Release 2.11 (2022-08-25)](release-notes/rl-2.11.md)
- [Release 2.10 (2022-07-11)](release-notes/rl-2.10.md)
- [Release 2.9 (2022-05-30)](release-notes/rl-2.9.md)
- [Release 2.8 (2022-04-19)](release-notes/rl-2.8.md)
- [Release 2.7 (2022-03-07)](release-notes/rl-2.7.md)
- [Release 2.6 (2022-01-24)](release-notes/rl-2.6.md)

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@ -101,7 +101,7 @@ In particular, notice the
`/nix/store/krpqk0l9ib0ibi1d2w52z293zw455cap-unstable.check` output. Nix
has copied the build results to that directory where you can examine it.
> **Note**
> []{#check-dirs-are-unregistered} **Note**
>
> Check paths are not protected against garbage collection, and this
> path will be deleted on the next garbage collection.

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@ -12,14 +12,14 @@ machine is accessible via SSH and that it has Nix installed. You can
test whether connecting to the remote Nix instance works, e.g.
```console
$ nix ping-store --store ssh://mac
$ nix store ping --store ssh://mac
```
will try to connect to the machine named `mac`. It is possible to
specify an SSH identity file as part of the remote store URI, e.g.
```console
$ nix ping-store --store ssh://mac?ssh-key=/home/alice/my-key
$ nix store ping --store ssh://mac?ssh-key=/home/alice/my-key
```
Since builds should be non-interactive, the key should not have a

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# Architecture
*(This chapter is unstable and a work in progress. Incoming links may rot.)*
This chapter describes how Nix works.
It should help users understand why Nix behaves as it does, and it should help developers understand how to modify Nix and how to write similar tools.
## Overview
Nix consists of [hierarchical layers][layer-architecture].
```
+-----------------------------------------------------------------+
| Nix |
| [ commmand line interface ]------, |
| | | |
| evaluates | |
| | manages |
| V | |
| [ configuration language ] | |
| | | |
| +-----------------------------|-------------------V-----------+ |
| | store evaluates to | |
| | | | |
| | referenced by V builds | |
| | [ build input ] ---> [ build plan ] ---> [ build result ] | |
| | | |
| +-------------------------------------------------------------+ |
+-----------------------------------------------------------------+
```
At the top is the [command line interface](../command-ref/command-ref.md), translating from invocations of Nix executables to interactions with the underlying layers.
Below that is the [Nix expression language](../expressions/expression-language.md), a [purely functional][purely-functional-programming] configuration language.
It is used to compose expressions which ultimately evaluate to self-contained *build plans*, used to derive *build results* from referenced *build inputs*.
The command line and Nix language are what users interact with most.
> **Note**
> The Nix language itself does not have a notion of *packages* or *configurations*.
> As far as we are concerned here, the inputs and results of a build plan are just data.
Underlying these is the [Nix store](./store/store.md), a mechanism to keep track of build plans, data, and references between them.
It can also execute build plans to produce new data.
A build plan is a series of *build tasks*.
Each build task has a special build input which is used as *build instructions*.
The result of a build task can be input to another build task.
```
+-----------------------------------------------------------------------------------------+
| store |
| ................................................. |
| : build plan : |
| : : |
| [ build input ]-----instructions-, : |
| : | : |
| : v : |
| [ build input ]----------->[ build task ]--instructions-, : |
| : | : |
| : | : |
| : v : |
| : [ build task ]----->[ build result ] |
| [ build input ]-----instructions-, ^ : |
| : | | : |
| : v | : |
| [ build input ]----------->[ build task ]---------------' : |
| : ^ : |
| : | : |
| [ build input ]------------------' : |
| : : |
| : : |
| :...............................................: |
| |
+-----------------------------------------------------------------------------------------+
```
[layer-architecture]: https://en.m.wikipedia.org/wiki/Multitier_architecture#Layers
[purely-functional-programming]: https://en.m.wikipedia.org/wiki/Purely_functional_programming

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# File System Object
The Nix store uses a simple file system model for the data it holds in [store objects](store.md#store-object).
Every file system object is one of the following:
- File: an executable flag, and arbitrary data for contents
- Directory: mapping of names to child file system objects
- [Symbolic link][symlink]: may point anywhere.
We call a store object's outermost file system object the *root*.
data FileSystemObject
= File { isExecutable :: Bool, contents :: Bytes }
| Directory { entries :: Map FileName FileSystemObject }
| SymLink { target :: Path }
Examples:
- a directory with contents
/nix/store/<hash>-hello-2.10
├── bin
│   └── hello
└── share
├── info
│   └── hello.info
└── man
└── man1
└── hello.1.gz
- a directory with relative symlink and other contents
/nix/store/<hash>-go-1.16.9
├── bin -> share/go/bin
├── nix-support/
└── share/
- a directory with absolute symlink
/nix/store/d3k...-nodejs
└── nix_node -> /nix/store/f20...-nodejs-10.24.
A bare file or symlink can be a root file system object.
Examples:
/nix/store/<hash>-hello-2.10.tar.gz
/nix/store/4j5...-pkg-config-wrapper-0.29.2-doc -> /nix/store/i99...-pkg-config-0.29.2-doc
Symlinks pointing outside of their own root or to a store object without a matching reference are allowed, but might not function as intended.
Examples:
- an arbitrarily symlinked file may change or not exist at all
/nix/store/<hash>-foo
└── foo -> /home/foo
- if a symlink to a store path was not automatically created by Nix, it may be invalid or get invalidated when the store object is deleted
/nix/store/<hash>-bar
└── bar -> /nix/store/abc...-foo
Nix file system objects do not support [hard links][hardlink]:
each file system object which is not the root has exactly one parent and one name.
However, as store objects are immutable, an underlying file system can use hard links for optimization.
[symlink]: https://en.m.wikipedia.org/wiki/Symbolic_link
[hardlink]: https://en.m.wikipedia.org/wiki/Hard_link

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# Store Path
Nix implements [references](store.md#reference) to [store objects](store.md#store-object) as *store paths*.
Store paths are pairs of
- a 20-byte [digest](#digest) for identification
- a symbolic name for people to read.
Example:
- digest: `b6gvzjyb2pg0kjfwrjmg1vfhh54ad73z`
- name: `firefox-33.1`
It is rendered to a file system path as the concatenation of
- [store directory](#store-directory)
- path-separator (`/`)
- [digest](#digest) rendered in a custom variant of [base-32](https://en.m.wikipedia.org/wiki/Base32) (20 arbitrary bytes become 32 ASCII characters)
- hyphen (`-`)
- name
Example:
/nix/store/b6gvzjyb2pg0kjfwrjmg1vfhh54ad73z-firefox-33.1
|--------| |------------------------------| |----------|
store directory digest name
## Store Directory
Every [store](./store.md) has a store directory.
If the store has a [file system representation](./store.md#files-and-processes), this directory contains the store’s [file system objects](#file-system-object), which can be addressed by [store paths](#store-path).
This means a store path is not just derived from the referenced store object itself, but depends on the store the store object is in.
> **Note**
> The store directory defaults to `/nix/store`, but is in principle arbitrary.
It is important which store a given store object belongs to:
Files in the store object can contain store paths, and processes may read these paths.
Nix can only guarantee [referential integrity](store/closure.md) if store paths do not cross store boundaries.
Therefore one can only copy store objects to a different store if
- the source and target stores' directories match
or
- the store object in question has no references, that is, contains no store paths.
One cannot copy a store object to a store with a different store directory.
Instead, it has to be rebuilt, together with all its dependencies.
It is in general not enough to replace the store directory string in file contents, as this may render executables unusable by invalidating their internal offsets or checksums.
# Digest
In a [store path](#store-path), the [digest][digest] is the output of a [cryptographic hash function][hash] of either all *inputs* involved in building the referenced store object or its actual *contents*.
Store objects are therefore said to be either [input-addressed](#input-addressing) or [content-addressed](#content-addressing).
> **Historical Note**
> The 20 byte restriction is because originally digests were [SHA-1][sha-1] hashes.
> Nix now uses [SHA-256][sha-256], and longer hashes are still reduced to 20 bytes for compatibility.
[digest]: https://en.m.wiktionary.org/wiki/digest#Noun
[hash]: https://en.m.wikipedia.org/wiki/Cryptographic_hash_function
[sha-1]: https://en.m.wikipedia.org/wiki/SHA-1
[sha-256]: https://en.m.wikipedia.org/wiki/SHA-256
### Reference scanning
When a new store object is built, Nix scans its file contents for store paths to construct its set of references.
The special format of a store path's [digest](#digest) allows reliably detecting it among arbitrary data.
Nix uses the [closure](store.md#closure) of build inputs to derive the list of allowed store paths, to avoid false positives.
This way, scanning files captures run time dependencies without the user having to declare them explicitly.
Doing it at build time and persisting references in the store object avoids repeating this time-consuming operation.
> **Note**
> In practice, it is sometimes still necessary for users to declare certain dependencies explicitly, if they are to be preserved in the build result's closure.
This depends on the specifics of the software to build and run.
>
> For example, Java programs are compressed after compilation, which obfuscates any store paths they may refer to and prevents Nix from automatically detecting them.
## Input Addressing
Input addressing means that the digest derives from how the store object was produced, namely its build inputs and build plan.
To compute the hash of a store object one needs a deterministic serialisation, i.e., a binary string representation which only changes if the store object changes.
Nix has a custom serialisation format called Nix Archive (NAR)
Store object references of this sort can *not* be validated from the content of the store object.
Rather, a cryptographic signature has to be used to indicate that someone is vouching for the store object really being produced from a build plan with that digest.
## Content Addressing
Content addressing means that the digest derives from the store object's contents, namely its file system objects and references.
If one knows content addressing was used, one can recalculate the reference and thus verify the store object.
Content addressing is currently only used for the special cases of source files and "fixed-output derivations", where the contents of a store object are known in advance.
Content addressing of build results is still an [experimental feature subject to some restrictions](https://github.com/tweag/rfcs/blob/cas-rfc/rfcs/0062-content-addressed-paths.md).

