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# Name
`nix.conf` - Nix configuration file
# Description
Nix supports a variety of configuration settings, which are read from configuration files or taken as command line flags.
## Configuration file
By default Nix reads settings from the following places, in that order:
1. The system-wide configuration file `sysconfdir/nix/nix.conf` (i.e. `/etc/nix/nix.conf` on most systems), or `$NIX_CONF_DIR/nix.conf` if [`NIX_CONF_DIR`](./env-common.md#env-NIX_CONF_DIR) is set.
Values loaded in this file are not forwarded to the Nix daemon.
The client assumes that the daemon has already loaded them.
1. If [`NIX_USER_CONF_FILES`](./env-common.md#env-NIX_USER_CONF_FILES) is set, then each path separated by `:` will be loaded in reverse order.
Otherwise it will look for `nix/nix.conf` files in `XDG_CONFIG_DIRS` and [`XDG_CONFIG_HOME`](./env-common.md#env-XDG_CONFIG_HOME).
If unset, `XDG_CONFIG_DIRS` defaults to `/etc/xdg`, and `XDG_CONFIG_HOME` defaults to `$HOME/.config` as per [XDG Base Directory Specification](https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html).
1. If [`NIX_CONFIG`](./env-common.md#env-NIX_CONFIG) is set, its contents are treated as the contents of a configuration file.
### File format
Configuration files consist of `name = value` pairs, one per line.
Comments start with a `#` character.
Example:
```
keep-outputs = true # Nice for developers
keep-derivations = true # Idem
```
Other files can be included with a line like `include <path>`, where `<path>` is interpreted relative to the current configuration file.
A missing file is an error unless `!include` is used instead.
A configuration setting usually overrides any previous value.
However, for settings that take a list of items, you can prefix the name of the setting by `extra-` to *append* to the previous value.
For instance,
```
substituters = a b
extra-substituters = c d
```
defines the `substituters` setting to be `a b c d`.
Unknown option names are not an error, and are simply ignored with a warning.
## Command line flags
Configuration options can be set on the command line, overriding the values set in the [configuration file](#configuration-file):
- Every configuration setting has corresponding command line flag (e.g. `--max-jobs 16`).
Boolean settings do not need an argument, and can be explicitly disabled with the `no-` prefix (e.g. `--keep-failed` and `--no-keep-failed`).
Unknown option names are invalid flags (unless there is already a flag with that name), and are rejected with an error.
- The flag `--option <name> <value>` is interpreted exactly like a `<name> = <value>` in a setting file.
Unknown option names are ignored with a warning.
The `extra-` prefix is supported for settings that take a list of items (e.g. `--extra-trusted users alice` or `--option extra-trusted-users alice`).
## Integer settings
Settings that have an integer type support the suffixes `K`, `M`, `G`
and `T`. These cause the specified value to be multiplied by 2^10,
2^20, 2^30 and 2^40, respectively. For instance, `--min-free 1M` is
equivalent to `--min-free 1048576`.
# Available settings

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# Common Environment Variables
Most Nix commands interpret the following environment variables:
- <span id="env-IN_NIX_SHELL">[`IN_NIX_SHELL`](#env-IN_NIX_SHELL)</span>
Indicator that tells if the current environment was set up by
`nix-shell`. It can have the values `pure` or `impure`.
- <span id="env-NIX_PATH">[`NIX_PATH`](#env-NIX_PATH)</span>
A colon-separated list of search path entries used to resolve [lookup paths](@docroot@/language/constructs/lookup-path.md).
This environment variable overrides the value of the [`nix-path` configuration setting](@docroot@/command-ref/conf-file.md#conf-nix-path).
It can be extended using the [`-I` option](@docroot@/command-ref/opt-common.md#opt-I).
> **Example**
>
> ```bash
> $ export NIX_PATH=`/home/eelco/Dev:nixos-config=/etc/nixos
> ```
If `NIX_PATH` is set to an empty string, resolving search paths will always fail.
> **Example**
>
> ```bash
> $ NIX_PATH= nix-instantiate --eval '<nixpkgs>'
> error: file 'nixpkgs' was not found in the Nix search path (add it using $NIX_PATH or -I)
> ```
- <span id="env-NIX_IGNORE_SYMLINK_STORE">[`NIX_IGNORE_SYMLINK_STORE`](#env-NIX_IGNORE_SYMLINK_STORE)</span>
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
resolving all symlink components. Thus, builds on different machines
(with `/nix/store` resolving to different locations) could yield
different results. This is generally not a problem, except when
builds are deployed to machines where `/nix/store` resolves
differently. If you are sure that you’re not going to do that, you
can set `NIX_IGNORE_SYMLINK_STORE` to `1`.
Note that if you’re symlinking the Nix store so that you can put it
on another file system than the root file system, on Linux you’re
better off using `bind` mount points, e.g.,
```console
$ mkdir /nix
$ mount -o bind /mnt/otherdisk/nix /nix
```
Consult the mount 8 manual page for details.
- <span id="env-NIX_STORE_DIR">[`NIX_STORE_DIR`](#env-NIX_STORE_DIR)</span>
Overrides the location of the Nix store (default `prefix/store`).
- <span id="env-NIX_DATA_DIR">[`NIX_DATA_DIR`](#env-NIX_DATA_DIR)</span>
Overrides the location of the Nix static data directory (default
`prefix/share`).
- <span id="env-NIX_LOG_DIR">[`NIX_LOG_DIR`](#env-NIX_LOG_DIR)</span>
Overrides the location of the Nix log directory (default
`prefix/var/log/nix`).
- <span id="env-NIX_STATE_DIR">[`NIX_STATE_DIR`](#env-NIX_STATE_DIR)</span>
Overrides the location of the Nix state directory (default
`prefix/var/nix`).
- <span id="env-NIX_CONF_DIR">[`NIX_CONF_DIR`](#env-NIX_CONF_DIR)</span>
Overrides the location of the system Nix configuration directory
(default `prefix/etc/nix`).
- <span id="env-NIX_CONFIG">[`NIX_CONFIG`](#env-NIX_CONFIG)</span>
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.
- <span id="env-NIX_USER_CONF_FILES">[`NIX_USER_CONF_FILES`](#env-NIX_USER_CONF_FILES)</span>
Overrides the location of the Nix user configuration files to load from.
The default are the locations according to the [XDG Base Directory Specification].
See the [XDG Base Directories](#xdg-base-directories) sub-section for details.
The variable is treated as a list separated by the `:` token.
- <span id="env-TMPDIR">[`TMPDIR`](#env-TMPDIR)</span>
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`.
- <span id="env-NIX_REMOTE">[`NIX_REMOTE`](#env-NIX_REMOTE)</span>
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](@docroot@/installation/multi-user.md). If the Nix
daemon's Unix socket is at some non-standard path, this variable
should be set to `unix://path/to/socket`. Otherwise, it should be
left unset.
- <span id="env-NIX_SHOW_STATS">[`NIX_SHOW_STATS`](#env-NIX_SHOW_STATS)</span>
If set to `1`, Nix will print some evaluation statistics, such as
the number of values allocated.
- <span id="env-NIX_COUNT_CALLS">[`NIX_COUNT_CALLS`](#env-NIX_COUNT_CALLS)</span>
If set to `1`, Nix will print how often functions were called during
Nix expression evaluation. This is useful for profiling your Nix
expressions.
- <span id="env-GC_INITIAL_HEAP_SIZE">[`GC_INITIAL_HEAP_SIZE`](#env-GC_INITIAL_HEAP_SIZE)</span>
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
will increase runtime due to the overhead of garbage collection.
## XDG Base Directories
Nix follows the [XDG Base Directory Specification].
For backwards compatibility, Nix commands will follow the standard only when [`use-xdg-base-directories`] is enabled.
[New Nix commands](@docroot@/command-ref/new-cli/nix.md) (experimental) conform to the standard by default.
The following environment variables are used to determine locations of various state and configuration files:
- [`XDG_CONFIG_HOME`]{#env-XDG_CONFIG_HOME} (default `~/.config`)
- [`XDG_STATE_HOME`]{#env-XDG_STATE_HOME} (default `~/.local/state`)
- [`XDG_CACHE_HOME`]{#env-XDG_CACHE_HOME} (default `~/.cache`)
[XDG Base Directory Specification]: https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html
[`use-xdg-base-directories`]: @docroot@/command-ref/conf-file.md#conf-use-xdg-base-directories
In addition, setting the following environment variables overrides the XDG base directories:
- [`NIX_CONFIG_HOME`]{#env-NIX_CONFIG_HOME} (default `$XDG_CONFIG_HOME/nix`)
- [`NIX_STATE_HOME`]{#env-NIX_STATE_HOME} (default `$XDG_STATE_HOME/nix`)
- [`NIX_CACHE_HOME`]{#env-NIX_CACHE_HOME} (default `$XDG_CACHE_HOME/nix`)
When [`use-xdg-base-directories`] is enabled, the configuration directory is:
1. `$NIX_CONFIG_HOME`, if it is defined
2. Otherwise, `$XDG_CONFIG_HOME/nix`, if `XDG_CONFIG_HOME` is defined
3. Otherwise, `~/.config/nix`.
Likewise for the state and cache directories.

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# Experimental Commands
This section lists [experimental commands](@docroot@/development/experimental-features.md#xp-feature-nix-command).
> **Warning**
>
> These commands may be removed in the future, or their syntax may
> change in incompatible ways.

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# Files
This section lists configuration files that you can use when you work
with Nix.

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## Channels
A directory containing symlinks to Nix channels, managed by [`nix-channel`]:
- `$XDG_STATE_HOME/nix/profiles/channels` for regular users
- `$NIX_STATE_DIR/profiles/per-user/root/channels` for `root`
[`nix-channel`] uses a [profile](@docroot@/command-ref/files/profiles.md) to store channels.
This profile contains symlinks to the contents of those channels.
## Subscribed channels
The list of subscribed channels is stored in
- `~/.nix-channels`
- `$XDG_STATE_HOME/nix/channels` if [`use-xdg-base-directories`] is set to `true`
in the following format:
```
<url> <name>
...
```
[`nix-channel`]: @docroot@/command-ref/nix-channel.md
[`use-xdg-base-directories`]: @docroot@/command-ref/conf-file.md#conf-use-xdg-base-directories

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## Default Nix expression
The source for the [Nix expressions](@docroot@/glossary.md#gloss-nix-expression) used by [`nix-env`] by default:
- `~/.nix-defexpr`
- `$XDG_STATE_HOME/nix/defexpr` if [`use-xdg-base-directories`] is set to `true`.
It is loaded as follows:
- If the default expression is a file, it is loaded as a Nix expression.
- If the default expression is a directory containing a `default.nix` file, that `default.nix` file is loaded as a Nix expression.
- If the default expression is a directory without a `default.nix` file, then its contents (both files and subdirectories) are loaded as Nix expressions.
The expressions are combined into a single attribute set, each expression under an attribute with the same name as the original file or subdirectory.
Subdirectories without a `default.nix` file are traversed recursively in search of more Nix expressions, but the names of these intermediate directories are not added to the attribute paths of the default Nix expression.
Then, the resulting expression is interpreted like this:
- If the expression is an attribute set, it is used as the default Nix expression.
- If the expression is a function, an empty set is passed as argument and the return value is used as the default Nix expression.
> **Example**
>
> If the default expression contains two files, `foo.nix` and `bar.nix`, then the default Nix expression will be equivalent to
>
> ```nix
> {
> foo = import ~/.nix-defexpr/foo.nix;
> bar = import ~/.nix-defexpr/bar.nix;
> }
> ```
The file [`manifest.nix`](@docroot@/command-ref/files/manifest.nix.md) is always ignored.
The command [`nix-channel`] places a symlink to the current user's [channels] in this directory, the [user channel link](#user-channel-link).
This makes all subscribed channels available as attributes in the default expression.
## User channel link
A symlink that ensures that [`nix-env`] can find the current user's [channels]:
- `~/.nix-defexpr/channels`
- `$XDG_STATE_HOME/defexpr/channels` if [`use-xdg-base-directories`] is set to `true`.
This symlink points to:
- `$XDG_STATE_HOME/profiles/channels` for regular users
- `$NIX_STATE_DIR/profiles/per-user/root/channels` for `root`
In a multi-user installation, you may also have `~/.nix-defexpr/channels_root`, which links to the channels of the root user.
[`nix-channel`]: @docroot@/command-ref/nix-channel.md
[`nix-env`]: @docroot@/command-ref/nix-env.md
[`use-xdg-base-directories`]: @docroot@/command-ref/conf-file.md#conf-use-xdg-base-directories
[channels]: @docroot@/command-ref/files/channels.md

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## `manifest.json`
The manifest file records the provenance of the packages that are installed in a [profile](./profiles.md) managed by [`nix profile`](@docroot@/command-ref/new-cli/nix3-profile.md) (experimental).
Here is an example of what the file might look like after installing `zoom-us` from Nixpkgs:
```json
{
"version": 1,
"elements": [
{
"active": true,
"attrPath": "legacyPackages.x86_64-linux.zoom-us",
"originalUrl": "flake:nixpkgs",
"storePaths": [
"/nix/store/wbhg2ga8f3h87s9h5k0slxk0m81m4cxl-zoom-us-5.3.469451.0927"
],
"uri": "github:NixOS/nixpkgs/13d0c311e3ae923a00f734b43fd1d35b47d8943a"
},
…
]
}
```
Each object in the array `elements` denotes an installed package and
has the following fields:
* `originalUrl`: The [flake reference](@docroot@/command-ref/new-cli/nix3-flake.md) specified by
the user at the time of installation (e.g. `nixpkgs`). This is also
the flake reference that will be used by `nix profile upgrade`.
* `uri`: The locked flake reference to which `originalUrl` resolved.
* `attrPath`: The flake output attribute that provided this
package. Note that this is not necessarily the attribute that the
user specified, but the one resulting from applying the default
attribute paths and prefixes; for instance, `hello` might resolve to
`packages.x86_64-linux.hello` and the empty string to
`packages.x86_64-linux.default`.
* `storePath`: The paths in the Nix store containing the package.
* `active`: Whether the profile contains symlinks to the files of this
package. If set to false, the package is kept in the Nix store, but
is not "visible" in the profile's symlink tree.