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# Store
A Nix store is a collection of *store objects* with references between them.
It supports operations to manipulate that collection.
The following concept map is a graphical outline of this chapter.
Arrows indicate suggested reading order.
```
,--------------[ store ]----------------,
| | |
v v v
[ store object ] [ closure ]--, [ operations ]
| | | | | |
v | | v v |
[ files and processes ] | | [ garbage collection ] |
/ \ | | |
v v | v v
[ file system object ] [ store path ] | [ derivation ]--->[ building ]
| ^ | | |
v | v v |
[ digest ]----' [ reference scanning ]<------------'
/ \
v v
[ input addressing ] [ content addressing ]
```
## Store Object
A store object can hold
- arbitrary *data*
- *references* to other store objects.
Store objects can be build inputs, build results, or build tasks.
Store objects are [immutable][immutable-object]: once created, they do not change until they are deleted.
## Reference
A store object reference is an [opaque][opaque-data-type], [unique identifier][unique-identifier]:
The only way to obtain references is by adding or building store objects.
A reference will always point to exactly one store object.
## Operations
A Nix store can *add*, *retrieve*, and *delete* store objects.
[ data ]
|
V
[ store ] ---> add ----> [ store' ]
|
V
[ reference ]
<!-- -->
[ reference ]
|
V
[ store ] ---> get
|
V
[ store object ]
<!-- -->
[ reference ]
|
V
[ store ] --> delete --> [ store' ]
It can *perform builds*, that is, create new store objects by transforming build inputs into build outputs, using instructions from the build tasks.
[ reference ]
|
V
[ store ] --> build --(maybe)--> [ store' ]
|
V
[ reference ]
As it keeps track of references, it can [garbage-collect][garbage-collection] unused store objects.
[ store ] --> collect garbage --> [ store' ]
## Files and Processes
Nix maps between its store model and the [Unix paradigm][unix-paradigm] of [files and processes][file-descriptor], by encoding immutable store objects and opaque identifiers as file system primitives: files and directories, and paths.
That allows processes to resolve references contained in files and thus access the contents of store objects.
Store objects are therefore implemented as the pair of
- a [file system object](fso.md) for data
- a set of [store paths](path.md) for references.
[unix-paradigm]: https://en.m.wikipedia.org/wiki/Everything_is_a_file
[file-descriptor]: https://en.m.wikipedia.org/wiki/File_descriptor
The following diagram shows a radical simplification of how Nix interacts with the operating system:
It uses files as build inputs, and build outputs are files again.
On the operating system, files can be run as processes, which in turn operate on files.
A build function also amounts to an operating system process (not depicted).
```
+-----------------------------------------------------------------+
| Nix |
| [ commmand line interface ]------, |
| | | |
| evaluates | |
| | manages |
| V | |
| [ configuration language ] | |
| | | |
| +-----------------------------|-------------------V-----------+ |
| | store evaluates to | |
| | | | |
| | referenced by V builds | |
| | [ build input ] ---> [ build plan ] ---> [ build result ] | |
| | ^ | | |
| +---------|----------------------------------------|----------+ |
+-----------|----------------------------------------|------------+
| |
file system object store path
| |
+-----------|----------------------------------------|------------+
| operating system +------------+ | |
| '------------ | | <-----------' |
| | file | |
| ,-- | | <-, |
| | +------------+ | |
| execute as | | read, write, execute |
| | +------------+ | |
| '-> | process | --' |
| +------------+ |
+-----------------------------------------------------------------+
```
There exist different types of stores, which all follow this model.
Examples:
- store on the local file system
- remote store accessible via SSH
- binary cache store accessible via HTTP
To make store objects accessible to processes, stores ultimately have to expose store objects through the file system.

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# A [Rosetta stone][rosetta-stone] for build system terminology
The Nix store's design is comparable to other build systems.
Usage of terms is, for historic reasons, not entirely consistent within the Nix ecosystem, and still subject to slow change.
The following translation table points out similarities and equivalent terms, to help clarify their meaning and inform consistent use in the future.
| generic build system | Nix | [Bazel][bazel] | [Build Systems à la Carte][bsalc] | programming language |
| -------------------------------- | ---------------- | -------------------------------------------------------------------- | --------------------------------- | ------------------------ |
| data (build input, build result) | store object | [artifact][bazel-artifact] | value | value |
| build instructions | builder | ([depends on action type][bazel-actions]) | function | function |
| build task | derivation | [action][bazel-action] | `Task` | [thunk][thunk] |
| build plan | derivation graph | [action graph][bazel-action-graph], [build graph][bazel-build-graph] | `Tasks` | [call graph][call-graph] |
| build | build | build | application of `Build` | evaluation |
| persistence layer | store | [action cache][bazel-action-cache] | `Store` | heap |
All of these systems share features of [declarative programming][declarative-programming] languages, a key insight first put forward by Eelco Dolstra et al. in [Imposing a Memory Management Discipline on Software Deployment][immdsd] (2004), elaborated in his PhD thesis [The Purely Functional Software Deployment Model][phd-thesis] (2006), and further refined by Andrey Mokhov et al. in [Build Systems à la Carte][bsalc] (2018).
[rosetta-stone]: https://en.m.wikipedia.org/wiki/Rosetta_Stone
[bazel]: https://bazel.build/start/bazel-intro
[bazel-artifact]: https://bazel.build/reference/glossary#artifact
[bazel-actions]: https://docs.bazel.build/versions/main/skylark/lib/actions.html
[bazel-action]: https://bazel.build/reference/glossary#action
[bazel-action-graph]: https://bazel.build/reference/glossary#action-graph
[bazel-build-graph]: https://bazel.build/reference/glossary#build-graph
[bazel-action-cache]: https://bazel.build/reference/glossary#action-cache
[thunk]: https://en.m.wikipedia.org/wiki/Thunk
[call-graph]: https://en.m.wikipedia.org/wiki/Call_graph
[declarative-programming]: https://en.m.wikipedia.org/wiki/Declarative_programming
[immdsd]: https://edolstra.github.io/pubs/immdsd-icse2004-final.pdf
[phd-thesis]: https://edolstra.github.io/pubs/phd-thesis.pdf
[bsalc]: https://www.microsoft.com/en-us/research/uploads/prod/2018/03/build-systems.pdf

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# Closure
Nix stores ensure [referential integrity][referential-integrity]: for each store object in the store, all the store objects it references must also be in the store.
The set of all store objects reachable by following references from a given initial set of store objects is called a *closure*.
Adding, building, copying and deleting store objects must be done in a way that preserves referential integrity:
- A newly added store object cannot have references, unless it is a build task.
- Build results must only refer to store objects in the closure of the build inputs.
Building a store object will add appropriate references, according to the build task.
- Store objects being copied must refer to objects already in the destination store.
Recursive copying must either proceed in dependency order or be atomic.
- We can only safely delete store objects which are not reachable from any reference still in use.
<!-- more details in section on garbage collection, link to it once it exists -->
[referential-integrity]: https://en.m.wikipedia.org/wiki/Referential_integrity
[garbage-collection]: https://en.m.wikipedia.org/wiki/Garbage_collection_(computer_science)
[immutable-object]: https://en.m.wikipedia.org/wiki/Immutable_object
[opaque-data-type]: https://en.m.wikipedia.org/wiki/Opaque_data_type
[unique-identifier]: https://en.m.wikipedia.org/wiki/Unique_identifier

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Most Nix commands interpret the following environment variables:
- `IN_NIX_SHELL`\
- [`IN_NIX_SHELL`]{#env-IN_NIX_SHELL}\
Indicator that tells if the current environment was set up by
`nix-shell`. Since Nix 2.0 the values are `"pure"` and `"impure"`
`nix-shell`. It can have the values `pure` or `impure`.
- `NIX_PATH`\
- [`NIX_PATH`]{#env-NIX_PATH}\
A colon-separated list of directories used to look up Nix
expressions enclosed in angle brackets (i.e., `<path>`). For
instance, the value
@ -44,7 +44,7 @@ Most Nix commands interpret the following environment variables:
The Nix search path can also be extended using the `-I` option to
many Nix commands, which takes precedence over `NIX_PATH`.
- `NIX_IGNORE_SYMLINK_STORE`\
- [`NIX_IGNORE_SYMLINK_STORE`]{#env-NIX_IGNORE_SYMLINK_STORE}\
Normally, the Nix store directory (typically `/nix/store`) is not
allowed to contain any symlink components. This is to prevent
“impure” builds. Builders sometimes “canonicalise” paths by
@ -66,41 +66,41 @@ Most Nix commands interpret the following environment variables:
Consult the mount 8 manual page for details.
- `NIX_STORE_DIR`\
- [`NIX_STORE_DIR`]{#env-NIX_STORE_DIR}\
Overrides the location of the Nix store (default `prefix/store`).
- `NIX_DATA_DIR`\
- [`NIX_DATA_DIR`]{#env-NIX_DATA_DIR}\
Overrides the location of the Nix static data directory (default
`prefix/share`).
- `NIX_LOG_DIR`\
- [`NIX_LOG_DIR`]{#env-NIX_LOG_DIR}\
Overrides the location of the Nix log directory (default
`prefix/var/log/nix`).
- `NIX_STATE_DIR`\
- [`NIX_STATE_DIR`]{#env-NIX_STATE_DIR}\
Overrides the location of the Nix state directory (default
`prefix/var/nix`).
- `NIX_CONF_DIR`\
- [`NIX_CONF_DIR`]{#env-NIX_CONF_DIR}\
Overrides the location of the system Nix configuration directory
(default `prefix/etc/nix`).
- `NIX_CONFIG`\
- [`NIX_CONFIG`]{#env-NIX_CONFIG}\
Applies settings from Nix configuration from the environment.
The content is treated as if it was read from a Nix configuration file.
Settings are separated by the newline character.
- `NIX_USER_CONF_FILES`\
- [`NIX_USER_CONF_FILES`]{#env-NIX_USER_CONF_FILES}\
Overrides the location of the user Nix configuration files to load
from (defaults to the XDG spec locations). The variable is treated
as a list separated by the `:` token.
- `TMPDIR`\
- [`TMPDIR`]{#env-TMPDIR}\
Use the specified directory to store temporary files. In particular,
this includes temporary build directories; these can take up
substantial amounts of disk space. The default is `/tmp`.
- `NIX_REMOTE`\
- [`NIX_REMOTE`]{#env-NIX_REMOTE}\
This variable should be set to `daemon` if you want to use the Nix
daemon to execute Nix operations. This is necessary in [multi-user
Nix installations](../installation/multi-user.md). If the Nix
@ -108,16 +108,16 @@ Most Nix commands interpret the following environment variables:
should be set to `unix://path/to/socket`. Otherwise, it should be
left unset.
- `NIX_SHOW_STATS`\
- [`NIX_SHOW_STATS`]{#env-NIX_SHOW_STATS}\
If set to `1`, Nix will print some evaluation statistics, such as
the number of values allocated.
- `NIX_COUNT_CALLS`\
- [`NIX_COUNT_CALLS`]{#env-NIX_COUNT_CALLS}\
If set to `1`, Nix will print how often functions were called during
Nix expression evaluation. This is useful for profiling your Nix
expressions.
- `GC_INITIAL_HEAP_SIZE`\
- [`GC_INITIAL_HEAP_SIZE`]{#env-GC_INITIAL_HEAP_SIZE}\
If Nix has been configured to use the Boehm garbage collector, this
variable sets the initial size of the heap in bytes. It defaults to
384 MiB. Setting it to a low value reduces memory consumption, but

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[`--dry-run`]
[{`--out-link` | `-o`} *outlink*]
# Disambiguation
This man page describes the command `nix-build`, which is distinct from `nix
build`. For documentation on the latter, run `nix build --help` or see `man
nix3-build`.
# Description
The `nix-build` command builds the derivations described by the Nix
@ -47,16 +53,16 @@ All options not listed here are passed to `nix-store
--realise`, except for `--arg` and `--attr` / `-A` which are passed to
`nix-instantiate`.
- `--no-out-link`\
- [`--no-out-link`]{#opt-no-out-link}\
Do not create a symlink to the output path. Note that as a result
the output does not become a root of the garbage collector, and so
might be deleted by `nix-store
--gc`.
- `--dry-run`\
- [`--dry-run`]{#opt-dry-run}\
Show what store paths would be built or downloaded.
- `--out-link` / `-o` *outlink*\
- [`--out-link`]{#opt-out-link} / `-o` *outlink*\
Change the name of the symlink to the output path created from
`result` to *outlink*.

6
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@ -31,7 +31,7 @@ subcommand to be performed. These are documented below.
Several commands, such as `nix-env -q` and `nix-env -i`, take a list of
arguments that specify the packages on which to operate. These are
extended regular expressions that must match the entire name of the
package. (For details on regular expressions, see regex7.) The match is
package. (For details on regular expressions, see **regex**(7).) The match is
case-sensitive. The regular expression can optionally be followed by a
dash and a version number; if omitted, any version of the package will
match. Here are some examples:
@ -198,7 +198,7 @@ a number of possible ways:
another.
- If `--from-expression` is given, *args* are Nix
[functions](../expressions/language-constructs.md#functions)
[functions](../language/constructs.md#functions)
that are called with the active Nix expression as their single
argument. The derivations returned by those function calls are
installed. This allows derivations to be specified in an
@ -412,7 +412,7 @@ The upgrade operation determines whether a derivation `y` is an upgrade
of a derivation `x` by looking at their respective `name` attributes.
The names (e.g., `gcc-3.3.1` are split into two parts: the package name
(`gcc`), and the version (`3.3.1`). The version part starts after the
first dash not followed by a letter. `x` is considered an upgrade of `y`
first dash not followed by a letter. `y` is considered an upgrade of `x`
if their package names match, and the version of `y` is higher than that
of `x`.

4
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@ -51,7 +51,7 @@ standard input.
- `--strict`\
When used with `--eval`, recursively evaluate list elements and
attributes. Normally, such sub-expressions are left unevaluated
(since the Nix expression language is lazy).
(since the Nix language is lazy).
> **Warning**
>
@ -66,7 +66,7 @@ standard input.
When used with `--eval`, print the resulting value as an XML
representation of the abstract syntax tree rather than as an ATerm.
The schema is the same as that used by the [`toXML`
built-in](../expressions/builtins.md).
built-in](../language/builtins.md).
- `--read-write-mode`\
When used with `--eval`, perform evaluation in read/write mode so

6
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@ -15,6 +15,12 @@
[`--keep` *name*]
{{`--packages` | `-p`} {*packages* | *expressions*} … | [*path*]}
# Disambiguation
This man page describes the command `nix-shell`, which is distinct from `nix
shell`. For documentation on the latter, run `nix shell --help` or see `man
nix3-shell`.
# Description
The command `nix-shell` will build the dependencies of the specified

4
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@ -22,7 +22,7 @@ This section lists the options that are common to all operations. These
options are allowed for every subcommand, though they may not always
have an effect.
- `--add-root` *path*\
- [`--add-root`]{#opt-add-root} *path*\
Causes the result of a realisation (`--realise` and
`--force-realise`) to be registered as a root of the garbage
collector. *path* will be created as a symlink to the resulting
@ -121,7 +121,7 @@ Special exit codes:
- `102`\
Hash mismatch, the build output was rejected because it does not
match the [`outputHash` attribute of the
derivation](../expressions/advanced-attributes.md).
derivation](../language/advanced-attributes.md).
- `104`\
Not deterministic, the build succeeded in check mode but the