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## `manifest.nix`
The manifest file records the provenance of the packages that are installed in a [profile](./profiles.md) managed by [`nix-env`](@docroot@/command-ref/nix-env.md).
Here is an example of how this file might look like after installing `hello` from Nixpkgs:
```nix
[{
meta = {
available = true;
broken = false;
changelog =
"https://git.savannah.gnu.org/cgit/hello.git/plain/NEWS?h=v2.12.1";
description = "A program that produces a familiar, friendly greeting";
homepage = "https://www.gnu.org/software/hello/manual/";
insecure = false;
license = {
deprecated = false;
free = true;
fullName = "GNU General Public License v3.0 or later";
redistributable = true;
shortName = "gpl3Plus";
spdxId = "GPL-3.0-or-later";
url = "https://spdx.org/licenses/GPL-3.0-or-later.html";
};
longDescription = ''
GNU Hello is a program that prints "Hello, world!" when you run it.
It is fully customizable.
'';
maintainers = [{
email = "edolstra+nixpkgs@gmail.com";
github = "edolstra";
githubId = 1148549;
name = "Eelco Dolstra";
}];
name = "hello-2.12.1";
outputsToInstall = [ "out" ];
platforms = [
"i686-cygwin"
"x86_64-cygwin"
"x86_64-darwin"
"i686-darwin"
"aarch64-darwin"
"armv7a-darwin"
"i686-freebsd13"
"x86_64-freebsd13"
"aarch64-genode"
"i686-genode"
"x86_64-genode"
"x86_64-solaris"
"js-ghcjs"
"aarch64-linux"
"armv5tel-linux"
"armv6l-linux"
"armv7a-linux"
"armv7l-linux"
"i686-linux"
"m68k-linux"
"microblaze-linux"
"microblazeel-linux"
"mipsel-linux"
"mips64el-linux"
"powerpc64-linux"
"powerpc64le-linux"
"riscv32-linux"
"riscv64-linux"
"s390-linux"
"s390x-linux"
"x86_64-linux"
"mmix-mmixware"
"aarch64-netbsd"
"armv6l-netbsd"
"armv7a-netbsd"
"armv7l-netbsd"
"i686-netbsd"
"m68k-netbsd"
"mipsel-netbsd"
"powerpc-netbsd"
"riscv32-netbsd"
"riscv64-netbsd"
"x86_64-netbsd"
"aarch64_be-none"
"aarch64-none"
"arm-none"
"armv6l-none"
"avr-none"
"i686-none"
"microblaze-none"
"microblazeel-none"
"msp430-none"
"or1k-none"
"m68k-none"
"powerpc-none"
"powerpcle-none"
"riscv32-none"
"riscv64-none"
"rx-none"
"s390-none"
"s390x-none"
"vc4-none"
"x86_64-none"
"i686-openbsd"
"x86_64-openbsd"
"x86_64-redox"
"wasm64-wasi"
"wasm32-wasi"
"x86_64-windows"
"i686-windows"
];
position =
"/nix/store/7niq32w715567hbph0q13m5lqna64c1s-nixos-unstable.tar.gz/nixos-unstable.tar.gz/pkgs/applications/misc/hello/default.nix:34";
unfree = false;
unsupported = false;
};
name = "hello-2.12.1";
out = {
outPath = "/nix/store/260q5867crm1xjs4khgqpl6vr9kywql1-hello-2.12.1";
};
outPath = "/nix/store/260q5867crm1xjs4khgqpl6vr9kywql1-hello-2.12.1";
outputs = [ "out" ];
system = "x86_64-linux";
type = "derivation";
}]
```
Each element in this list corresponds to an installed package.
It incorporates some attributes of the original derivation, including `meta`, `name`, `out`, `outPath`, `outputs`, `system`.
This information is used by Nix for querying and updating the package.

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## Profiles
A directory that contains links to profiles managed by [`nix-env`] and [`nix profile`]:
- `$XDG_STATE_HOME/nix/profiles` for regular users
- `$NIX_STATE_DIR/profiles/per-user/root` if the user is `root`
A profile is a directory of symlinks to files in the Nix store.
### Filesystem layout
Profiles are versioned as follows. When using a profile named *path*, *path* is a symlink to *path*`-`*N*`-link`, where *N* is the version of the profile.
In turn, *path*`-`*N*`-link` is a symlink to a path in the Nix store.
For example:
```console
$ ls -l ~alice/.local/state/nix/profiles/profile*
lrwxrwxrwx 1 alice users 14 Nov 25 14:35 /home/alice/.local/state/nix/profiles/profile -> profile-7-link
lrwxrwxrwx 1 alice users 51 Oct 28 16:18 /home/alice/.local/state/nix/profiles/profile-5-link -> /nix/store/q69xad13ghpf7ir87h0b2gd28lafjj1j-profile
lrwxrwxrwx 1 alice users 51 Oct 29 13:20 /home/alice/.local/state/nix/profiles/profile-6-link -> /nix/store/6bvhpysd7vwz7k3b0pndn7ifi5xr32dg-profile
lrwxrwxrwx 1 alice users 51 Nov 25 14:35 /home/alice/.local/state/nix/profiles/profile-7-link -> /nix/store/mp0x6xnsg0b8qhswy6riqvimai4gm677-profile
```
Each of these symlinks is a root for the Nix garbage collector.
The contents of the store path corresponding to each version of the
profile is a tree of symlinks to the files of the installed packages,
e.g.
```console
$ ll -R ~eelco/.local/state/nix/profiles/profile-7-link/
/home/eelco/.local/state/nix/profiles/profile-7-link/:
total 20
dr-xr-xr-x 2 root root 4096 Jan 1 1970 bin
-r--r--r-- 2 root root 1402 Jan 1 1970 manifest.nix
dr-xr-xr-x 4 root root 4096 Jan 1 1970 share
/home/eelco/.local/state/nix/profiles/profile-7-link/bin:
total 20
lrwxrwxrwx 5 root root 79 Jan 1 1970 chromium -> /nix/store/ijm5k0zqisvkdwjkc77mb9qzb35xfi4m-chromium-86.0.4240.111/bin/chromium
lrwxrwxrwx 7 root root 87 Jan 1 1970 spotify -> /nix/store/w9182874m1bl56smps3m5zjj36jhp3rn-spotify-1.1.26.501.gbe11e53b-15/bin/spotify
lrwxrwxrwx 3 root root 79 Jan 1 1970 zoom-us -> /nix/store/wbhg2ga8f3h87s9h5k0slxk0m81m4cxl-zoom-us-5.3.469451.0927/bin/zoom-us
/home/eelco/.local/state/nix/profiles/profile-7-link/share/applications:
total 12
lrwxrwxrwx 4 root root 120 Jan 1 1970 chromium-browser.desktop -> /nix/store/4cf803y4vzfm3gyk3vzhzb2327v0kl8a-chromium-unwrapped-86.0.4240.111/share/applications/chromium-browser.desktop
lrwxrwxrwx 7 root root 110 Jan 1 1970 spotify.desktop -> /nix/store/w9182874m1bl56smps3m5zjj36jhp3rn-spotify-1.1.26.501.gbe11e53b-15/share/applications/spotify.desktop
lrwxrwxrwx 3 root root 107 Jan 1 1970 us.zoom.Zoom.desktop -> /nix/store/wbhg2ga8f3h87s9h5k0slxk0m81m4cxl-zoom-us-5.3.469451.0927/share/applications/us.zoom.Zoom.desktop
…
```
Each profile version contains a manifest file:
- [`manifest.nix`](@docroot@/command-ref/files/manifest.nix.md) used by [`nix-env`](@docroot@/command-ref/nix-env.md).
- [`manifest.json`](@docroot@/command-ref/files/manifest.json.md) used by [`nix profile`](@docroot@/command-ref/new-cli/nix3-profile.md) (experimental).
## User profile link
A symbolic link to the user's current profile:
- `~/.nix-profile`
- `$XDG_STATE_HOME/nix/profile` if [`use-xdg-base-directories`] is set to `true`.
By default, this symlink points to:
- `$XDG_STATE_HOME/nix/profiles/profile` for regular users
- `$NIX_STATE_DIR/profiles/per-user/root/profile` for `root`
The `PATH` environment variable should include `/bin` subdirectory of the profile link (e.g. `~/.nix-profile/bin`) for the user environment to be visible to the user.
The [installer](@docroot@/installation/installing-binary.md) sets this up by default, unless you enable [`use-xdg-base-directories`].
[`nix-env`]: @docroot@/command-ref/nix-env.md
[`nix profile`]: @docroot@/command-ref/new-cli/nix3-profile.md
[`use-xdg-base-directories`]: @docroot@/command-ref/conf-file.md#conf-use-xdg-base-directories

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This section lists commands and options that you can use when you work
with Nix.

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# Main Commands
This section lists commands and options that you can use when you work
with Nix.

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xp_features_json = custom_target(
command : [nix, '__dump-xp-features'],
capture : true,
output : 'xp-features.json',
)
experimental_features_shortlist_md = custom_target(
command : nix_eval_for_docs + [
'--expr',
'import @INPUT0@ (builtins.fromJSON (builtins.readFile ./@INPUT1@))',
],
input : [
'../../generate-xp-features-shortlist.nix',
xp_features_json,
],
output : 'experimental-features-shortlist.md',
capture : true,
env : nix_env_for_docs,
)
nix3_cli_files = custom_target(
command : [
python.full_path(),
'@INPUT0@',
'@OUTPUT@',
'--'
] + nix_eval_for_docs + [
'--expr',
'import @INPUT1@ true (builtins.readFile ./@INPUT2@)',
],
input : [
'../../remove_before_wrapper.py',
'../../generate-manpage.nix',
nix3_cli_json,
],
output : 'new-cli',
env : nix_env_for_docs,
)
conf_file_md_body = custom_target(
command : [
nix_eval_for_docs,
'--expr',
'import @INPUT0@ { prefix = "conf"; } (builtins.fromJSON (builtins.readFile ./@INPUT1@))',
],
capture : true,
input : [
'../../generate-settings.nix',
conf_file_json,
],
output : 'conf-file.body.md',
env : nix_env_for_docs,
)
conf_file_md = custom_target(
command : [ 'cat', '@INPUT0@', '@INPUT1@' ],
capture : true,
input : [
'conf-file-prefix.md',
conf_file_md_body,
],
output : 'conf-file.md',
)

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# Name
`nix-build` - build a Nix expression
# Synopsis
`nix-build` [*paths…*]
[`--arg` *name* *value*]
[`--argstr` *name* *value*]
[{`--attr` | `-A`} *attrPath*]
[`--no-out-link`]
[`--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
expressions in *paths*. If the build succeeds, it places a symlink to
the result in the current directory. The symlink is called `result`. If
there are multiple Nix expressions, or the Nix expressions evaluate to
multiple derivations, multiple sequentially numbered symlinks are
created (`result`, `result-2`, and so on).
If no *paths* are specified, then `nix-build` will use `default.nix` in
the current directory, if it exists.
If an element of *paths* starts with `http://` or `https://`, it is
interpreted as the URL of a tarball that will be downloaded and unpacked
to a temporary location. The tarball must include a single top-level
directory containing at least a file named `default.nix`.
`nix-build` is essentially a wrapper around
[`nix-instantiate`](nix-instantiate.md) (to translate a high-level Nix
expression to a low-level [store derivation]) and [`nix-store
--realise`](@docroot@/command-ref/nix-store/realise.md) (to build the store
derivation).
[store derivation]: @docroot@/glossary.md#gloss-store-derivation
> **Warning**
>
> The result of the build is automatically registered as a root of the
> Nix garbage collector. This root disappears automatically when the
> `result` symlink is deleted or renamed. So don’t rename the symlink.
# Options
All options not listed here are passed to
[`nix-store --realise`](nix-store/realise.md),
except for `--arg` and `--attr` / `-A` which are passed to [`nix-instantiate`](nix-instantiate.md).
- <span id="opt-no-out-link">[`--no-out-link`](#opt-no-out-link)<span>
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`.
- <span id="opt-dry-run">[`--dry-run`](#opt-dry-run)</span>
Show what store paths would be built or downloaded.
- <span id="opt-out-link">[`--out-link`](#opt-out-link)</span> / `-o` *outlink*
Change the name of the symlink to the output path created from
`result` to *outlink*.
{{#include ./status-build-failure.md}}
{{#include ./opt-common.md}}
{{#include ./env-common.md}}
# Examples
```console
$ nix-build '<nixpkgs>' --attr firefox
store derivation is /nix/store/qybprl8sz2lc...-firefox-1.5.0.7.drv
/nix/store/d18hyl92g30l...-firefox-1.5.0.7
$ ls -l result
lrwxrwxrwx ... result -> /nix/store/d18hyl92g30l...-firefox-1.5.0.7
$ ls ./result/bin/
firefox firefox-config
```
If a derivation has multiple outputs, `nix-build` will build the default
(first) output. You can also build all outputs:
```console
$ nix-build '<nixpkgs>' --attr openssl.all
```
This will create a symlink for each output named `result-outputname`.
The suffix is omitted if the output name is `out`. So if `openssl` has
outputs `out`, `bin` and `man`, `nix-build` will create symlinks
`result`, `result-bin` and `result-man`. It’s also possible to build a
specific output:
```console
$ nix-build '<nixpkgs>' --attr openssl.man
```
This will create a symlink `result-man`.
Build a Nix expression given on the command line:
```console
$ nix-build --expr 'with import <nixpkgs> { }; runCommand "foo" { } "echo bar > $out"'
$ cat ./result
bar
```
Build the GNU Hello package from the latest revision of the master
branch of Nixpkgs:
```console
$ nix-build https://github.com/NixOS/nixpkgs/archive/master.tar.gz --attr hello
```