46
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@ -2,13 +2,13 @@
Most Nix commands accept the following command-line options:
- `--help`\
- [`--help`]{#opt-help}\
Prints out a summary of the command syntax and exits.
- `--version`\
- [`--version`]{#opt-version}\
Prints out the Nix version number on standard output and exits.
- `--verbose` / `-v`\
- [`--verbose`]{#opt-verbose} / `-v`\
Increases the level of verbosity of diagnostic messages printed on
standard error. For each Nix operation, the information printed on
standard output is well-defined; any diagnostic information is
@ -37,14 +37,14 @@ Most Nix commands accept the following command-line options:
- 5\
“Vomit”: print vast amounts of debug information.
- `--quiet`\
- [`--quiet`]{#opt-quiet}\
Decreases the level of verbosity of diagnostic messages printed on
standard error. This is the inverse option to `-v` / `--verbose`.
This option may be specified repeatedly. See the previous verbosity
levels list.
- `--log-format` *format*\
- [`--log-format`]{#opt-log-format} *format*\
This option can be used to change the output of the log format, with
*format* being one of:
@ -66,14 +66,14 @@ Most Nix commands accept the following command-line options:
- bar-with-logs\
Display the raw logs, with the progress bar at the bottom.
- `--no-build-output` / `-Q`\
- [`--no-build-output`]{#opt-no-build-output} / `-Q`\
By default, output written by builders to standard output and
standard error is echoed to the Nix command's standard error. This
option suppresses this behaviour. Note that the builder's standard
output and error are always written to a log file in
`prefix/nix/var/log/nix`.
- `--max-jobs` / `-j` *number*\
- [`--max-jobs`]{#opt-max-jobs} / `-j` *number*\
Sets the maximum number of build jobs that Nix will perform in
parallel to the specified number. Specify `auto` to use the number
of CPUs in the system. The default is specified by the `max-jobs`
@ -83,7 +83,7 @@ Most Nix commands accept the following command-line options:
Setting it to `0` disallows building on the local machine, which is
useful when you want builds to happen only on remote builders.
- `--cores`\
- [`--cores`]{#opt-cores}\
Sets the value of the `NIX_BUILD_CORES` environment variable in
the invocation of builders. Builders can use this variable at
their discretion to control the maximum amount of parallelism. For
@ -94,18 +94,18 @@ Most Nix commands accept the following command-line options:
means that the builder should use all available CPU cores in the
system.
- `--max-silent-time`\
- [`--max-silent-time`]{#opt-max-silent-time}\
Sets the maximum number of seconds that a builder can go without
producing any data on standard output or standard error. The
default is specified by the `max-silent-time` configuration
setting. `0` means no time-out.
- `--timeout`\
- [`--timeout`]{#opt-timeout}\
Sets the maximum number of seconds that a builder can run. The
default is specified by the `timeout` configuration setting. `0`
means no timeout.
- `--keep-going` / `-k`\
- [`--keep-going`]{#opt-keep-going} / `-k`\
Keep going in case of failed builds, to the greatest extent
possible. That is, if building an input of some derivation fails,
Nix will still build the other inputs, but not the derivation
@ -113,13 +113,13 @@ Most Nix commands accept the following command-line options:
for builds of substitutes), possibly killing builds in progress (in
case of parallel or distributed builds).
- `--keep-failed` / `-K`\
- [`--keep-failed`]{#opt-keep-failed} / `-K`\
Specifies that in case of a build failure, the temporary directory
(usually in `/tmp`) in which the build takes place should not be
deleted. The path of the build directory is printed as an
informational message.
- `--fallback`\
- [`--fallback`]{#opt-fallback}\
Whenever Nix attempts to build a derivation for which substitutes
are known for each output path, but realising the output paths
through the substitutes fails, fall back on building the derivation.
@ -134,18 +134,18 @@ Most Nix commands accept the following command-line options:
failure in obtaining the substitutes to lead to a full build from
source (with the related consumption of resources).
- `--readonly-mode`\
- [`--readonly-mode`]{#opt-readonly-mode}\
When this option is used, no attempt is made to open the Nix
database. Most Nix operations do need database access, so those
operations will fail.
- `--arg` *name* *value*\
- [`--arg`]{#opt-arg} *name* *value*\
This option is accepted by `nix-env`, `nix-instantiate`,
`nix-shell` and `nix-build`. When evaluating Nix expressions, the
expression evaluator will automatically try to call functions that
it encounters. It can automatically call functions for which every
argument has a [default
value](../expressions/language-constructs.md#functions) (e.g.,
value](../language/constructs.md#functions) (e.g.,
`{ argName ? defaultValue }: ...`). With `--arg`, you can also
call functions that have arguments without a default value (or
override a default value). That is, if the evaluator encounters a
@ -164,19 +164,19 @@ Most Nix commands accept the following command-line options:
So if you call this Nix expression (e.g., when you do `nix-env -iA
pkgname`), the function will be called automatically using the
value [`builtins.currentSystem`](../expressions/builtins.md) for
value [`builtins.currentSystem`](../language/builtins.md) for
the `system` argument. You can override this using `--arg`, e.g.,
`nix-env -iA pkgname --arg system \"i686-freebsd\"`. (Note that
since the argument is a Nix string literal, you have to escape the
quotes.)
- `--argstr` *name* *value*\
- [`--argstr`]{#opt-argstr} *name* *value*\
This option is like `--arg`, only the value is not a Nix
expression but a string. So instead of `--arg system
\"i686-linux\"` (the outer quotes are to keep the shell happy) you
can say `--argstr system i686-linux`.
- `--attr` / `-A` *attrPath*\
- [`--attr`]{#opt-attr} / `-A` *attrPath*\
Select an attribute from the top-level Nix expression being
evaluated. (`nix-env`, `nix-instantiate`, `nix-build` and
`nix-shell` only.) The *attribute path* *attrPath* is a sequence
@ -191,7 +191,7 @@ Most Nix commands accept the following command-line options:
attribute of the fourth element of the array in the `foo` attribute
of the top-level expression.
- `--expr` / `-E`\
- [`--expr`]{#opt-expr} / `-E`\
Interpret the command line arguments as a list of Nix expressions to
be parsed and evaluated, rather than as a list of file names of Nix
expressions. (`nix-instantiate`, `nix-build` and `nix-shell` only.)
@ -202,17 +202,17 @@ Most Nix commands accept the following command-line options:
use, give your expression to the `nix-shell -p` convenience flag
instead.
- `-I` *path*\
- [`-I`]{#opt-I} *path*\
Add a path to the Nix expression search path. This option may be
given multiple times. See the `NIX_PATH` environment variable for
information on the semantics of the Nix search path. Paths added
through `-I` take precedence over `NIX_PATH`.
- `--option` *name* *value*\
- [`--option`]{#opt-option} *name* *value*\
Set the Nix configuration option *name* to *value*. This overrides
settings in the Nix configuration file (see nix.conf5).
- `--repair`\
- [`--repair`]{#opt-repair}\
Fix corrupted or missing store paths by redownloading or rebuilding
them. Note that this is slow because it requires computing a
cryptographic hash of the contents of every path in the closure of

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@ -71,18 +71,6 @@ To install it in `$(pwd)/outputs` and test it:
nix (Nix) 3.0
```
To run a functional test:
```console
make tests/test-name-should-auto-complete.sh.test
```
To run the unit-tests for C++ code:
```
make check
```
If you have a flakes-enabled Nix you can replace:
```console
@ -94,3 +82,29 @@ by:
```console
$ nix develop
```
## Testing
Nix comes with three different flavors of tests: unit, functional and integration.
### Unit-tests
The unit-tests for each Nix library (`libexpr`, `libstore`, etc..) are defined
under `src/{library_name}/tests` using the
[googletest](https://google.github.io/googletest/) framework.
You can run the whole testsuite with `make check`, or the tests for a specific component with `make libfoo-tests_RUN`. Finer-grained filtering is also possible using the [--gtest_filter](https://google.github.io/googletest/advanced.html#running-a-subset-of-the-tests) command-line option.
### Functional tests
The functional tests reside under the `tests` directory and are listed in `tests/local.mk`.
The whole testsuite can be run with `make install && make installcheck`.
Individual tests can be run with `make tests/{testName}.sh.test`.
### Integration tests
The integration tests are defined in the Nix flake under the `hydraJobs.tests` attribute.
These tests include everything that needs to interact with external services or run Nix in a non-trivial distributed setup.
Because these tests are expensive and require more than what the standard github-actions setup provides, they only run on the master branch (on <https://hydra.nixos.org/jobset/nix/master>).
You can run them manually with `nix build .#hydraJobs.tests.{testName}` or `nix-build -A hydraJobs.tests.{testName}`

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@ -1,80 +0,0 @@
# Arguments and Variables
The [Nix expression for GNU Hello](expression-syntax.md) 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/top-level/all-packages.nix`, where all Nix expressions for
packages are imported and called with the appropriate arguments. Here
are some fragments of `all-packages.nix`, with annotations of what
they mean:
```nix
...
rec { ①
hello = import ../applications/misc/hello/ex-1 ② { ③
inherit fetchurl stdenv perl;
};
perl = import ../development/interpreters/perl { ④
inherit fetchurl stdenv;
};
fetchurl = import ../build-support/fetchurl {
inherit stdenv; ...
};
stdenv = ...;
}
```
1. 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 packages into each other.
2. 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 the Nix expression
for GNU Hello in `all-packages.nix` at this point. That would be
completely equivalent, but it would make `all-packages.nix` 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.
3. This is where the actual composition takes place. Here we *call* the
function imported from `../applications/misc/hello/ex-1` with a 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 the [Nix expression for GNU Hello](expression-syntax.md)).
> **Note**
>
> Nixpkgs has a convenience function `callPackage` that imports and
> calls a function, filling in any missing arguments by passing the
> corresponding attribute from the Nixpkgs set, like this:
>
> ```nix
> hello = callPackage ../applications/misc/hello/ex-1 { };
> ```
>
> If necessary, you can set or override arguments:
>
> ```nix
> hello = callPackage ../applications/misc/hello/ex-1 { stdenv = myStdenv; };
> ```
4. Likewise, we have to instantiate Perl, `fetchurl`, and the standard
environment.

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@ -1,70 +0,0 @@
# Build Script
Here is the builder referenced from Hello's Nix expression (stored in
`pkgs/applications/misc/hello/ex-1/builder.sh`):
```bash
source $stdenv/setup ①
PATH=$perl/bin:$PATH ②
tar xvfz $src ③
cd hello-*
./configure --prefix=$out ④
make ⑤
make install
```
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:
1. When Nix runs a builder, it initially completely clears the
environment (except for the attributes declared in the derivation).
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 Hello's Nix expression, but `mkDerivation` adds
it automatically.)
2. 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 package (since it was passed in as an attribute to the
derivation), so `$perl/bin` is the directory containing the Perl
interpreter.
3. Now we have to unpack the sources. The `src` attribute was bound to
the result of fetching the Hello source tarball from the network, so
the `src` environment variable points to the location in the Nix
store to which the tarball was downloaded. After unpacking, we `cd`
to the resulting source directory.
The whole build is performed in a temporary directory created in
`/tmp`, by the way. This directory is removed after the builder
finishes, so there is no need to clean up the sources afterwards.
Also, the temporary directory is always newly created, so you don't
have to worry about files from previous builds interfering with the
current build.
4. GNU Hello is a typical Autoconf-based package, so we first have to
run its `configure` script. In Nix every package is stored in a
separate location in the Nix store, for instance
`/nix/store/9a54ba97fb71b65fda531012d0443ce2-hello-2.1.1`. Nix
computes this path by cryptographically hashing all attributes of
the derivation. The path is passed to the builder through the `out`
environment variable. So here we give `configure` the parameter
`--prefix=$out` to cause Hello to be installed in the expected
location.
5. 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 `-e` option, which causes the script to
be aborted if any command fails without an error check.

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@ -1,12 +0,0 @@
# Nix Expression Language
The 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. Its main job is to describe
packages, compositions of packages, and the variability within packages.
This section presents the various features of the language.

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@ -1,93 +0,0 @@
# Expression Syntax
Here is a Nix expression for GNU Hello:
```nix
{ stdenv, fetchurl, perl }: ①
stdenv.mkDerivation { ②
name = "hello-2.1.1"; ③
builder = ./builder.sh; ④
src = fetchurl { ⑤
url = "ftp://ftp.nluug.nl/pub/gnu/hello/hello-2.1.1.tar.gz";
sha256 = "1md7jsfd8pa45z73bz1kszpp01yw6x5ljkjk2hx7wl800any6465";
};
inherit perl; ⑥
}
```
This file is 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):
1. 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 package that
is used by almost all Nix Packages; 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 package.
2. So we have to build a package. 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 package from a set of
*attributes*. A set is just a list of key/value pairs where each
key is a string and each value is an arbitrary Nix
expression. They take the general form `{ name1 = expr1; ...
nameN = exprN; }`.
3. The attribute `name` specifies the symbolic name and version of
the package. 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 packages. This attribute is required by `mkDerivation`.
4. 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 the [next section](build-script.md),
discussed below.
5. The builder has to know what the sources of the package are. Here,
the attribute `src` is bound to the result of a call to the
`fetchurl` function. Given a URL and a SHA-256 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).
6. 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
```nix
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.

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@ -1,66 +0,0 @@
# Generic Builder Syntax
Recall that the [build script for GNU Hello](build-script.md) looked
something like this:
```bash
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. Here is what a builder using
the generic build facilities looks like:
```bash
buildInputs="$perl" ①
source $stdenv/setup ②
genericBuild ③
```
Here is what each line means:
1. The `buildInputs` variable tells `setup` to use the indicated
packages as “inputs”. This means that if a package provides a `bin`
subdirectory, it's added to `PATH`; if it has a `include`
subdirectory, it's added to GCC's header search path; and so on.
(This is implemented in a modular way: `setup` tries to source the
file `pkg/nix-support/setup-hook` of all dependencies. These “setup
hooks” can then set up whatever environment variables they want; for
instance, the setup hook for Perl sets the `PERL5LIB` environment
variable to contain the `lib/site_perl` directories of all inputs.)
2. The function `genericBuild` is defined in the file `$stdenv/setup`.
3. The final step calls the shell function `genericBuild`, which
performs the steps that were done explicitly in the previous build
script. 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 the Nixpkgs manual for details.
Discerning readers will note that the `buildInputs` could just as well
have been set in the Nix expression, like this:
```nix
buildInputs = [ perl ];
```
The `perl` attribute can then be removed, and the builder becomes even
shorter:
```bash
source $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.

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@ -1,251 +0,0 @@
# Values
## Simple Values
Nix has the following basic data types:
- *Strings* can be written in three ways.
The most common way is to enclose the string between double quotes,
e.g., `"foo bar"`. Strings can span multiple lines. The special
characters `"` and `\` and the character sequence `${` must be
escaped by prefixing them with a backslash (`\`). Newlines, carriage
returns and tabs can be written as `\n`, `\r` and `\t`,
respectively.
You can include the result of an expression into a string by
enclosing it in `${...}`, a feature known as *antiquotation*. The
enclosed expression must evaluate to something that can be coerced
into a string (meaning that it must be a string, a path, or a
derivation). For instance, rather than writing
```nix
"--with-freetype2-library=" + freetype + "/lib"
```
(where `freetype` is a derivation), you can instead write the more
natural
```nix
"--with-freetype2-library=${freetype}/lib"
```
The latter is automatically translated to the former. A more
complicated example (from the Nix expression for
[Qt](http://www.trolltech.com/products/qt)):
```nix
configureFlags = "
-system-zlib -system-libpng -system-libjpeg
${if openglSupport then "-dlopen-opengl
-L${mesa}/lib -I${mesa}/include
-L${libXmu}/lib -I${libXmu}/include" else ""}
${if threadSupport then "-thread" else "-no-thread"}
";
```
Note that Nix expressions and strings can be arbitrarily nested; in
this case the outer string contains various antiquotations that
themselves contain strings (e.g., `"-thread"`), some of which in
turn contain expressions (e.g., `${mesa}`).
The second way to write string literals is as an *indented string*,
which is enclosed between pairs of *double single-quotes*, like so:
```nix
''
This is the first line.
This is the second line.
This is the third line.
''
```
This kind of string literal intelligently strips indentation from
the start of each line. To be precise, it strips from each line a
number of spaces equal to the minimal indentation of the string as a
whole (disregarding the indentation of empty lines). For instance,
the first and second line are indented two spaces, while the third
line is indented four spaces. Thus, two spaces are stripped from
each line, so the resulting string is
```nix
"This is the first line.\nThis is the second line.\n This is the third line.\n"
```
Note that the whitespace and newline following the opening `''` is
ignored if there is no non-whitespace text on the initial line.
Antiquotation (`${expr}`) is supported in indented strings.
Since `${` and `''` have special meaning in indented strings, you
need a way to quote them. `$` can be escaped by prefixing it with
`''` (that is, two single quotes), i.e., `''$`. `''` can be escaped
by prefixing it with `'`, i.e., `'''`. `$` removes any special
meaning from the following `$`. Linefeed, carriage-return and tab
characters can be written as `''\n`, `''\r`, `''\t`, and `''\`
escapes any other character.
Indented strings are primarily useful in that they allow multi-line
string literals to follow the indentation of the enclosing Nix
expression, and that less escaping is typically necessary for
strings representing languages such as shell scripts and
configuration files because `''` is much less common than `"`.
Example:
```nix
stdenv.mkDerivation {
...
postInstall =
''
mkdir $out/bin $out/etc
cp foo $out/bin
echo "Hello World" > $out/etc/foo.conf
${if enableBar then "cp bar $out/bin" else ""}
'';
...
}
```
Finally, as a convenience, *URIs* as defined in appendix B of
[RFC 2396](http://www.ietf.org/rfc/rfc2396.txt) can be written *as
is*, without quotes. For instance, the string
`"http://example.org/foo.tar.bz2"` can also be written as
`http://example.org/foo.tar.bz2`.
- Numbers, which can be *integers* (like `123`) or *floating point*
(like `123.43` or `.27e13`).
Numbers are type-compatible: pure integer operations will always
return integers, whereas any operation involving at least one
floating point number will have a floating point number as a result.
- *Paths*, e.g., `/bin/sh` or `./builder.sh`. A path must contain at
least one slash to be recognised as such. For instance, `builder.sh`
is not a path: it's parsed as an expression that selects the
attribute `sh` from the variable `builder`. If the file name is
relative, i.e., if it does not begin with a slash, it is made
absolute at parse time relative to the directory of the Nix
expression that contained it. For instance, if a Nix expression in
`/foo/bar/bla.nix` refers to `../xyzzy/fnord.nix`, the absolute path
is `/foo/xyzzy/fnord.nix`.
If the first component of a path is a `~`, it is interpreted as if
the rest of the path were relative to the user's home directory.
e.g. `~/foo` would be equivalent to `/home/edolstra/foo` for a user
whose home directory is `/home/edolstra`.
Paths can also be specified between angle brackets, e.g.
`<nixpkgs>`. This means that the directories listed in the
environment variable `NIX_PATH` will be searched for the given file
or directory name.
Antiquotation is supported in any paths except those in angle brackets.
`./${foo}-${bar}.nix` is a more convenient way of writing
`./. + "/" + foo + "-" + bar + ".nix"` or `./. + "/${foo}-${bar}.nix"`. At
least one slash must appear *before* any antiquotations for this to be
recognized as a path. `a.${foo}/b.${bar}` is a syntactically valid division
operation. `./a.${foo}/b.${bar}` is a path.
- *Booleans* with values `true` and `false`.
- The null value, denoted as `null`.
## Lists
Lists are formed by enclosing a whitespace-separated list of values
between square brackets. For example,
```nix
[ 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.,
```nix
[ 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 a set.
Note that lists are only lazy in values, and they are strict in length.
## Sets
Sets are really the core of the language, since ultimately the Nix
language is all about creating derivations, which are really just sets
of attributes to be passed to build scripts.
Sets are just a list of name/value pairs (called *attributes*) enclosed
in curly brackets, where each value is an arbitrary expression
terminated by a semicolon. For example:
```nix
{ x = 123;
text = "Hello";
y = f { bla = 456; };
}
```
This defines a set with attributes named `x`, `text`, `y`. The order of
the attributes is irrelevant. An attribute name may only occur once.
Attributes can be selected from a set using the `.` operator. For
instance,
```nix
{ a = "Foo"; b = "Bar"; }.a
```
evaluates to `"Foo"`. It is possible to provide a default value in an
attribute selection using the `or` keyword. For example,
```nix
{ a = "Foo"; b = "Bar"; }.c or "Xyzzy"
```
will evaluate to `"Xyzzy"` because there is no `c` attribute in the set.
You can use arbitrary double-quoted strings as attribute names:
```nix
{ "foo ${bar}" = 123; "nix-1.0" = 456; }."foo ${bar}"
```
This will evaluate to `123` (Assuming `bar` is antiquotable). In the
case where an attribute name is just a single antiquotation, the quotes
can be dropped:
```nix
{ foo = 123; }.${bar} or 456
```
This will evaluate to `123` if `bar` evaluates to `"foo"` when coerced
to a string and `456` otherwise (again assuming `bar` is antiquotable).
In the special case where an attribute name inside of a set declaration
evaluates to `null` (which is normally an error, as `null` is not
antiquotable), that attribute is simply not added to the set:
```nix
{ ${if foo then "bar" else null} = true; }
```
This will evaluate to `{}` if `foo` evaluates to `false`.
A set that has a `__functor` attribute whose value is callable (i.e. is
itself a function or a set with a `__functor` attribute whose value is
callable) can be applied as if it were a function, with the set itself
passed in first , e.g.,
```nix
let add = { __functor = self: x: x + self.x; };
inc = add // { x = 1; };
in inc 1
```
evaluates to `2`. This can be used to attach metadata to a function
without the caller needing to treat it specially, or to implement a form
of object-oriented programming, for example.

61
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@ -1,61 +0,0 @@