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# Name
`nix-channel` - manage Nix channels
# Synopsis
`nix-channel` {`--add` url [*name*] | `--remove` *name* | `--list` | `--update` [*names…*] | `--list-generations` | `--rollback` [*generation*] }
# Description
Channels are a mechanism for referencing remote Nix expressions and conveniently retrieving their latest version.
The moving parts of channels are:
- The official channels listed at <https://nixos.org/channels>
- The user-specific list of [subscribed channels](#subscribed-channels)
- The [downloaded channel contents](#channels)
- The [Nix expression search path](@docroot@/command-ref/conf-file.md#conf-nix-path), set with the [`-I` option](#opt-i) or the [`NIX_PATH` environment variable](#env-NIX_PATH)
> **Note**
>
> The state of a subscribed channel is external to the Nix expressions relying on it.
> This may limit reproducibility.
>
> Dependencies on other Nix expressions can be declared explicitly with:
> - [`fetchurl`](@docroot@/language/builtins.md#builtins-fetchurl), [`fetchTarball`](@docroot@/language/builtins.md#builtins-fetchTarball), or [`fetchGit`](@docroot@/language/builtins.md#builtins-fetchGit) in Nix expressions
> - the [`-I` option](@docroot@/command-ref/opt-common.md#opt-I) in command line invocations
This command has the following operations:
- `--add` *url* \[*name*\]
Add a channel *name* located at *url* to the list of subscribed channels.
If *name* is omitted, default to the last component of *url*, with the suffixes `-stable` or `-unstable` removed.
> **Note**
>
> `--add` does not automatically perform an update.
> Use `--update` explicitly.
A channel URL must point to a directory containing a file `nixexprs.tar.gz`.
At the top level, that tarball must contain a single directory with a `default.nix` file that serves as the channel’s entry point.
- `--remove` *name*
Remove the channel *name* from the list of subscribed channels.
- `--list`
Print the names and URLs of all subscribed channels on standard output.
- `--update` \[*names*…\]
Download the Nix expressions of subscribed channels and create a new generation.
Update all channels if none is specified, and only those included in *names* otherwise.
- `--list-generations`
Prints a list of all the current existing generations for the
channel profile.
Works the same way as
```
nix-env --profile /nix/var/nix/profiles/per-user/$USER/channels --list-generations
```
- `--rollback` \[*generation*\]
Revert channels to the state before the last call to `nix-channel --update`.
Optionally, you can specify a specific channel *generation* number to restore.
{{#include ./opt-common.md}}
{{#include ./env-common.md}}
# Files
`nix-channel` operates on the following files.
{{#include ./files/channels.md}}
# Examples
Subscribe to the Nixpkgs channel and run `hello` from the GNU Hello package:
```console
$ nix-channel --add https://nixos.org/channels/nixpkgs-unstable
$ nix-channel --list
nixpkgs https://nixos.org/channels/nixpkgs
$ nix-channel --update
$ nix-shell -p hello --run hello
hello
```
Revert channel updates using `--rollback`:
```console
$ nix-instantiate --eval '<nixpkgs>' --attr lib.version
"22.11pre296212.530a53dcbc9"
$ nix-channel --rollback
switching from generation 483 to 482
$ nix-instantiate --eval '<nixpkgs>' --attr lib.version
"22.11pre281526.d0419badfad"
```
Remove a channel:
```console
$ nix-channel --remove nixpkgs
$ nix-channel --list
```

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# Name
`nix-collect-garbage` - delete unreachable [store objects]
# Synopsis
`nix-collect-garbage` [`--delete-old`] [`-d`] [`--delete-older-than` *period*] [`--max-freed` *bytes*] [`--dry-run`]
# Description
The command `nix-collect-garbage` is mostly an alias of [`nix-store --gc`](@docroot@/command-ref/nix-store/gc.md).
That is, it deletes all unreachable [store objects] in the Nix store to clean up your system.
However, it provides two additional options,
[`--delete-old`](#opt-delete-old) and [`--delete-older-than`](#opt-delete-older-than),
which also delete old [profiles], allowing potentially more [store objects] to be deleted because profiles are also garbage collection roots.
These options are the equivalent of running
[`nix-env --delete-generations`](@docroot@/command-ref/nix-env/delete-generations.md)
with various augments on multiple profiles,
prior to running `nix-collect-garbage` (or just `nix-store --gc`) without any flags.
> **Note**
>
> Deleting previous configurations makes rollbacks to them impossible.
These flags should be used with care, because they potentially delete generations of profiles used by other users on the system.
## Locations searched for profiles
`nix-collect-garbage` cannot know about all profiles; that information doesn't exist.
Instead, it looks in a few locations, and acts on all profiles it finds there:
1. The default profile locations as specified in the [profiles] section of the manual.
2. > **NOTE**
>
> Not stable; subject to change
>
> Do not rely on this functionality; it just exists for migration purposes and is may change in the future.
> These deprecated paths remain a private implementation detail of Nix.
`$NIX_STATE_DIR/profiles` and `$NIX_STATE_DIR/profiles/per-user`.
With the exception of `$NIX_STATE_DIR/profiles/per-user/root` and `$NIX_STATE_DIR/profiles/default`, these directories are no longer used by other commands.
`nix-collect-garbage` looks there anyways in order to clean up profiles from older versions of Nix.
# Options
These options are for deleting old [profiles] prior to deleting unreachable [store objects].
- <span id="opt-delete-old">[`--delete-old`](#opt-delete-old)</span> / `-d`
Delete all old generations of profiles.
This is the equivalent of invoking [`nix-env --delete-generations old`](@docroot@/command-ref/nix-env/delete-generations.md#generations-old) on each found profile.
- <span id="opt-delete-older-than">[`--delete-older-than`](#opt-delete-older-than)</span> *period*
Delete all generations of profiles older than the specified amount (except for the generations that were active at that point in time).
*period* is a value such as `30d`, which would mean 30 days.
This is the equivalent of invoking [`nix-env --delete-generations <period>`](@docroot@/command-ref/nix-env/delete-generations.md#generations-time) on each found profile.
See the documentation of that command for additional information about the *period* argument.
{{#include ./opt-common.md}}
{{#include ./env-common.md}}
# Example
To delete from the Nix store everything that is not used by the current
generations of each profile, do
```console
$ nix-collect-garbage -d
```
[profiles]: @docroot@/command-ref/files/profiles.md
[store objects]: @docroot@/store/store-object.md

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# Name
`nix-copy-closure` - copy store objects to or from a remote machine via SSH
# Synopsis
`nix-copy-closure`
[`--to` | `--from` ]
[`--gzip`]
[`--include-outputs`]
[`--use-substitutes` | `-s`]
[`-v`]
[_user_@]_machine_[:_port_] _paths_
# Description
Given _paths_ from one machine, `nix-copy-closure` computes the [closure](@docroot@/glossary.md#gloss-closure) of those paths (i.e. all their dependencies in the Nix store), and copies [store objects](@docroot@/glossary.md#gloss-store-object) in that closure to another machine via SSH.
It doesn’t copy store objects that are already present on the other machine.
> **Note**
>
> While the Nix store to use on the local machine can be specified on the command line with the [`--store`](@docroot@/command-ref/conf-file.md#conf-store) option, the Nix store to be accessed on the remote machine can only be [configured statically](@docroot@/command-ref/conf-file.md#configuration-file) on that remote machine.
Since `nix-copy-closure` calls `ssh`, you may need to authenticate with the remote machine.
In fact, you may be asked for authentication _twice_ because `nix-copy-closure` currently connects twice to the remote machine: first to get the set of paths missing on the target machine, and second to send the dump of those paths.
When using public key authentication, you can avoid typing the passphrase with `ssh-agent`.
# Options
- `--to`
Copy the closure of _paths_ from a Nix store accessible from the local machine to the Nix store on the remote _machine_.
This is the default behavior.
- `--from`
Copy the closure of _paths_ from the Nix store on the remote _machine_ to the local machine's specified Nix store.
- `--gzip`
Enable compression of the SSH connection.
- `--include-outputs`
Also copy the outputs of [store derivation]s included in the closure.
[store derivation]: @docroot@/glossary.md#gloss-store-derivation
- `--use-substitutes` / `-s`
Attempt to download missing store objects on the target from [substituters](@docroot@/command-ref/conf-file.md#conf-substituters).
Any store objects that cannot be substituted on the target are still copied normally from the source.
This is useful, for instance, if the connection between the source and target machine is slow, but the connection between the target machine and `cache.nixos.org` (the default binary cache server) is fast.
{{#include ./opt-common.md}}
# Environment variables
- `NIX_SSHOPTS`
Additional options to be passed to `ssh` on the command line.
{{#include ./env-common.md}}
# Examples
> **Example**
>
> Copy GNU Hello with all its dependencies to a remote machine:
>
> ```shell-session
> $ storePath="$(nix-build '<nixpkgs>' -I nixpkgs=channel:nixpkgs-unstable -A hello --no-out-link)"
> $ nix-copy-closure --to alice@itchy.example.org "$storePath"
> copying 5 paths...
> copying path '/nix/store/nrwkk6ak3rgkrxbqhsscb01jpzmslf2r-xgcc-13.2.0-libgcc' to 'ssh://alice@itchy.example.org'...
> copying path '/nix/store/gm61h1y42pqyl6178g90x8zm22n6pyy5-libunistring-1.1' to 'ssh://alice@itchy.example.org'...
> copying path '/nix/store/ddfzjdykw67s20c35i7a6624by3iz5jv-libidn2-2.3.7' to 'ssh://alice@itchy.example.org'...
> copying path '/nix/store/apab5i73dqa09wx0q27b6fbhd1r18ihl-glibc-2.39-31' to 'ssh://alice@itchy.example.org'...
> copying path '/nix/store/g1n2vryg06amvcc1avb2mcq36faly0mh-hello-2.12.1' to 'ssh://alice@itchy.example.org'...
> ```
> **Example**
>
> Copy GNU Hello from a remote machine using a known store path, and run it:
>
> ```shell-session
> $ storePath="$(nix-instantiate --eval '<nixpkgs>' -I nixpkgs=channel:nixpkgs-unstable -A hello.outPath | tr -d '"')"
> $ nix-copy-closure --from alice@itchy.example.org "$storePath"
> $ "$storePath"/bin/hello
> Hello, world!
> ```

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# Name
`nix-daemon` - Nix multi-user support daemon
# Synopsis
`nix-daemon`
# Description
The Nix daemon is necessary in multi-user Nix installations. It runs
build tasks and other operations on the Nix store on behalf of
unprivileged users.