# Building and Testing
You can now try to build Hello. Of course, you could do `nix-env -f . -iA
hello`, but you may not want to install a possibly broken package just
yet. The best way to test the package is by using the command
`nix-build`, which builds a Nix expression and creates a symlink named
`result` in the current directory:
```console
$ nix-build -A hello
building path `/nix/store/632d2b22514d...-hello-2.1.1'
hello-2.1.1/
hello-2.1.1/intl/
hello-2.1.1/intl/ChangeLog
...
$ ls -l result
lrwxrwxrwx ... 2006-09-29 10:43 result -> /nix/store/632d2b22514d...-hello-2.1.1
$ ./result/bin/hello
Hello, world!
```
The `-A` option selects the `hello` attribute. This is faster than
using the symbolic package name specified by the `name` attribute
(which also happens to be `hello`) and is unambiguous (there can be
multiple packages with the symbolic name `hello`, but there can be
only one attribute in a set named `hello`).
`nix-build` registers the `./result` symlink as a garbage collection
root, so unless and until you delete the `./result` symlink, the output
of the build will be safely kept on your system. You can use
`nix-build`’s `-o` switch to give the symlink another name.
Nix has 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 paths will not be registered as valid. If you try to build the
derivation again, Nix will remove the output paths if they exist (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:
```console
$ nix-build -A hello
waiting for lock on `/nix/store/0h5b7hp8d4hqfrw8igvx97x1xawrjnac-hello-2.1.1x'
```
So it is always safe to run multiple instances of Nix in parallel (which
isn’t the case with, say, `make`).

23
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View file

@ -1,23 +0,0 @@
# A Simple Nix Expression
This section shows how to add and test the [GNU Hello
package](http://www.gnu.org/software/hello/hello.html) to the Nix
Packages collection. Hello is a program that prints out the text “Hello,
world\!”.
To add a package to the Nix Packages collection, you generally need to
do three things:
1. Write a Nix expression for the package. This is a file that
describes all the inputs involved in building the package, such as
dependencies, sources, and so on.
2. Write a *builder*. This is a shell script that builds the package
from the inputs. (In fact, it can be written in any language, but
typically it's a `bash` shell script.)
3. Add the package to the file `pkgs/top-level/all-packages.nix`. The
Nix expression written in the first step is a *function*; it
requires other packages 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 package.

12
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View file

@ -1,12 +0,0 @@
This chapter shows you how to write Nix expressions, which instruct Nix
how to build packages. 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.
> **Note**
>
> This chapter is mostly about the Nix expression language. For more
> extensive information on adding packages to the Nix Packages
> collection (such as functions in the standard environment and coding
> conventions), please consult [its
> manual](http://nixos.org/nixpkgs/manual/).

38
doc/manual/src/glossary.md
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@ -1,48 +1,48 @@
# Glossary
- derivation\
- [derivation]{#gloss-derivation}\
A description of a build action. The result of a derivation is a
store object. Derivations are typically specified in Nix expressions
using the [`derivation` primitive](expressions/derivations.md). These are
using the [`derivation` primitive](language/derivations.md). These are
translated into low-level *store derivations* (implicitly by
`nix-env` and `nix-build`, or explicitly by `nix-instantiate`).
- store\
- [store]{#gloss-store}\
The location in the file system where store objects live. Typically
`/nix/store`.
- store path\
- [store path]{#gloss-store-path}\
The location in the file system of a store object, i.e., an
immediate child of the Nix store directory.
- store object\
- [store object]{#gloss-store-object}\
A file that is an immediate child of the Nix store directory. These
can be regular files, but also entire directory trees. Store objects
can be sources (objects copied from outside of the store),
derivation outputs (objects produced by running a build action), or
derivations (files describing a build action).
- substitute\
- [substitute]{#gloss-substitute}\
A substitute is a command invocation stored in the Nix database that
describes how to build a store object, bypassing the normal build
mechanism (i.e., derivations). Typically, the substitute builds the
store object by downloading a pre-built version of the store object
from some server.
- purity\
- [purity]{#gloss-purity}\
The assumption that equal Nix derivations when run always produce
the same output. This cannot be guaranteed in general (e.g., a
builder can rely on external inputs such as the network or the
system time) but the Nix model assumes it.
- Nix expression\
- [Nix expression]{#gloss-nix-expression}\
A high-level description of software packages and compositions
thereof. Deploying software using Nix entails writing Nix
expressions for your packages. Nix expressions are translated to
derivations that are stored in the Nix store. These derivations can
then be built.
- reference\
- [reference]{#gloss-reference}\
A store path `P` is said to have a reference to a store path `Q` if
the store object at `P` contains the path `Q` somewhere. The
*references* of a store path are the set of store paths to which it
@ -52,11 +52,11 @@
output paths), whereas an output path only references other output
paths.
- reachable\
- [reachable]{#gloss-reachable}\
A store path `Q` is reachable from another store path `P` if `Q`
is in the *closure* of the *references* relation.
- closure\
- [closure]{#gloss-closure}\
The closure of a store path is the set of store paths that are
directly or indirectly “reachable” from that store path; that is,
it’s the closure of the path under the *references* relation. For
@ -71,34 +71,34 @@
to path `Q`, then `Q` is in the closure of `P`. Further, if `Q`
references `R` then `R` is also in the closure of `P`.
- output path\
- [output path]{#gloss-output-path}\
A store path produced by a derivation.
- deriver\
- [deriver]{#gloss-deriver}\
The deriver of an *output path* is the store
derivation that built it.
- validity\
- [validity]{#gloss-validity}\
A store path is considered *valid* if it exists in the file system,
is listed in the Nix database as being valid, and if all paths in
its closure are also valid.
- user environment\
- [user environment]{#gloss-user-env}\
An automatically generated store object that consists of a set of
symlinks to “active” applications, i.e., other store paths. These
are generated automatically by
[`nix-env`](command-ref/nix-env.md). See *profiles*.
- profile\
- [profile]{#gloss-profile}\
A symlink to the current *user environment* of a user, e.g.,
`/nix/var/nix/profiles/default`.
- NAR\
- [NAR]{#gloss-nar}\
A *N*ix *AR*chive. This is a serialisation of a path in the Nix
store. It can contain regular files, directories and symbolic
links. NARs are generated and unpacked using `nix-store --dump`
and `nix-store --restore`.
- `∅` \
- [`∅`]{#gloss-emtpy-set}\
The empty set symbol. In the context of profile history, this denotes a package is not present in a particular version of the profile.
- `ε` \
- [`ε`]{#gloss-epsilon}\
The epsilon symbol. In the context of a package, this means the version is empty. More precisely, the derivation does not have a version attribute.

35
doc/manual/src/installation/installing-binary.md
View file

@ -13,7 +13,7 @@ for your platform:
- multi-user on macOS
> **Notes on read-only filesystem root in macOS 10.15 Catalina +**
>
>
> - It took some time to support this cleanly. You may see posts,
> examples, and tutorials using obsolete workarounds.
> - Supporting it cleanly made macOS installs too complex to qualify
@ -31,8 +31,8 @@ $ sh <(curl -L https://nixos.org/nix/install) --no-daemon
```
This will perform a single-user installation of Nix, meaning that `/nix`
is owned by the invoking user. You should run this under your usual user
account, *not* as root. The script will invoke `sudo` to create `/nix`
is owned by the invoking user. You can run this under your usual user
account or root. The script will invoke `sudo` to create `/nix`
if it doesn’t already exist. If you don’t have `sudo`, you should
manually create `/nix` first as root, e.g.:
@ -71,11 +71,11 @@ $ sh <(curl -L https://nixos.org/nix/install) --daemon
The multi-user installation of Nix will create build users between the
user IDs 30001 and 30032, and a group with the group ID 30000. You
should run this under your usual user account, *not* as root. The script
can run this under your usual user account or root. The script
will invoke `sudo` as needed.
> **Note**
>
>
> If you need Nix to use a different group ID or user ID set, you will
> have to download the tarball manually and [edit the install
> script](#installing-from-a-binary-tarball).
@ -148,7 +148,8 @@ and `/etc/zshrc` which you may remove.
This will remove all the build users that no longer serve a purpose.
4. Edit fstab using `sudo vifs` to remove the line mounting the Nix Store
volume on `/nix`, which looks like this,
volume on `/nix`, which looks like
`UUID=<uuid> /nix apfs rw,noauto,nobrowse,suid,owners` or
`LABEL=Nix\040Store /nix apfs rw,nobrowse`. This will prevent automatic
mounting of the Nix Store volume.
@ -167,7 +168,7 @@ and `/etc/zshrc` which you may remove.
removed next.
7. Remove the Nix Store volume:
```console
sudo diskutil apfs deleteVolume /nix
```
@ -175,8 +176,20 @@ and `/etc/zshrc` which you may remove.
This will remove the Nix Store volume and everything that was added to the
store.
If the output indicates that the command couldn't remove the volume, you should
make sure you don't have an _unmounted_ Nix Store volume. Look for a
"Nix Store" volume in the output of the following command:
```console
diskutil list
```
If you _do_ see a "Nix Store" volume, delete it by re-running the diskutil
deleteVolume command, but replace `/nix` with the store volume's `diskXsY`
identifier.
> **Note**
>
>
> After you complete the steps here, you will still have an empty `/nix`
> directory. This is an expected sign of a successful uninstall. The empty
> `/nix` directory will disappear the next time you reboot.
@ -186,12 +199,12 @@ and `/etc/zshrc` which you may remove.
> read-only root will prevent you from manually deleting the empty `/nix`
> mountpoint.
# macOS Installation <a name="sect-macos-installation-change-store-prefix"></a><a name="sect-macos-installation-encrypted-volume"></a><a name="sect-macos-installation-symlink"></a><a name="sect-macos-installation-recommended-notes"></a>
# macOS Installation
[]{#sect-macos-installation-change-store-prefix}[]{#sect-macos-installation-encrypted-volume}[]{#sect-macos-installation-symlink}[]{#sect-macos-installation-recommended-notes}
<!-- Note: anchors above to catch permalinks to old explanations -->
We believe we have ironed out how to cleanly support the read-only root
on modern macOS. New installs will do this automatically, and you can
also re-run a new installer to convert your existing setup.
on modern macOS. New installs will do this automatically.
This section previously detailed the situation, options, and trade-offs,
but it now only outlines what the installer does. You don't need to know

22
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View file

@ -2,7 +2,7 @@
Derivations can declare some infrequently used optional attributes.
- `allowedReferences`\
- [`allowedReferences`]{#adv-attr-allowedReferences}\
The optional attribute `allowedReferences` specifies a list of legal
references (dependencies) of the output of the builder. For example,
@ -17,7 +17,7 @@ Derivations can declare some infrequently used optional attributes.
booting Linux don’t have accidental dependencies on other paths in
the Nix store.
- `allowedRequisites`\
- [`allowedRequisites`]{#adv-attr-allowedRequisites}\
This attribute is similar to `allowedReferences`, but it specifies
the legal requisites of the whole closure, so all the dependencies
recursively. For example,
@ -30,7 +30,7 @@ Derivations can declare some infrequently used optional attributes.
runtime dependency than `foobar`, and in addition it enforces that
`foobar` itself doesn't introduce any other dependency itself.
- `disallowedReferences`\
- [`disallowedReferences`]{#adv-attr-disallowedReferences}\
The optional attribute `disallowedReferences` specifies a list of
illegal references (dependencies) of the output of the builder. For
example,
@ -42,7 +42,7 @@ Derivations can declare some infrequently used optional attributes.
enforces that the output of a derivation cannot have a direct
runtime dependencies on the derivation `foo`.
- `disallowedRequisites`\
- [`disallowedRequisites`]{#adv-attr-disallowedRequisites}\
This attribute is similar to `disallowedReferences`, but it
specifies illegal requisites for the whole closure, so all the
dependencies recursively. For example,
@ -55,7 +55,7 @@ Derivations can declare some infrequently used optional attributes.
dependency on `foobar` or any other derivation depending recursively
on `foobar`.
- `exportReferencesGraph`\
- [`exportReferencesGraph`]{#adv-attr-exportReferencesGraph}\
This attribute allows builders access to the references graph of
their inputs. The attribute is a list of inputs in the Nix store
whose references graph the builder needs to know. The value of
@ -84,7 +84,7 @@ Derivations can declare some infrequently used optional attributes.
with a Nix store containing the closure of a bootable NixOS
configuration).
- `impureEnvVars`\
- [`impureEnvVars`]{#adv-attr-impureEnvVars}\
This attribute allows you to specify a list of environment variables
that should be passed from the environment of the calling user to
the builder. Usually, the environment is cleared completely when the
@ -112,7 +112,7 @@ Derivations can declare some infrequently used optional attributes.
> environmental variables come from the environment of the
> `nix-build`.
- `outputHash`; `outputHashAlgo`; `outputHashMode`\
- [`outputHash`]{#adv-attr-outputHash}; [`outputHashAlgo`]{#adv-attr-outputHashAlgo}; [`outputHashMode`]{#adv-attr-outputHashMode}\
These attributes declare that the derivation is a so-called
*fixed-output derivation*, which means that a cryptographic hash of
the output is already known in advance. When the build of a
@ -208,7 +208,7 @@ Derivations can declare some infrequently used optional attributes.
[`nix-hash` command](../command-ref/nix-hash.md) for information
about converting to and from base-32 notation.)
- `__contentAddressed`
- [`__contentAddressed`]{#adv-attr-__contentAddressed}
If this **experimental** attribute is set to true, then the derivation
outputs will be stored in a content-addressed location rather than the
traditional input-addressed one.
@ -216,7 +216,7 @@ Derivations can declare some infrequently used optional attributes.
Setting this attribute also requires setting `outputHashMode` and `outputHashAlgo` like for *fixed-output derivations* (see above).
- `passAsFile`\
- [`passAsFile`]{#adv-attr-passAsFile}\
A list of names of attributes that should be passed via files rather
than environment variables. For example, if you have
@ -234,7 +234,7 @@ Derivations can declare some infrequently used optional attributes.
builder, since most operating systems impose a limit on the size
of the environment (typically, a few hundred kilobyte).
- `preferLocalBuild`\
- [`preferLocalBuild`]{#adv-attr-preferLocalBuild}\
If this attribute is set to `true` and [distributed building is
enabled](../advanced-topics/distributed-builds.md), then, if
possible, the derivation will be built locally instead of forwarded
@ -242,7 +242,7 @@ Derivations can declare some infrequently used optional attributes.
where the cost of doing a download or remote build would exceed
the cost of building locally.
- `allowSubstitutes`\
- [`allowSubstitutes`]{#adv-attr-allowSubstitutes}\
If this attribute is set to `false`, then Nix will always build this
derivation; it will not try to substitute its outputs. This is
useful for very trivial derivations (such as `writeText` in Nixpkgs)

2
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@ -14,7 +14,7 @@ Here are the constants built into the Nix expression evaluator:
This allows a Nix expression to fall back gracefully on older Nix
installations that don’t have the desired built-in function.
- `builtins.currentSystem`\
- [`builtins.currentSystem`]{#builtins-currentSystem}\
The built-in value `currentSystem` evaluates to the Nix platform
identifier for the Nix installation on which the expression is being
evaluated, such as `"i686-linux"` or `"x86_64-darwin"`.

0
doc/manual/src/expressions/builtins-prefix.md → doc/manual/src/language/builtins-prefix.md
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0
doc/manual/src/expressions/builtins-suffix.md → doc/manual/src/language/builtins-suffix.md
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0
doc/manual/src/expressions/language-constructs.md → doc/manual/src/language/constructs.md
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2
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@ -4,7 +4,7 @@ The most important built-in function is `derivation`, which is used to
describe a single derivation (a build action). It takes as input a set,
the attributes of which specify the inputs of the build.
- There must be an attribute named `system` whose value must be a
- There must be an attribute named [`system`]{#attr-system} whose value must be a
string specifying a Nix system type, such as `"i686-linux"` or
`"x86_64-darwin"`. (To figure out your system type, run `nix -vv
--version`.) The build can only be performed on a machine and

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# Nix Language
The Nix language is
- *domain-specific*
It only exists for the Nix package manager:
to describe packages and configurations as well as their variants and compositions.
It is not intended for general purpose use.
- *declarative*
There is no notion of executing sequential steps.
Dependencies between operations are established only through data.
- *pure*
Values cannot change during computation.
Functions always produce the same output if their input does not change.
- *functional*
Functions are like any other value.
Functions can be assigned to names, taken as arguments, or returned by functions.
- *lazy*
Expressions are only evaluated when their value is needed.
- *dynamically typed*
Type errors are only detected when expressions are evaluated.

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# Operators
The table below lists the operators in the Nix expression language, in
The table below lists the operators in the Nix language, in
order of precedence (from strongest to weakest binding).
| Name | Syntax | Associativity | Description | Precedence |