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# Name
`nix-env` - manipulate or query Nix user environments
# Synopsis
`nix-env` *operation* [*options*] [*arguments…*]
[`--option` *name* *value*]
[`--arg` *name* *value*]
[`--argstr` *name* *value*]
[{`--file` | `-f`} *path*]
[{`--profile` | `-p`} *path*]
[`--system-filter` *system*]
[`--dry-run`]
# Description
The command `nix-env` is used to manipulate Nix user environments. User
environments are sets of software packages available to a user at some
point in time. In other words, they are a synthesised view of the
programs available in the Nix store. There may be many user
environments: different users can have different environments, and
individual users can switch between different environments.
`nix-env` takes exactly one *operation* flag which indicates the
subcommand to be performed. The following operations are available:
- [`--install`](./nix-env/install.md)
- [`--upgrade`](./nix-env/upgrade.md)
- [`--uninstall`](./nix-env/uninstall.md)
- [`--set`](./nix-env/set.md)
- [`--set-flag`](./nix-env/set-flag.md)
- [`--query`](./nix-env/query.md)
- [`--switch-profile`](./nix-env/switch-profile.md)
- [`--list-generations`](./nix-env/list-generations.md)
- [`--delete-generations`](./nix-env/delete-generations.md)
- [`--switch-generation`](./nix-env/switch-generation.md)
- [`--rollback`](./nix-env/rollback.md)
These pages can be viewed offline:
- `man nix-env-<operation>`.
Example: `man nix-env-install`
- `nix-env --help --<operation>`
Example: `nix-env --help --install`
# Package sources
`nix-env` can obtain packages from multiple sources:
- An attribute set of derivations from:
- The [default Nix expression](@docroot@/command-ref/files/default-nix-expression.md) (by default)
- A Nix file, specified via `--file`
- A [profile](@docroot@/command-ref/files/profiles.md), specified via `--from-profile`
- A Nix expression that is a function which takes default expression as argument, specified via `--from-expression`
- A [store path](@docroot@/store/store-path.md)
# Selectors
Several operations, such as [`nix-env --query`](./nix-env/query.md) and [`nix-env --install`](./nix-env/install.md), take a list of *arguments* that specify the packages on which to operate.
Packages are identified based on a `name` part and a `version` part of a [symbolic derivation name](@docroot@/language/derivations.md#attr-name):
- `name`: Everything up to but not including the first dash (`-`) that is *not* followed by a letter.
- `version`: The rest, excluding the separating dash.
> **Example**
>
> `nix-env` parses the symbolic derivation name `apache-httpd-2.0.48` as:
>
> ```json
> {
> "name": "apache-httpd",
> "version": "2.0.48"
> }
> ```
> **Example**
>
> `nix-env` parses the symbolic derivation name `firefox.*` as:
>
> ```json
> {
> "name": "firefox.*",
> "version": ""
> }
> ```
The `name` parts of the *arguments* to `nix-env` are treated as extended regular expressions and matched against the `name` parts of derivation names in the package source.
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.
For details on regular expressions, see [**regex**(7)](https://linux.die.net/man/7/regex).
> **Example**
>
> Common patterns for finding package names with `nix-env`:
>
> - `firefox`
>
> Matches the package name `firefox` and any version.
>
> - `firefox-32.0`
>
> Matches the package name `firefox` and version `32.0`.
>
> - `gtk\\+`
>
> Matches the package name `gtk+`.
> The `+` character must be escaped using a backslash (`\`) to prevent it from being interpreted as a quantifier, and the backslash must be escaped in turn with another backslash to ensure that the shell passes it on.
>
> - `.\*`
>
> Matches any package name.
> This is the default for most commands.
>
> - `'.*zip.*'`
>
> Matches any package name containing the string `zip`.
> Note the dots: `'*zip*'` does not work, because in a regular expression, the character `*` is interpreted as a quantifier.
>
> - `'.*(firefox|chromium).*'`
>
> Matches any package name containing the strings `firefox` or `chromium`.
# Files
`nix-env` operates on the following files.
{{#include ./files/default-nix-expression.md}}
{{#include ./files/profiles.md}}

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# Name
`nix-env --delete-generations` - delete profile generations
# Synopsis
`nix-env` `--delete-generations` *generations*
# Description
This operation deletes the specified generations of the current profile.
*generations* can be a one of the following:
- <span id="generations-list">[`<number>...`](#generations-list)</span>
A list of generation numbers, each one a separate command-line argument.
Delete exactly the profile generations given by their generation number.
Deleting the current generation is not allowed.
- <span id="generations-old">[The special value `old`](#generations-old)</span>
Delete all generations except the current one.
> **WARNING**
>
> Older *and newer* generations will be deleted by this operation.
>
> One might expect this to just delete older generations than the curent one, but that is only true if the current generation is also the latest.
> Because one can roll back to a previous generation, it is possible to have generations newer than the current one.
> They will also be deleted.
- <span id="generations-time">[`<number>d`](#generations-time)</span>
The last *number* days
*Example*: `30d`
Delete all generations created more than *number* days ago, except the most recent one of them.
This allows rolling back to generations that were available within the specified period.
- <span id="generations-count">[`+<number>`](#generations-count)</span>
The last *number* generations up to the present
*Example*: `+5`
Keep the last *number* generations, along with any newer than current.
Periodically deleting old generations is important to make garbage collection
effective.
The is because profiles are also garbage collection roots — any [store object] reachable from a profile is "alive" and ineligible for deletion.
[store object]: @docroot@/store/store-object.md
{{#include ./opt-common.md}}
{{#include ../opt-common.md}}
{{#include ./env-common.md}}
{{#include ../env-common.md}}
# Examples
## Delete explicit generation numbers
```console
$ nix-env --delete-generations 3 4 8
```
Delete the generations numbered 3, 4, and 8, so long as the current active generation is not any of those.
## Keep most-recent by count (number of generations)
```console
$ nix-env --delete-generations +5
```
Suppose `30` is the current generation, and we currently have generations numbered `20` through `32`.
Then this command will delete generations `20` through `25` (`<= 30 - 5`),
and keep generations `26` through `31` (`> 30 - 5`).
## Keep most-recent by time (number of days)
```console
$ nix-env --delete-generations 30d
```
This command will delete all generations older than 30 days, except for the generation that was active 30 days ago (if it currently exists).
## Delete all older
```console
$ nix-env --profile other_profile --delete-generations old
```

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# Environment variables
- `NIX_PROFILE`
Location of the Nix profile. Defaults to the target of the symlink
`~/.nix-profile`, if it exists, or `/nix/var/nix/profiles/default`
otherwise.

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# Name
`nix-env --install` - add packages to user environment
# Synopsis
`nix-env` {`--install` | `-i`} *args…*
[{`--prebuilt-only` | `-b`}]
[{`--attr` | `-A`}]
[`--from-expression`] [`-E`]
[`--from-profile` *path*]
[`--preserve-installed` | `-P`]
[`--remove-all` | `-r`]
# Description
The `--install` operation creates a new user environment.
It is based on the current generation of the active [profile](@docroot@/command-ref/files/profiles.md), to which a set of [store paths] described by *args* is added.
[store paths]: @docroot@/store/store-path.md
The arguments *args* map to store paths in a number of possible ways:
- By default, *args* is a set of [derivation] names denoting derivations in the [default Nix expression].
These are [realised], and the resulting output paths are installed.
Currently installed derivations with a name equal to the name of a derivation being added are removed unless the option `--preserve-installed` is specified.
[derivation]: @docroot@/glossary.md#gloss-derivation
[default Nix expression]: @docroot@/command-ref/files/default-nix-expression.md
[realised]: @docroot@/glossary.md#gloss-realise
If there are multiple derivations matching a name in *args* that
have the same name (e.g., `gcc-3.3.6` and `gcc-4.1.1`), then the
derivation with the highest *priority* is used. A derivation can
define a priority by declaring the `meta.priority` attribute. This
attribute should be a number, with a higher value denoting a lower
priority. The default priority is `5`.
If there are multiple matching derivations with the same priority,
then the derivation with the highest version will be installed.
You can force the installation of multiple derivations with the same
name by being specific about the versions. For instance, `nix-env --install
gcc-3.3.6 gcc-4.1.1` will install both version of GCC (and will
probably cause a user environment conflict\!).
- If [`--attr`](#opt-attr) / `-A` is specified, the arguments are *attribute paths* that select attributes from the [default Nix expression].
This is faster than using derivation names and unambiguous.
Show the attribute paths of available packages with [`nix-env --query`](./query.md):
```console
nix-env --query --available --attr-path
```
- If `--from-profile` *path* is given, *args* is a set of names
denoting installed [store paths] in the profile *path*. This is an
easy way to copy user environment elements from one profile to
another.
- If `--from-expression` is given, *args* are [Nix language functions](@docroot@/language/syntax.md#functions) that are called with the [default Nix expression] as their single argument.
The derivations returned by those function calls are installed.
This allows derivations to be specified in an unambiguous way, which is necessary if there are multiple derivations with the same name.
- If *args* are [store derivations](@docroot@/glossary.md#gloss-store-derivation), then these are [realised], and the resulting output paths are installed.
- If *args* are [store paths] that are not store derivations, then these are [realised] and installed.
- By default all [outputs](@docroot@/language/derivations.md#attr-outputs) are installed for each [derivation].
This can be overridden by adding a `meta.outputsToInstall` attribute on the derivation listing a subset of the output names.
Example:
The file `example.nix` defines a derivation with two outputs `foo` and `bar`, each containing a file.
```nix
# example.nix
let
pkgs = import <nixpkgs> {};
command = ''
${pkgs.coreutils}/bin/mkdir -p $foo $bar
echo foo > $foo/foo-file
echo bar > $bar/bar-file
'';
in
derivation {
name = "example";
builder = "${pkgs.bash}/bin/bash";
args = [ "-c" command ];
outputs = [ "foo" "bar" ];
system = builtins.currentSystem;
}
```
Installing from this Nix expression will make files from both outputs appear in the current profile.
```console
$ nix-env --install --file example.nix
installing 'example'
$ ls ~/.nix-profile
foo-file
bar-file
manifest.nix
```
Adding `meta.outputsToInstall` to that derivation will make `nix-env` only install files from the specified outputs.
```nix
# example-outputs.nix
import ./example.nix // { meta.outputsToInstall = [ "bar" ]; }
```
```console
$ nix-env --install --file example-outputs.nix
installing 'example'
$ ls ~/.nix-profile
bar-file
manifest.nix
```
# Options
- `--prebuilt-only` / `-b`
Use only derivations for which a substitute is registered, i.e.,
there is a pre-built binary available that can be downloaded in lieu
of building the derivation. Thus, no packages will be built from
source.
- `--preserve-installed` / `-P`
Do not remove derivations with a name matching one of the
derivations being installed. Usually, trying to have two versions of
the same package installed in the same generation of a profile will
lead to an error in building the generation, due to file name
clashes between the two versions. However, this is not the case for
all packages.
- `--remove-all` / `-r`
Remove all previously installed packages first. This is equivalent
to running `nix-env --uninstall '.*'` first, except that everything happens
in a single transaction.
{{#include ./opt-common.md}}
{{#include ../opt-common.md}}
{{#include ./env-common.md}}
{{#include ../env-common.md}}
# Examples
To install a package using a specific attribute path from the active Nix expression:
```console
$ nix-env --install --attr gcc40mips
installing `gcc-4.0.2'
$ nix-env --install --attr xorg.xorgserver
installing `xorg-server-1.2.0'
```
To install a specific version of `gcc` using the derivation name:
```console
$ nix-env --install gcc-3.3.2
installing `gcc-3.3.2'
uninstalling `gcc-3.1'
```
Using attribute path for selecting a package is preferred,
as it is much faster and there will not be multiple matches.
Note the previously installed version is removed, since
`--preserve-installed` was not specified.
To install an arbitrary version:
```console
$ nix-env --install gcc
installing `gcc-3.3.2'
```
To install all derivations in the Nix expression `foo.nix`:
```console
$ nix-env --file ~/foo.nix --install '.*'
```
To copy the store path with symbolic name `gcc` from another profile:
```console
$ nix-env --install --from-profile /nix/var/nix/profiles/foo gcc
```
To install a specific [store derivation] (typically created by
`nix-instantiate`):
```console
$ nix-env --install /nix/store/fibjb1bfbpm5mrsxc4mh2d8n37sxh91i-gcc-3.4.3.drv
```
To install a specific output path:
```console
$ nix-env --install /nix/store/y3cgx0xj1p4iv9x0pnnmdhr8iyg741vk-gcc-3.4.3
```
To install from a Nix expression specified on the command-line:
```console
$ nix-env --file ./foo.nix --install --expr \
'f: (f {system = "i686-linux";}).subversionWithJava'
```
I.e., this evaluates to `(f: (f {system =
"i686-linux";}).subversionWithJava) (import ./foo.nix)`, thus selecting
the `subversionWithJava` attribute from the set returned by calling the
function defined in `./foo.nix`.
A dry-run tells you which paths will be downloaded or built from source:
```console
$ nix-env --file '<nixpkgs>' --install --attr hello --dry-run
(dry run; not doing anything)
installing ‘hello-2.10’
this path will be fetched (0.04 MiB download, 0.19 MiB unpacked):
/nix/store/wkhdf9jinag5750mqlax6z2zbwhqb76n-hello-2.10
...
```
To install Firefox from the latest revision in the Nixpkgs/NixOS 14.12
channel:
```console
$ nix-env --file https://github.com/NixOS/nixpkgs/archive/nixos-14.12.tar.gz --install --attr firefox
```

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# Name
`nix-env --list-generations` - list profile generations
# Synopsis
`nix-env` `--list-generations`
# Description
This operation print a list of all the currently existing generations
for the active profile. These may be switched to using the
`--switch-generation` operation. It also prints the creation date of the
generation, and indicates the current generation.
{{#include ./opt-common.md}}
{{#include ../opt-common.md}}
{{#include ./env-common.md}}
{{#include ../env-common.md}}
# Examples
```console
$ nix-env --list-generations
95 2004-02-06 11:48:24
96 2004-02-06 11:49:01
97 2004-02-06 16:22:45
98 2004-02-06 16:24:33 (current)
```

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# Options
The following options are allowed for all `nix-env` operations, but may not always have an effect.
- `--file` / `-f` *path*
Specifies the Nix expression (designated below as the *active Nix
expression*) used by the `--install`, `--upgrade`, and `--query
--available` operations to obtain derivations. The default is
`~/.nix-defexpr`.
If the argument starts with `http://` or `https://`, it is
interpreted as the URL of a tarball that will be downloaded and
unpacked to a temporary location. The tarball must include a single
top-level directory containing at least a file named `default.nix`.
- `--profile` / `-p` *path*
Specifies the profile to be used by those operations that operate on
a profile (designated below as the *active profile*). A profile is a
sequence of user environments called *generations*, one of which is
the *current generation*.
- `--dry-run`
For the `--install`, `--upgrade`, `--uninstall`,
`--switch-generation`, `--delete-generations` and `--rollback`
operations, this flag will cause `nix-env` to print what *would* be
done if this flag had not been specified, without actually doing it.
`--dry-run` also prints out which paths will be
[substituted](@docroot@/glossary.md) (i.e., downloaded) and which paths
will be built from source (because no substitute is available).
- `--system-filter` *system*
By default, operations such as `--query --available` show derivations matching any platform. This option
allows you to use derivations for the specified platform *system*.

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# Name
`nix-env --query` - display information about packages
# Synopsis
`nix-env` {`--query` | `-q`} *names…*
[`--installed` | `--available` | `-a`]
[{`--status` | `-s`}]
[{`--attr-path` | `-P`}]
[`--no-name`]
[{`--compare-versions` | `-c`}]
[`--system`]
[`--drv-path`]
[`--out-path`]
[`--description`]
[`--meta`]
[`--xml`]
[`--json`]
[{`--prebuilt-only` | `-b`}]
[{`--attr` | `-A`} *attribute-path*]
# Description
The query operation displays information about either the store paths
that are installed in the current generation of the active profile
(`--installed`), or the derivations that are available for installation
in the active Nix expression (`--available`). It only prints information
about derivations whose symbolic name matches one of *names*.
The derivations are sorted by their `name` attributes.
# Source selection
The following flags specify the set of things on which the query
operates.
- `--installed`
The query operates on the store paths that are installed in the
current generation of the active profile. This is the default.
- `--available` / `-a`
The query operates on the derivations that are available in the
active Nix expression.
# Queries
The following flags specify what information to display about the
selected derivations. Multiple flags may be specified, in which case the
information is shown in the order given here. Note that the name of the
derivation is shown unless `--no-name` is specified.
- `--xml`
Print the result in an XML representation suitable for automatic
processing by other tools. The root element is called `items`, which
contains a `item` element for each available or installed
derivation. The fields discussed below are all stored in attributes
of the `item` elements.
- `--json`
Print the result in a JSON representation suitable for automatic
processing by other tools.
- `--prebuilt-only` / `-b`
Show only derivations for which a substitute is registered, i.e.,
there is a pre-built binary available that can be downloaded in lieu
of building the derivation. Thus, this shows all packages that
probably can be installed quickly.
- `--status` / `-s`
Print the *status* of the derivation. The status consists of three
characters. The first is `I` or `-`, indicating whether the
derivation is currently installed in the current generation of the
active profile. This is by definition the case for `--installed`,
but not for `--available`. The second is `P` or `-`, indicating
whether the derivation is present on the system. This indicates
whether installation of an available derivation will require the
derivation to be built. The third is `S` or `-`, indicating whether
a substitute is available for the derivation.
- `--attr-path` / `-P`
Print the *attribute path* of the derivation, which can be used to
unambiguously select it using the `--attr` option available in
commands that install derivations like `nix-env --install`. This
option only works together with `--available`
- `--no-name`
Suppress printing of the `name` attribute of each derivation.
- `--compare-versions` / `-c`
Compare installed versions to available versions, or vice versa (if
`--available` is given). This is useful for quickly seeing whether
upgrades for installed packages are available in a Nix expression. A
column is added with the following meaning:
- `<` *version*
A newer version of the package is available or installed.
- `=` *version*
At most the same version of the package is available or
installed.
- `>` *version*
Only older versions of the package are available or installed.
- `- ?`
No version of the package is available or installed.
- `--system`
Print the `system` attribute of the derivation.
- `--drv-path`
Print the path of the [store derivation](@docroot@/glossary.md#gloss-store-derivation).
- `--out-path`
Print the output path of the derivation.
- `--description`
Print a short (one-line) description of the derivation, if
available. The description is taken from the `meta.description`
attribute of the derivation.
- `--meta`
Print all of the meta-attributes of the derivation. This option is
only available with `--xml` or `--json`.
{{#include ./opt-common.md}}
{{#include ../opt-common.md}}
{{#include ./env-common.md}}
{{#include ../env-common.md}}
# Examples
To show installed packages:
```console
$ nix-env --query
bison-1.875c
docbook-xml-4.2
firefox-1.0.4
MPlayer-1.0pre7
ORBit2-2.8.3
…
```
To show available packages:
```console
$ nix-env --query --available
firefox-1.0.7
GConf-2.4.0.1
MPlayer-1.0pre7
ORBit2-2.8.3
…
```
To show the status of available packages:
```console
$ nix-env --query --available --status
-P- firefox-1.0.7 (not installed but present)
--S GConf-2.4.0.1 (not present, but there is a substitute for fast installation)
--S MPlayer-1.0pre3 (i.e., this is not the installed MPlayer, even though the version is the same!)
IP- ORBit2-2.8.3 (installed and by definition present)
…
```
To show available packages in the Nix expression `foo.nix`:
```console
$ nix-env --file ./foo.nix --query --available
foo-1.2.3
```
To compare installed versions to what’s available:
```console
$ nix-env --query --compare-versions
...
acrobat-reader-7.0 - ? (package is not available at all)
autoconf-2.59 = 2.59 (same version)
firefox-1.0.4 < 1.0.7 (a more recent version is available)
...
```
To show all packages with “`zip`” in the name:
```console
$ nix-env --query --available '.*zip.*'
bzip2-1.0.6
gzip-1.6
zip-3.0
…
```
To show all packages with “`firefox`” or “`chromium`” in the name:
```console
$ nix-env --query --available '.*(firefox|chromium).*'
chromium-37.0.2062.94
chromium-beta-38.0.2125.24
firefox-32.0.3
firefox-with-plugins-13.0.1
…
```
To show all packages in the latest revision of the Nixpkgs repository:
```console
$ nix-env --file https://github.com/NixOS/nixpkgs/archive/master.tar.gz --query --available
```

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# Name
`nix-env --rollback` - set user environment to previous generation
# Synopsis
`nix-env` `--rollback`
# Description
This operation switches to the “previous” generation of the active
profile, that is, the highest numbered generation lower than the current
generation, if it exists. It is just a convenience wrapper around
`--list-generations` and `--switch-generation`.
{{#include ./opt-common.md}}
{{#include ../opt-common.md}}
{{#include ./env-common.md}}
{{#include ../env-common.md}}
# Examples
```console
$ nix-env --rollback
switching from generation 92 to 91
```
```console
$ nix-env --rollback
error: no generation older than the current (91) exists
```

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# Name
`nix-env --set-flag` - modify meta attributes of installed packages
# Synopsis
`nix-env` `--set-flag` *name* *value* *drvnames*
# Description
The `--set-flag` operation allows meta attributes of installed packages
to be modified. There are several attributes that can be usefully
modified, because they affect the behaviour of `nix-env` or the user
environment build script:
- `priority` can be changed to resolve filename clashes. The user
environment build script uses the `meta.priority` attribute of
derivations to resolve filename collisions between packages. Lower
priority values denote a higher priority. For instance, the GCC
wrapper package and the Binutils package in Nixpkgs both have a file
`bin/ld`, so previously if you tried to install both you would get a
collision. Now, on the other hand, the GCC wrapper declares a higher
priority than Binutils, so the former’s `bin/ld` is symlinked in the
user environment.
- `keep` can be set to `true` to prevent the package from being
upgraded or replaced. This is useful if you want to hang on to an
older version of a package.
- `active` can be set to `false` to “disable” the package. That is, no
symlinks will be generated to the files of the package, but it
remains part of the profile (so it won’t be garbage-collected). It
can be set back to `true` to re-enable the package.
{{#include ./opt-common.md}}
{{#include ../opt-common.md}}
{{#include ../env-common.md}}
# Examples
To prevent the currently installed Firefox from being upgraded:
```console
$ nix-env --set-flag keep true firefox
```
After this, `nix-env --upgrade ` will ignore Firefox.
To disable the currently installed Firefox, then install a new Firefox
while the old remains part of the profile:
```console
$ nix-env --query
firefox-2.0.0.9 (the current one)
$ nix-env --preserve-installed --install firefox-2.0.0.11
installing `firefox-2.0.0.11'
building path(s) `/nix/store/myy0y59q3ig70dgq37jqwg1j0rsapzsl-user-environment'
collision between `/nix/store/...-firefox-2.0.0.11/bin/firefox'
and `/nix/store/...-firefox-2.0.0.9/bin/firefox'.
(i.e., can’t have two active at the same time)
$ nix-env --set-flag active false firefox
setting flag on `firefox-2.0.0.9'
$ nix-env --preserve-installed --install firefox-2.0.0.11
installing `firefox-2.0.0.11'
$ nix-env --query
firefox-2.0.0.11 (the enabled one)
firefox-2.0.0.9 (the disabled one)
```
To make files from `binutils` take precedence over files from `gcc`:
```console
$ nix-env --set-flag priority 5 binutils
$ nix-env --set-flag priority 10 gcc
```

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# Name
`nix-env --set` - set profile to contain a specified derivation
## Synopsis
`nix-env` `--set` *drvname*
## Description
The `--set` operation modifies the current generation of a profile so
that it contains exactly the specified derivation, and nothing else.
{{#include ./opt-common.md}}
{{#include ../opt-common.md}}
{{#include ./env-common.md}}
{{#include ../env-common.md}}
## Examples
The following updates a profile such that its current generation will
contain just Firefox:
```console
$ nix-env --profile /nix/var/nix/profiles/browser --set firefox
```

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# Name
`nix-env --switch-generation` - set user environment to given profile generation
# Synopsis
`nix-env` {`--switch-generation` | `-G`} *generation*
# Description
This operation makes generation number *generation* the current
generation of the active profile. That is, if the `profile` is the path
to the active profile, then the symlink `profile` is made to point to
`profile-generation-link`, which is in turn a symlink to the actual user
environment in the Nix store.
Switching will fail if the specified generation does not exist.
{{#include ./opt-common.md}}
{{#include ../opt-common.md}}
{{#include ./env-common.md}}
{{#include ../env-common.md}}
# Examples
```console
$ nix-env --switch-generation 42
switching from generation 50 to 42
```

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# Name
`nix-env --switch-profile` - set user environment to given profile
# Synopsis
`nix-env` {`--switch-profile` | `-S`} *path*
# Description
This operation makes *path* the current profile for the user. That is,
the symlink `~/.nix-profile` is made to point to *path*.
{{#include ./opt-common.md}}
{{#include ../opt-common.md}}
{{#include ./env-common.md}}
{{#include ../env-common.md}}
# Examples
```console
$ nix-env --switch-profile ~/my-profile
```

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# Name
`nix-env --uninstall` - remove packages from user environment
# Synopsis
`nix-env` {`--uninstall` | `-e`} *drvnames…*
# Description
The uninstall operation creates a new user environment, based on the
current generation of the active profile, from which the store paths
designated by the symbolic names *drvnames* are removed.
{{#include ./opt-common.md}}
{{#include ../opt-common.md}}
{{#include ./env-common.md}}
{{#include ../env-common.md}}
# Examples
```console
$ nix-env --uninstall gcc
$ nix-env --uninstall '.*' (remove everything)
```