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# Data Types
## Primitives
- <a id="type-string" href="#type-string">String</a>
*Strings* can be written in three ways.
The most common way is to enclose the string between double quotes,
e.g., `"foo bar"`. Strings can span multiple lines. The special
characters `"` and `\` and the character sequence `${` must be
escaped by prefixing them with a backslash (`\`). Newlines, carriage
returns and tabs can be written as `\n`, `\r` and `\t`,
respectively.
You can include the result of an expression into a string by
enclosing it in `${...}`, a feature known as *antiquotation*. The
enclosed expression must evaluate to something that can be coerced
into a string (meaning that it must be a string, a path, or a
derivation). For instance, rather than writing
```nix
"--with-freetype2-library=" + freetype + "/lib"
```
(where `freetype` is a derivation), you can instead write the more
natural
```nix
"--with-freetype2-library=${freetype}/lib"
```
The latter is automatically translated to the former. A more
complicated example (from the Nix expression for
[Qt](http://www.trolltech.com/products/qt)):
```nix
configureFlags = "
-system-zlib -system-libpng -system-libjpeg
${if openglSupport then "-dlopen-opengl
-L${mesa}/lib -I${mesa}/include
-L${libXmu}/lib -I${libXmu}/include" else ""}
${if threadSupport then "-thread" else "-no-thread"}
";
```
Note that Nix expressions and strings can be arbitrarily nested; in
this case the outer string contains various antiquotations that
themselves contain strings (e.g., `"-thread"`), some of which in
turn contain expressions (e.g., `${mesa}`).
The second way to write string literals is as an *indented string*,
which is enclosed between pairs of *double single-quotes*, like so:
```nix
''
This is the first line.
This is the second line.
This is the third line.
''
```
This kind of string literal intelligently strips indentation from
the start of each line. To be precise, it strips from each line a
number of spaces equal to the minimal indentation of the string as a
whole (disregarding the indentation of empty lines). For instance,
the first and second line are indented two spaces, while the third
line is indented four spaces. Thus, two spaces are stripped from
each line, so the resulting string is
```nix
"This is the first line.\nThis is the second line.\n This is the third line.\n"
```
Note that the whitespace and newline following the opening `''` is
ignored if there is no non-whitespace text on the initial line.
Antiquotation (`${expr}`) is supported in indented strings.
Since `${` and `''` have special meaning in indented strings, you
need a way to quote them. `$` can be escaped by prefixing it with
`''` (that is, two single quotes), i.e., `''$`. `''` can be escaped
by prefixing it with `'`, i.e., `'''`. `$` removes any special
meaning from the following `$`. Linefeed, carriage-return and tab
characters can be written as `''\n`, `''\r`, `''\t`, and `''\`
escapes any other character.
Indented strings are primarily useful in that they allow multi-line
string literals to follow the indentation of the enclosing Nix
expression, and that less escaping is typically necessary for
strings representing languages such as shell scripts and
configuration files because `''` is much less common than `"`.
Example:
```nix
stdenv.mkDerivation {
...
postInstall =
''
mkdir $out/bin $out/etc
cp foo $out/bin
echo "Hello World" > $out/etc/foo.conf
${if enableBar then "cp bar $out/bin" else ""}
'';
...
}
```
Finally, as a convenience, *URIs* as defined in appendix B of
[RFC 2396](http://www.ietf.org/rfc/rfc2396.txt) can be written *as
is*, without quotes. For instance, the string
`"http://example.org/foo.tar.bz2"` can also be written as
`http://example.org/foo.tar.bz2`.
- <a id="type-number" href="#type-number">Number</a>
Numbers, which can be *integers* (like `123`) or *floating point*
(like `123.43` or `.27e13`).
Numbers are type-compatible: pure integer operations will always
return integers, whereas any operation involving at least one
floating point number will have a floating point number as a result.
- <a id="type-path" href="#type-path">Path</a>
*Paths*, e.g., `/bin/sh` or `./builder.sh`. A path must contain at
least one slash to be recognised as such. For instance, `builder.sh`
is not a path: it's parsed as an expression that selects the
attribute `sh` from the variable `builder`. If the file name is
relative, i.e., if it does not begin with a slash, it is made
absolute at parse time relative to the directory of the Nix
expression that contained it. For instance, if a Nix expression in
`/foo/bar/bla.nix` refers to `../xyzzy/fnord.nix`, the absolute path
is `/foo/xyzzy/fnord.nix`.
If the first component of a path is a `~`, it is interpreted as if
the rest of the path were relative to the user's home directory.
e.g. `~/foo` would be equivalent to `/home/edolstra/foo` for a user
whose home directory is `/home/edolstra`.
Paths can also be specified between angle brackets, e.g.
`<nixpkgs>`. This means that the directories listed in the
environment variable `NIX_PATH` will be searched for the given file
or directory name.
Antiquotation is supported in any paths except those in angle brackets.
`./${foo}-${bar}.nix` is a more convenient way of writing
`./. + "/" + foo + "-" + bar + ".nix"` or `./. + "/${foo}-${bar}.nix"`. At
least one slash must appear *before* any antiquotations for this to be
recognized as a path. `a.${foo}/b.${bar}` is a syntactically valid division
operation. `./a.${foo}/b.${bar}` is a path.
- <a id="type-boolean" href="#type-boolean">Boolean</a>
*Booleans* with values `true` and `false`.
- <a id="type-null" href="#type-null">Null</a>
The null value, denoted as `null`.
## List
Lists are formed by enclosing a whitespace-separated list of values
between square brackets. For example,
```nix
[ 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.,
```nix
[ 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 a set.
Note that lists are only lazy in values, and they are strict in length.
## Attribute Set
An attribute set is a collection of name-value-pairs (called *attributes*) enclosed in curly brackets (`{ }`).
Names and values are separated by an equal sign (`=`).
Each value is an arbitrary expression terminated by a semicolon (`;`).
Attributes can appear in any order.
An attribute name may only occur once.
Example:
```nix
{
x = 123;
text = "Hello";
y = f { bla = 456; };
}
```
This defines a set with attributes named `x`, `text`, `y`.
Attributes can be selected from a set using the `.` operator. For
instance,
```nix
{ a = "Foo"; b = "Bar"; }.a
```
evaluates to `"Foo"`. It is possible to provide a default value in an
attribute selection using the `or` keyword. For example,
```nix
{ a = "Foo"; b = "Bar"; }.c or "Xyzzy"
```
will evaluate to `"Xyzzy"` because there is no `c` attribute in the set.
You can use arbitrary double-quoted strings as attribute names:
```nix
{ "foo ${bar}" = 123; "nix-1.0" = 456; }."foo ${bar}"
```
This will evaluate to `123` (Assuming `bar` is antiquotable). In the
case where an attribute name is just a single antiquotation, the quotes
can be dropped:
```nix
{ foo = 123; }.${bar} or 456
```
This will evaluate to `123` if `bar` evaluates to `"foo"` when coerced
to a string and `456` otherwise (again assuming `bar` is antiquotable).
In the special case where an attribute name inside of a set declaration
evaluates to `null` (which is normally an error, as `null` is not
antiquotable), that attribute is simply not added to the set:
```nix
{ ${if foo then "bar" else null} = true; }
```
This will evaluate to `{}` if `foo` evaluates to `false`.
A set that has a `__functor` attribute whose value is callable (i.e. is
itself a function or a set with a `__functor` attribute whose value is
callable) can be applied as if it were a function, with the set itself
passed in first , e.g.,
```nix
let add = { __functor = self: x: x + self.x; };
inc = add // { x = 1; };
in inc 1
```
evaluates to `2`. This can be used to attach metadata to a function
without the caller needing to treat it specially, or to implement a form
of object-oriented programming, for example.

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This chapter discusses how to do package management with Nix, i.e.,
how to obtain, install, upgrade, and erase packages. This is the
“user’s” perspective of the Nix system — people who want to *create*
packages should consult the [chapter on writing Nix
expressions](../expressions/writing-nix-expressions.md).
packages should consult the chapter on the [Nix language](../language/index.md).

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# Release 2.10 (2022-07-11)
* `nix repl` now takes installables on the command line, unifying the usage
with other commands that use `--file` and `--expr`. Primary breaking change
is for the common usage of `nix repl '<nixpkgs>'` which can be recovered with
`nix repl --file '<nixpkgs>'` or `nix repl --expr 'import <nixpkgs>{}'`.
This is currently guarded by the `repl-flake` experimental feature.
* A new function `builtins.traceVerbose` is available. It is similar
to `builtins.trace` if the `trace-verbose` setting is set to true,
and it is a no-op otherwise.
* `nix search` has a new flag `--exclude` to filter out packages.
* On Linux, if `/nix` doesn't exist and cannot be created and you're
not running as root, Nix will automatically use
`~/.local/share/nix/root` as a chroot store. This enables non-root
users to download the statically linked Nix binary and have it work
out of the box, e.g.
```
# ~/nix run nixpkgs#hello
warning: '/nix' does not exists, so Nix will use '/home/ubuntu/.local/share/nix/root' as a chroot store
Hello, world!
```
* `flake-registry.json` is now fetched from `channels.nixos.org`.
* Nix can now be built with LTO by passing `--enable-lto` to `configure`.
LTO is currently only supported when building with GCC.

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# Release 2.11 (2022-08-24)
* `nix copy` now copies the store paths in parallel as much as possible (again).
This doesn't apply for the `daemon` and `ssh-ng` stores which copy everything
in one batch to avoid latencies issues.

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# Release 2.9 (2022-05-30)
* Running Nix with the new `--debugger` flag will cause it to start a
repl session if an exception is thrown during evaluation, or if
`builtins.break` is called. From there you can inspect the values
of variables and evaluate Nix expressions. In debug mode, the
following new repl commands are available:
```
:env Show env stack
:bt Show trace stack
:st Show current trace
:st <idx> Change to another trace in the stack
:c Go until end of program, exception, or builtins.break().
:s Go one step
```
Read more about the debugger
[here](https://www.zknotes.com/note/5970).
* Nix now provides better integration with zsh's `run-help`
feature. It is now included in the Nix installation in the form of
an autoloadable shell function, `run-help-nix`. It picks up Nix
subcommands from the currently typed in command and directs the user
to the associated man pages.
* `nix repl` has a new build-and-link (`:bl`) command that builds a
derivation while creating GC root symlinks.
* The path produced by `builtins.toFile` is now allowed to be imported
or read even with restricted evaluation. Note that this will not
work with a read-only store.
* `nix build` has a new `--print-out-paths` flag to print the
resulting output paths. This matches the default behaviour of
`nix-build`.
* You can now specify which outputs of a derivation `nix` should
operate on using the syntax `installable^outputs`,
e.g. `nixpkgs#glibc^dev,static` or `nixpkgs#glibc^*`. By default,
`nix` will use the outputs specified by the derivation's
`meta.outputsToInstall` attribute if it exists, or all outputs
otherwise.
* `builtins.fetchTree` (and flake inputs) can now be used to fetch
plain files over the `http(s)` and `file` protocols in addition to
directory tarballs.

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# Release X.Y (202?-??-??)
* Nix now provides better integration with zsh's run-help feature. It is now
included in the Nix installation in the form of an autoloadable shell
function, run-help-nix. It picks up Nix subcommands from the currently typed
in command and directs the user to the associated man pages.
* `nix repl` has a new build-'n-link (`:bl`) command that builds a derivation
while creating GC root symlinks.
* The path produced by `builtins.toFile` is now allowed to be imported or read
even with restricted evaluation. Note that this will not work with a
read-only store.
* `nix build` has a new `--print-out-paths` flag to print the resulting output paths.
This matches the default behaviour of `nix-build`.
* Error traces have been reworked to provide detailed explanations and more
accurate error locations. A short excerpt of the trace is now shown by
default when an error occurs.