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# Name
`nix-env --upgrade` - upgrade packages in user environment
# Synopsis
`nix-env` {`--upgrade` | `-u`} *args*
[`--lt` | `--leq` | `--eq` | `--always`]
[{`--prebuilt-only` | `-b`}]
[{`--attr` | `-A`}]
[`--from-expression`] [`-E`]
[`--from-profile` *path*]
[`--preserve-installed` | `-P`]
# Description
The upgrade operation creates a new user environment, based on the
current generation of the active profile, in which all store paths are
replaced for which there are newer versions in the set of paths
described by *args*. Paths for which there are no newer versions are
left untouched; this is not an error. It is also not an error if an
element of *args* matches no installed derivations.
For a description of how *args* is mapped to a set of store paths, see
[`--install`](#operation---install). If *args* describes multiple
store paths with the same symbolic name, only the one with the highest
version is installed.
# Flags
- `--lt`
Only upgrade a derivation to newer versions. This is the default.
- `--leq`
In addition to upgrading to newer versions, also “upgrade” to
derivations that have the same version. Version are not a unique
identification of a derivation, so there may be many derivations
that have the same version. This flag may be useful to force
“synchronisation” between the installed and available derivations.
- `--eq`
*Only* “upgrade” to derivations that have the same version. This may
not seem very useful, but it actually is, e.g., when there is a new
release of Nixpkgs and you want to replace installed applications
with the same versions built against newer dependencies (to reduce
the number of dependencies floating around on your system).
- `--always`
In addition to upgrading to newer versions, also “upgrade” to
derivations that have the same or a lower version. I.e., derivations
may actually be downgraded depending on what is available in the
active Nix expression.
- `--prebuilt-only` / `-b`
Use only derivations for which a substitute is registered, i.e.,
there is a pre-built binary available that can be downloaded in lieu
of building the derivation. Thus, no packages will be built from
source.
- `--preserve-installed` / `-P`
Do not remove derivations with a name matching one of the
derivations being installed. Usually, trying to have two versions of
the same package installed in the same generation of a profile will
lead to an error in building the generation, due to file name
clashes between the two versions. However, this is not the case for
all packages.
{{#include ./opt-common.md}}
{{#include ../opt-common.md}}
{{#include ./env-common.md}}
{{#include ../env-common.md}}
# Examples
```console
$ nix-env --upgrade --attr nixpkgs.gcc
upgrading `gcc-3.3.1' to `gcc-3.4'
```
When there are no updates available, nothing will happen:
```console
$ nix-env --upgrade --attr nixpkgs.pan
```
Using `-A` is preferred when possible, as it is faster and unambiguous but
it is also possible to upgrade to a specific version by matching the derivation name:
```console
$ nix-env --upgrade gcc-3.3.2 --always
upgrading `gcc-3.4' to `gcc-3.3.2'
```
To try to upgrade everything
(matching packages based on the part of the derivation name without version):
```console
$ nix-env --upgrade
upgrading `hello-2.1.2' to `hello-2.1.3'
upgrading `mozilla-1.2' to `mozilla-1.4'
```
# Versions
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. `y` is considered an upgrade of `x`
if their package names match, and the version of `y` is higher than that
of `x`.
The versions are compared by splitting them into contiguous components
of numbers and letters. E.g., `3.3.1pre5` is split into `[3, 3, 1,
"pre", 5]`. These lists are then compared lexicographically (from left
to right). Corresponding components `a` and `b` are compared as follows.
If they are both numbers, integer comparison is used. If `a` is an empty
string and `b` is a number, `a` is considered less than `b`. The special
string component `pre` (for *pre-release*) is considered to be less than
other components. String components are considered less than number
components. Otherwise, they are compared lexicographically (i.e., using
case-sensitive string comparison).
This is illustrated by the following examples:
1.0 < 2.3
2.1 < 2.3
2.3 = 2.3
2.5 > 2.3
3.1 > 2.3
2.3.1 > 2.3
2.3.1 > 2.3a
2.3pre1 < 2.3
2.3pre3 < 2.3pre12
2.3a < 2.3c
2.3pre1 < 2.3c
2.3pre1 < 2.3q

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# Name
`nix-hash` - compute the cryptographic hash of a path
# Synopsis
`nix-hash` [`--flat`] [`--base32`] [`--truncate`] [`--type` *hashAlgo*] *path…*
`nix-hash` [`--to-base16`|`--to-base32`|`--to-base64`|`--to-sri`] [`--type` *hashAlgo*] *hash…*
# Description
The command `nix-hash` computes the cryptographic hash of the contents
of each *path* and prints it on standard output. By default, it computes
an MD5 hash, but other hash algorithms are available as well. The hash
is printed in hexadecimal. To generate the same hash as
`nix-prefetch-url` you have to specify multiple arguments, see below for
an example.
The hash is computed over a *serialisation* of each path: a dump of
the file system tree rooted at the path. This allows directories and
symlinks to be hashed as well as regular files. The dump is in the
*[Nix Archive (NAR)][Nix Archive] format* produced by [`nix-store
--dump`](@docroot@/command-ref/nix-store/dump.md). Thus, `nix-hash path`
yields the same cryptographic hash as `nix-store --dump path |
md5sum`.
[Nix Archive]: @docroot@/store/file-system-object/content-address.md#serial-nix-archive
# Options
- `--flat`
Print the cryptographic hash of the contents of each regular file *path*.
That is, instead of computing
the hash of the [Nix Archive (NAR)](@docroot@/store/file-system-object/content-address.md#serial-nix-archive) of *path*,
just [directly hash]((@docroot@/store/file-system-object/content-address.md#serial-flat) *path* as is.
This requires *path* to resolve to a regular file rather than directory.
The result is identical to that produced by the GNU commands
`md5sum` and `sha1sum`.
- `--base16`
Print the hash in a hexadecimal representation (default).
- `--base32`
Print the hash in a base-32 representation rather than hexadecimal.
This base-32 representation is more compact and can be used in Nix
expressions (such as in calls to `fetchurl`).
- `--base64`
Similar to --base32, but print the hash in a base-64 representation,
which is more compact than the base-32 one.
- `--sri`
Print the hash in SRI format with base-64 encoding.
The type of hash algorithm will be prepended to the hash string,
followed by a hyphen (-) and the base-64 hash body.
- `--truncate`
Truncate hashes longer than 160 bits (such as SHA-256) to 160 bits.
- `--type` *hashAlgo*
Use the specified cryptographic hash algorithm, which can be one of
`md5`, `sha1`, `sha256`, and `sha512`.
- `--to-base16`
Don’t hash anything, but convert the base-32 hash representation
*hash* to hexadecimal.
- `--to-base32`
Don’t hash anything, but convert the hexadecimal hash representation
*hash* to base-32.
- `--to-base64`
Don’t hash anything, but convert the hexadecimal hash representation
*hash* to base-64.
- `--to-sri`
Don’t hash anything, but convert the hexadecimal hash representation
*hash* to SRI.
# Examples
Computing the same hash as `nix-prefetch-url`:
```console
$ nix-prefetch-url file://<(echo test)
1lkgqb6fclns49861dwk9rzb6xnfkxbpws74mxnx01z9qyv1pjpj
$ nix-hash --type sha256 --flat --base32 <(echo test)
1lkgqb6fclns49861dwk9rzb6xnfkxbpws74mxnx01z9qyv1pjpj
```
Computing hashes:
```console
$ mkdir test
$ echo "hello" > test/world
$ nix-hash test/ (MD5 hash; default)
8179d3caeff1869b5ba1744e5a245c04
$ nix-store --dump test/ | md5sum (for comparison)
8179d3caeff1869b5ba1744e5a245c04 -
$ nix-hash --type sha1 test/
e4fd8ba5f7bbeaea5ace89fe10255536cd60dab6
$ nix-hash --type sha1 --base16 test/
e4fd8ba5f7bbeaea5ace89fe10255536cd60dab6
$ nix-hash --type sha1 --base32 test/
nvd61k9nalji1zl9rrdfmsmvyyjqpzg4
$ nix-hash --type sha1 --base64 test/
5P2Lpfe76upazon+ECVVNs1g2rY=
$ nix-hash --type sha1 --sri test/
sha1-5P2Lpfe76upazon+ECVVNs1g2rY=
$ nix-hash --type sha256 --flat test/
error: reading file `test/': Is a directory
$ nix-hash --type sha256 --flat test/world
5891b5b522d5df086d0ff0b110fbd9d21bb4fc7163af34d08286a2e846f6be03
```
Converting between hexadecimal, base-32, base-64, and SRI:
```console
$ nix-hash --type sha1 --to-base32 e4fd8ba5f7bbeaea5ace89fe10255536cd60dab6
nvd61k9nalji1zl9rrdfmsmvyyjqpzg4
$ nix-hash --type sha1 --to-base16 nvd61k9nalji1zl9rrdfmsmvyyjqpzg4
e4fd8ba5f7bbeaea5ace89fe10255536cd60dab6
$ nix-hash --type sha1 --to-base64 e4fd8ba5f7bbeaea5ace89fe10255536cd60dab6
5P2Lpfe76upazon+ECVVNs1g2rY=
$ nix-hash --type sha1 --to-sri nvd61k9nalji1zl9rrdfmsmvyyjqpzg4
sha1-5P2Lpfe76upazon+ECVVNs1g2rY=
$ nix-hash --to-base16 sha1-5P2Lpfe76upazon+ECVVNs1g2rY=
e4fd8ba5f7bbeaea5ace89fe10255536cd60dab6
```

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# Name
`nix-instantiate` - instantiate store derivations from Nix expressions
# Synopsis
`nix-instantiate`
[`--parse` | `--eval` [`--strict`] [`--json`] [`--xml`] ]
[`--read-write-mode`]
[`--arg` *name* *value*]
[{`--attr`| `-A`} *attrPath*]
[`--add-root` *path*]
[`--expr` | `-E`]
*files…*
`nix-instantiate` `--find-file` *files…*
# Description
The command `nix-instantiate` produces [store derivation]s from (high-level) Nix expressions.
It evaluates the Nix expressions in each of *files* (which defaults to
*./default.nix*). Each top-level expression should evaluate to a
derivation, a list of derivations, or a set of derivations. The paths
of the resulting store derivations are printed on standard output.
[store derivation]: @docroot@/glossary.md#gloss-store-derivation
If *files* is the character `-`, then a Nix expression will be read from
standard input.
# Options
- `--add-root` *path*
See the [corresponding option](nix-store.md) in `nix-store`.
- `--parse`
Just parse the input files, and print their abstract syntax trees on
standard output as a Nix expression.
- `--eval`
Just parse and evaluate the input files, and print the resulting
values on standard output. No instantiation of store derivations
takes place.
> **Warning**
>
> This option produces output which can be parsed as a Nix expression which
> will produce a different result than the input expression when evaluated.
> For example, these two Nix expressions print the same result despite
> having different meaning:
>
> ```console
> $ nix-instantiate --eval --expr '{ a = {}; }'
> { a = <CODE>; }
> $ nix-instantiate --eval --expr '{ a = <CODE>; }'
> { a = <CODE>; }
> ```
>
> For human-readable output, `nix eval` (experimental) is more informative:
>
> ```console
> $ nix-instantiate --eval --expr 'a: a'
> <LAMBDA>
> $ nix eval --expr 'a: a'
> «lambda @ «string»:1:1»
> ```
>
> For machine-readable output, the `--xml` option produces unambiguous
> output:
>
> ```console
> $ nix-instantiate --eval --xml --expr '{ foo = <CODE>; }'
> <?xml version='1.0' encoding='utf-8'?>
> <expr>
> <attrs>
> <attr column="3" line="1" name="foo">
> <unevaluated />
> </attr>
> </attrs>
> </expr>
> ```
- `--find-file`
Look up the given files in Nix’s search path (as specified by the
`NIX_PATH` environment variable). If found, print the corresponding
absolute paths on standard output. For instance, if `NIX_PATH` is
`nixpkgs=/home/alice/nixpkgs`, then `nix-instantiate --find-file
nixpkgs/default.nix` will print `/home/alice/nixpkgs/default.nix`.
- `--strict`
When used with `--eval`, recursively evaluate list elements and
attributes. Normally, such sub-expressions are left unevaluated
(since the Nix language is lazy).
> **Warning**
>
> This option can cause non-termination, because lazy data
> structures can be infinitely large.
- `--json`
When used with `--eval`, print the resulting value as an JSON
representation of the abstract syntax tree rather than as a Nix expression.
- `--xml`
When used with `--eval`, print the resulting value as an XML
representation of the abstract syntax tree rather than as a Nix expression.
The schema is the same as that used by the [`toXML`
built-in](../language/builtins.md).
- `--read-write-mode`
When used with `--eval`, perform evaluation in read/write mode so
nix language features that require it will still work (at the cost
of needing to do instantiation of every evaluated derivation). If
this option is not enabled, there may be uninstantiated store paths
in the final output.
{{#include ./opt-common.md}}
{{#include ./env-common.md}}
# Examples
Instantiate [store derivation]s from a Nix expression, and build them using `nix-store`:
```console
$ nix-instantiate test.nix (instantiate)
/nix/store/cigxbmvy6dzix98dxxh9b6shg7ar5bvs-perl-BerkeleyDB-0.26.drv
$ nix-store --realise $(nix-instantiate test.nix) (build)
...
/nix/store/qhqk4n8ci095g3sdp93x7rgwyh9rdvgk-perl-BerkeleyDB-0.26 (output path)
$ ls -l /nix/store/qhqk4n8ci095g3sdp93x7rgwyh9rdvgk-perl-BerkeleyDB-0.26
dr-xr-xr-x 2 eelco users 4096 1970-01-01 01:00 lib
...
```
You can also give a Nix expression on the command line:
```console
$ nix-instantiate --expr 'with import <nixpkgs> { }; hello'
/nix/store/j8s4zyv75a724q38cb0r87rlczaiag4y-hello-2.8.drv
```
This is equivalent to:
```console
$ nix-instantiate '<nixpkgs>' --attr hello
```
Parsing and evaluating Nix expressions:
```console
$ nix-instantiate --parse --expr '1 + 2'
1 + 2
```
```console
$ nix-instantiate --eval --expr '1 + 2'
3
```
```console
$ nix-instantiate --eval --xml --expr '1 + 2'
<?xml version='1.0' encoding='utf-8'?>
<expr>
<int value="3" />
</expr>
```
The difference between non-strict and strict evaluation:
```console
$ nix-instantiate --eval --xml --expr '{ x = {}; }'
<?xml version='1.0' encoding='utf-8'?>
<expr>
<attrs>
<attr column="3" line="1" name="x">
<unevaluated />
</attr>
</attrs>
</expr>
$ nix-instantiate --eval --xml --strict --expr '{ x = {}; }'
<?xml version='1.0' encoding='utf-8'?>
<expr>
<attrs>
<attr column="3" line="1" name="x">
<attrs>
</attrs>
</attr>
</attrs>
</expr>
```

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# Name
`nix-prefetch-url` - copy a file from a URL into the store and print its hash
# Synopsis
`nix-prefetch-url` *url* [*hash*]
[`--type` *hashAlgo*]
[`--print-path`]
[`--unpack`]
[`--name` *name*]
# Description
The command `nix-prefetch-url` downloads the file referenced by the URL
*url*, prints its cryptographic hash, and copies it into the Nix store.
The file name in the store is `hash-baseName`, where *baseName* is
everything following the final slash in *url*.
This command is just a convenience for Nix expression writers. Often a
Nix expression fetches some source distribution from the network using
the `fetchurl` expression contained in Nixpkgs. However, `fetchurl`
requires a cryptographic hash. If you don't know the hash, you would
have to download the file first, and then `fetchurl` would download it
again when you build your Nix expression. Since `fetchurl` uses the same
name for the downloaded file as `nix-prefetch-url`, the redundant
download can be avoided.
If *hash* is specified, then a download is not performed if the Nix
store already contains a file with the same hash and base name.
Otherwise, the file is downloaded, and an error is signaled if the
actual hash of the file does not match the specified hash.
This command prints the hash on standard output.
The hash is printed using base-32 unless `--type md5` is specified,
in which case it's printed using base-16.
Additionally, if the option `--print-path` is used,
the path of the downloaded file in the Nix store is also printed.
# Options
- `--type` *hashAlgo*
Use the specified cryptographic hash algorithm,
which can be one of `md5`, `sha1`, `sha256`, and `sha512`.
The default is `sha256`.
- `--print-path`
Print the store path of the downloaded file on standard output.
- `--unpack`
Unpack the archive (which must be a tarball or zip file) and add the
result to the Nix store. The resulting hash can be used with
functions such as Nixpkgs’s `fetchzip` or `fetchFromGitHub`.
- `--executable`
Set the executable bit on the downloaded file.
- `--name` *name*
Override the name of the file in the Nix store. By default, this is
`hash-basename`, where *basename* is the last component of *url*.
Overriding the name is necessary when *basename* contains characters
that are not allowed in Nix store paths.
# Examples
```console
$ nix-prefetch-url ftp://ftp.gnu.org/pub/gnu/hello/hello-2.10.tar.gz
0ssi1wpaf7plaswqqjwigppsg5fyh99vdlb9kzl7c9lng89ndq1i
```
```console
$ nix-prefetch-url --print-path mirror://gnu/hello/hello-2.10.tar.gz
0ssi1wpaf7plaswqqjwigppsg5fyh99vdlb9kzl7c9lng89ndq1i
/nix/store/3x7dwzq014bblazs7kq20p9hyzz0qh8g-hello-2.10.tar.gz
```
```console
$ nix-prefetch-url --unpack --print-path https://github.com/NixOS/patchelf/archive/0.8.tar.gz
079agjlv0hrv7fxnx9ngipx14gyncbkllxrp9cccnh3a50fxcmy7
/nix/store/19zrmhm3m40xxaw81c8cqm6aljgrnwj2-0.8.tar.gz
```

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# Name
`nix-shell` - start an interactive shell based on a Nix expression
# Synopsis
`nix-shell`
[`--arg` *name* *value*]
[`--argstr` *name* *value*]
[{`--attr` | `-A`} *attrPath*]
[`--command` *cmd*]
[`--run` *cmd*]
[`--exclude` *regexp*]
[`--pure`]
[`--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
derivation, but not the derivation itself. It will then start an
interactive shell in which all environment variables defined by the
derivation *path* have been set to their corresponding values, and the
script `$stdenv/setup` has been sourced. This is useful for reproducing
the environment of a derivation for development.
If *path* is not given, `nix-shell` defaults to `shell.nix` if it
exists, and `default.nix` otherwise.
If *path* starts with `http://` or `https://`, it is interpreted as the
URL of a tarball that will be downloaded and unpacked to a temporary
location. The tarball must include a single top-level directory
containing at least a file named `default.nix`.
If the derivation defines the variable `shellHook`, it will be run
after `$stdenv/setup` has been sourced. Since this hook is not executed
by regular Nix builds, it allows you to perform initialisation specific
to `nix-shell`. For example, the derivation attribute
```nix
shellHook =
''
echo "Hello shell"
export SOME_API_TOKEN="$(cat ~/.config/some-app/api-token)"
'';
```
will cause `nix-shell` to print `Hello shell` and set the `SOME_API_TOKEN`
environment variable to a user-configured value.
# Options
All options not listed here are passed to `nix-store
--realise`, except for `--arg` and `--attr` / `-A` which are passed to
`nix-instantiate`.
- `--command` *cmd*
In the environment of the derivation, run the shell command *cmd*.
This command is executed in an interactive shell. (Use `--run` to
use a non-interactive shell instead.) However, a call to `exit` is
implicitly added to the command, so the shell will exit after
running the command. To prevent this, add `return` at the end;
e.g. `--command "echo Hello; return"` will print `Hello` and then
drop you into the interactive shell. This can be useful for doing
any additional initialisation.
- `--run` *cmd*
Like `--command`, but executes the command in a non-interactive
shell. This means (among other things) that if you hit Ctrl-C while
the command is running, the shell exits.
- `--exclude` *regexp*
Do not build any dependencies whose store path matches the regular
expression *regexp*. This option may be specified multiple times.
- `--pure`
If this flag is specified, the environment is almost entirely
cleared before the interactive shell is started, so you get an
environment that more closely corresponds to the “real” Nix build. A
few variables, in particular `HOME`, `USER` and `DISPLAY`, are
retained.
- `--packages` / `-p` *packages*…
Set up an environment in which the specified packages are present.
The command line arguments are interpreted as attribute names inside
the Nix Packages collection. Thus, `nix-shell --packages libjpeg openjdk`
will start a shell in which the packages denoted by the attribute
names `libjpeg` and `openjdk` are present.
- `-i` *interpreter*
The chained script interpreter to be invoked by `nix-shell`. Only
applicable in `#!`-scripts (described below).
- `--keep` *name*
When a `--pure` shell is started, keep the listed environment
variables.
{{#include ./opt-common.md}}
# Environment variables
- `NIX_BUILD_SHELL`
Shell used to start the interactive environment. Defaults to the
`bash` found in `<nixpkgs>`, falling back to the `bash` found in
`PATH` if not found.
{{#include ./env-common.md}}
# Examples
To build the dependencies of the package Pan, and start an interactive
shell in which to build it:
```console
$ nix-shell '<nixpkgs>' --attr pan
[nix-shell]$ eval ${unpackPhase:-unpackPhase}
[nix-shell]$ cd $sourceRoot
[nix-shell]$ eval ${patchPhase:-patchPhase}
[nix-shell]$ eval ${configurePhase:-configurePhase}
[nix-shell]$ eval ${buildPhase:-buildPhase}
[nix-shell]$ ./pan/gui/pan
```
The reason we use form `eval ${configurePhase:-configurePhase}` here is because
those packages that override these phases do so by exporting the overridden
values in the environment variable of the same name.
Here bash is being told to either evaluate the contents of 'configurePhase',
if it exists as a variable, otherwise evaluate the configurePhase function.
To clear the environment first, and do some additional automatic
initialisation of the interactive shell:
```console
$ nix-shell '<nixpkgs>' --attr pan --pure \
--command 'export NIX_DEBUG=1; export NIX_CORES=8; return'
```
Nix expressions can also be given on the command line using the `-E` and
`-p` flags. For instance, the following starts a shell containing the
packages `sqlite` and `libX11`:
```console
$ nix-shell --expr 'with import <nixpkgs> { }; runCommand "dummy" { buildInputs = [ sqlite xorg.libX11 ]; } ""'
```
A shorter way to do the same is:
```console
$ nix-shell --packages sqlite xorg.libX11
[nix-shell]$ echo $NIX_LDFLAGS
… -L/nix/store/j1zg5v…-sqlite-3.8.0.2/lib -L/nix/store/0gmcz9…-libX11-1.6.1/lib …
```
Note that `-p` accepts multiple full nix expressions that are valid in
the `buildInputs = [ ... ]` shown above, not only package names. So the
following is also legal:
```console
$ nix-shell --packages sqlite 'git.override { withManual = false; }'
```
The `-p` flag looks up Nixpkgs in the Nix search path. You can override
it by passing `-I` or setting `NIX_PATH`. For example, the following
gives you a shell containing the Pan package from a specific revision of
Nixpkgs:
```console
$ nix-shell --packages pan -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/8a3eea054838b55aca962c3fbde9c83c102b8bf2.tar.gz
[nix-shell:~]$ pan --version
Pan 0.139
```
# Use as a `#!`-interpreter
You can use `nix-shell` as a script interpreter to allow scripts written
in arbitrary languages to obtain their own dependencies via Nix. This is
done by starting the script with the following lines:
```bash
#! /usr/bin/env nix-shell
#! nix-shell -i real-interpreter --packages packages
```
where *real-interpreter* is the “real” script interpreter that will be
invoked by `nix-shell` after it has obtained the dependencies and
initialised the environment, and *packages* are the attribute names of
the dependencies in Nixpkgs.
The lines starting with `#! nix-shell` specify `nix-shell` options (see
above). Note that you cannot write `#! /usr/bin/env nix-shell -i ...`
because many operating systems only allow one argument in `#!` lines.
For example, here is a Python script that depends on Python and the
`prettytable` package:
```python
#! /usr/bin/env nix-shell
#! nix-shell -i python3 --packages python3 python3Packages.prettytable
import prettytable
# Print a simple table.
t = prettytable.PrettyTable(["N", "N^2"])
for n in range(1, 10): t.add_row([n, n * n])
print(t)
```
Similarly, the following is a Perl script that specifies that it
requires Perl and the `HTML::TokeParser::Simple` and `LWP` packages:
```perl
#! /usr/bin/env nix-shell
#! nix-shell -i perl --packages perl perlPackages.HTMLTokeParserSimple perlPackages.LWP
use HTML::TokeParser::Simple;
# Fetch nixos.org and print all hrefs.
my $p = HTML::TokeParser::Simple->new(url => 'http://nixos.org/');
while (my $token = $p->get_tag("a")) {
my $href = $token->get_attr("href");
print "$href\n" if $href;
}
```
Sometimes you need to pass a simple Nix expression to customize a
package like Terraform:
```bash
#! /usr/bin/env nix-shell
#! nix-shell -i bash --packages 'terraform.withPlugins (plugins: [ plugins.openstack ])'
terraform apply
```
> **Note**
>
> You must use single or double quotes (`'`, `"`) when passing a simple Nix expression
> in a nix-shell shebang.
Finally, using the merging of multiple nix-shell shebangs the following
Haskell script uses a specific branch of Nixpkgs/NixOS (the 20.03 stable
branch):
```haskell
#! /usr/bin/env nix-shell
#! nix-shell -i runghc --packages 'haskellPackages.ghcWithPackages (ps: [ps.download-curl ps.tagsoup])'
#! nix-shell -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/nixos-20.03.tar.gz
import Network.Curl.Download
import Text.HTML.TagSoup
import Data.Either
import Data.ByteString.Char8 (unpack)
-- Fetch nixos.org and print all hrefs.
main = do
resp <- openURI "https://nixos.org/"
let tags = filter (isTagOpenName "a") $ parseTags $ unpack $ fromRight undefined resp
let tags' = map (fromAttrib "href") tags
mapM_ putStrLn $ filter (/= "") tags'
```
If you want to be even more precise, you can specify a specific revision
of Nixpkgs:
#! nix-shell -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/0672315759b3e15e2121365f067c1c8c56bb4722.tar.gz
The examples above all used `-p` to get dependencies from Nixpkgs. You
can also use a Nix expression to build your own dependencies. For
example, the Python example could have been written as:
```python
#! /usr/bin/env nix-shell
#! nix-shell deps.nix -i python
```
where the file `deps.nix` in the same directory as the `#!`-script
contains:
```nix
with import <nixpkgs> {};
runCommand "dummy" { buildInputs = [ python pythonPackages.prettytable ]; } ""
```
The script's file name is passed as the first argument to the interpreter specified by the `-i` flag.
Aside from the very first line, which is a directive to the operating system, the additional `#! nix-shell` lines do not need to be at the beginning of the file.
This allows wrapping them in block comments for languages where `#` does not start a comment, such as ECMAScript, Erlang, PHP, or Ruby.

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# Name
`nix-store` - manipulate or query the Nix store
# Synopsis
`nix-store` *operation* [*options…*] [*arguments…*]
[`--option` *name* *value*]
[`--add-root` *path*]
# Description
The command `nix-store` performs primitive operations on the Nix store.
You generally do not need to run this command manually.
`nix-store` takes exactly one *operation* flag which indicates the subcommand to be performed. The following operations are available:
- [`--realise`](./nix-store/realise.md)
- [`--serve`](./nix-store/serve.md)
- [`--gc`](./nix-store/gc.md)
- [`--delete`](./nix-store/delete.md)
- [`--query`](./nix-store/query.md)
- [`--add`](./nix-store/add.md)
- [`--add-fixed`](./nix-store/add-fixed.md)
- [`--verify`](./nix-store/verify.md)
- [`--verify-path`](./nix-store/verify-path.md)
- [`--repair-path`](./nix-store/repair-path.md)
- [`--dump`](./nix-store/dump.md)
- [`--restore`](./nix-store/restore.md)
- [`--export`](./nix-store/export.md)
- [`--import`](./nix-store/import.md)
- [`--optimise`](./nix-store/optimise.md)
- [`--read-log`](./nix-store/read-log.md)
- [`--dump-db`](./nix-store/dump-db.md)
- [`--load-db`](./nix-store/load-db.md)
- [`--print-env`](./nix-store/print-env.md)
- [`--generate-binary-cache-key`](./nix-store/generate-binary-cache-key.md)
These pages can be viewed offline:
- `man nix-store-<operation>`.
Example: `man nix-store-realise`
- `nix-store --help --<operation>`
Example: `nix-store --help --realise`

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# Name
`nix-store --add-fixed` - add paths to store using given hashing algorithm
## Synopsis
`nix-store` `--add-fixed` [`--recursive`] *algorithm* *paths…*
## Description
The operation `--add-fixed` adds the specified paths to the Nix store.
Unlike `--add` paths are registered using the specified hashing
algorithm, resulting in the same output path as a fixed-output
derivation. This can be used for sources that are not available from a
public url or broke since the download expression was written.
This operation has the following options:
- `--recursive`
Use recursive instead of flat hashing mode, used when adding
directories to the store.
{{#include ./opt-common.md}}
{{#include ../opt-common.md}}
{{#include ../env-common.md}}
## Example
```console
$ nix-store --add-fixed sha256 ./hello-2.10.tar.gz
/nix/store/3x7dwzq014bblazs7kq20p9hyzz0qh8g-hello-2.10.tar.gz
```

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# Name
`nix-store --add` - add paths to Nix store
# Synopsis
`nix-store` `--add` *paths…*
# Description
The operation `--add` adds the specified paths to the Nix store. It
prints the resulting paths in the Nix store on standard output.
{{#include ./opt-common.md}}
{{#include ../opt-common.md}}
{{#include ../env-common.md}}
# Example
```console
$ nix-store --add ./foo.c
/nix/store/m7lrha58ph6rcnv109yzx1nk1cj7k7zf-foo.c
```

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# Name
`nix-store --delete` - delete store paths
# Synopsis
`nix-store` `--delete` [`--ignore-liveness`] *paths…*
# Description
The operation `--delete` deletes the store paths *paths* from the Nix
store, but only if it is safe to do so; that is, when the path is not
reachable from a root of the garbage collector. This means that you can
only delete paths that would also be deleted by `nix-store --gc`. Thus,
`--delete` is a more targeted version of `--gc`.
With the option `--ignore-liveness`, reachability from the roots is
ignored. However, the path still won’t be deleted if there are other
paths in the store that refer to it (i.e., depend on it).
{{#include ./opt-common.md}}
{{#include ../opt-common.md}}
{{#include ../env-common.md}}
# Example
```console
$ nix-store --delete /nix/store/zq0h41l75vlb4z45kzgjjmsjxvcv1qk7-mesa-6.4
0 bytes freed (0.00 MiB)
error: cannot delete path `/nix/store/zq0h41l75vlb4z45kzgjjmsjxvcv1qk7-mesa-6.4' since it is still alive
```

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# Name
`nix-store --dump-db` - export Nix database
# Synopsis
`nix-store` `--dump-db` [*paths…*]
# Description
The operation `--dump-db` writes a dump of the Nix database to standard
output. It can be loaded into an empty Nix store using `--load-db`. This
is useful for making backups and when migrating to different database
schemas.
By default, `--dump-db` will dump the entire Nix database. When one or
more store paths is passed, only the subset of the Nix database for
those store paths is dumped. As with `--export`, the user is responsible
for passing all the store paths for a closure. See `--export` for an
example.
{{#include ./opt-common.md}}
{{#include ../opt-common.md}}
{{#include ../env-common.md}}

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# Name
`nix-store --dump` - write a single path to a [Nix Archive]
## Synopsis
`nix-store` `--dump` *path*
## Description
The operation `--dump` produces a [Nix archive](@docroot@/glossary.md#gloss-nar) (NAR) file containing the
contents of the file system tree rooted at *path*. The archive is
written to standard output.
A NAR archive is like a TAR or Zip archive, but it contains only the
information that Nix considers important. For instance, timestamps are
elided because all files in the Nix store have their timestamp set to 0
anyway. Likewise, all permissions are left out except for the execute
bit, because all files in the Nix store have 444 or 555 permission.
Also, a NAR archive is *canonical*, meaning that “equal” paths always
produce the same NAR archive. For instance, directory entries are
always sorted so that the actual on-disk order doesn’t influence the
result. This means that the cryptographic hash of a NAR dump of a
path is usable as a fingerprint of the contents of the path. Indeed,
the hashes of store paths stored in Nix’s database (see `nix-store --query
--hash`) are SHA-256 hashes of the NAR dump of each store path.
NAR archives support filenames of unlimited length and 64-bit file
sizes. They can contain regular files, directories, and symbolic links,
but not other types of files (such as device nodes).
A Nix archive can be unpacked using [`nix-store --restore`](@docroot@/command-ref/nix-store/restore.md).
[Nix Archive]: @docroot@/store/file-system-object/content-address.md#serial-nix-archive
{{#include ./opt-common.md}}
{{#include ../opt-common.md}}
{{#include ../env-common.md}}

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# Name
`nix-store --export` - export store paths to a [Nix Archive]
## Synopsis
`nix-store` `--export` *paths…*
## Description
The operation `--export` writes a serialisation of the given [store objects](@docroot@/glossary.md#gloss-store-object) to standard output in a format that can be imported into another [Nix store](@docroot@/store/index.md) with [`nix-store --import`](./import.md).
> **Warning**
>
> This command *does not* produce a [closure](@docroot@/glossary.md#gloss-closure) of the specified store paths.
> Trying to import a store object that refers to store paths not available in the target Nix store will fail.
>
> Use [`nix-store --query`](@docroot@/command-ref/nix-store/query.md) to obtain the closure of a store path.
This command is different from [`nix-store --dump`](./dump.md), which produces a [Nix archive](@docroot@/glossary.md#gloss-nar) that *does not* contain the set of [references](@docroot@/glossary.md#gloss-reference) of a given store path.
> **Note**
>
> For efficient transfer of closures to remote machines over SSH, use [`nix-copy-closure`](@docroot@/command-ref/nix-copy-closure.md).
[Nix Archive]: @docroot@/store/file-system-object/content-address.md#serial-nix-archive
{{#include ./opt-common.md}}
{{#include ../opt-common.md}}
{{#include ../env-common.md}}
# Examples
> **Example**
>
> Deploy GNU Hello to an airgapped machine via USB stick.
>
> Write the closure to the block device on a machine with internet connection:
>
> ```shell-session
> [alice@itchy]$ storePath=$(nix-build '<nixpkgs>' -I nixpkgs=channel:nixpkgs-unstable -A hello --no-out-link)
> [alice@itchy]$ nix-store --export $(nix-store --query --requisites $storePath) | sudo dd of=/dev/usb
> ```
>
> Read the closure from the block device on the machine without internet connection:
>
> ```shell-session
> [bob@scratchy]$ hello=$(sudo dd if=/dev/usb | nix-store --import | tail -1)
> [bob@scratchy]$ $hello/bin/hello
> Hello, world!
> ```

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# Name
`nix-store --gc` - run garbage collection
# Synopsis
`nix-store` `--gc` [`--print-roots` | `--print-live` | `--print-dead`] [`--max-freed` *bytes*]
# Description
Without additional flags, the operation `--gc` performs a garbage
collection on the Nix store. That is, all paths in the Nix store not
reachable via file system references from a set of “roots”, are deleted.
The following suboperations may be specified:
- `--print-roots`
This operation prints on standard output the set of roots used by
the garbage collector.
- `--print-live`
This operation prints on standard output the set of “live” store
paths, which are all the store paths reachable from the roots. Live
paths should never be deleted, since that would break consistency —
it would become possible that applications are installed that
reference things that are no longer present in the store.
- `--print-dead`
This operation prints out on standard output the set of “dead” store
paths, which is just the opposite of the set of live paths: any path
in the store that is not live (with respect to the roots) is dead.
By default, all unreachable paths are deleted. The following options
control what gets deleted and in what order:
- `--max-freed` *bytes*
Keep deleting paths until at least *bytes* bytes have been deleted,
then stop. The argument *bytes* can be followed by the
multiplicative suffix `K`, `M`, `G` or `T`, denoting KiB, MiB, GiB
or TiB units.
The behaviour of the collector is also influenced by the
`keep-outputs` and `keep-derivations` settings in the Nix
configuration file.
By default, the collector prints the total number of freed bytes when it
finishes (or when it is interrupted). With `--print-dead`, it prints the
number of bytes that would be freed.
{{#include ./opt-common.md}}
{{#include ../opt-common.md}}
{{#include ../env-common.md}}
# Examples
To delete all unreachable paths, just do:
```console
$ nix-store --gc
deleting `/nix/store/kq82idx6g0nyzsp2s14gfsc38npai7lf-cairo-1.0.4.tar.gz.drv'
...
8825586 bytes freed (8.42 MiB)
```
To delete at least 100 MiBs of unreachable paths:
```console
$ nix-store --gc --max-freed $((100 * 1024 * 1024))
```

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# Name
`nix-store --generate-binary-cache-key` - generate key pair to use for a binary cache
## Synopsis
`nix-store` `--generate-binary-cache-key` *key-name* *secret-key-file* *public-key-file*
## Description
This command generates an [Ed25519 key pair](http://ed25519.cr.yp.to/)
that can be used to create a signed binary cache. It takes three
mandatory parameters:
1. A key name, such as `cache.example.org-1`, that is used to look up
keys on the client when it verifies signatures. It can be anything,
but it’s suggested to use the host name of your cache (e.g.
`cache.example.org`) with a suffix denoting the number of the key
(to be incremented every time you need to revoke a key).
2. The file name where the secret key is to be stored.
3. The file name where the public key is to be stored.
{{#include ./opt-common.md}}
{{#include ../opt-common.md}}
{{#include ../env-common.md}}

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# Name
`nix-store --import` - import [Nix Archive] into the store
[Nix Archive]: @docroot@/store/file-system-object/content-address.md#serial-nix-archive
# Synopsis
`nix-store` `--import`
# Description
The operation `--import` reads a serialisation of a set of [store objects](@docroot@/glossary.md#gloss-store-object) produced by [`nix-store --export`](./export.md) from standard input, and adds those store objects to the specified [Nix store](@docroot@/store/index.md).
Paths that already exist in the target Nix store are ignored.
If a path [refers](@docroot@/glossary.md#gloss-reference) to another path that doesn’t exist in the target Nix store, the import fails.
> **Note**
>
> For efficient transfer of closures to remote machines over SSH, use [`nix-copy-closure`](@docroot@/command-ref/nix-copy-closure.md).
{{#include ./opt-common.md}}
{{#include ../opt-common.md}}
{{#include ../env-common.md}}
# Examples
> **Example**
>
> Given a closure of GNU Hello as a file:
>
> ```shell-session
> $ storePath="$(nix-build '<nixpkgs>' -I nixpkgs=channel:nixpkgs-unstable -A hello --no-out-link)"
> $ nix-store --export $(nix-store --query --requisites $storePath) > hello.closure
> ```
>
> Import the closure into a [remote SSH store](@docroot@/store/types/ssh-store.md) using the [`--store`](@docroot@/command-ref/conf-file.md#conf-store) option:
>
> ```console
> $ nix-store --import --store ssh://alice@itchy.example.org < hello.closure
> ```

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# Name
`nix-store --load-db` - import Nix database
# Synopsis
`nix-store` `--load-db`
# Description
The operation `--load-db` reads a dump of the Nix database created by
`--dump-db` from standard input and loads it into the Nix database.
{{#include ./opt-common.md}}
{{#include ../opt-common.md}}
{{#include ../env-common.md}}

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# Options
The following options are allowed for all `nix-store` operations, but may not always have an effect.
- <span id="opt-add-root">[`--add-root`](#opt-add-root)</span> *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
store path. In addition, a uniquely named symlink to *path* will
be created in `/nix/var/nix/gcroots/auto/`. For instance,
```console
$ nix-store --add-root /home/eelco/bla/result --realise ...
$ ls -l /nix/var/nix/gcroots/auto
lrwxrwxrwx 1 ... 2005-03-13 21:10 dn54lcypm8f8... -> /home/eelco/bla/result
$ ls -l /home/eelco/bla/result
lrwxrwxrwx 1 ... 2005-03-13 21:10 /home/eelco/bla/result -> /nix/store/1r11343n6qd4...-f-spot-0.0.10
```
Thus, when `/home/eelco/bla/result` is removed, the GC root in the
`auto` directory becomes a dangling symlink and will be ignored by
the collector.
> **Warning**
>
> Note that it is not possible to move or rename GC roots, since
> the symlink in the `auto` directory will still point to the old
> location.
If there are multiple results, then multiple symlinks will be
created by sequentially numbering symlinks beyond the first one
(e.g., `foo`, `foo-2`, `foo-3`, and so on).

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# Name
`nix-store --optimise` - reduce disk space usage
## Synopsis
`nix-store` `--optimise`
## Description
The operation `--optimise` reduces Nix store disk space usage by finding
identical files in the store and hard-linking them to each other. It
typically reduces the size of the store by something like 25-35%. Only
regular files and symlinks are hard-linked in this manner. Files are
considered identical when they have the same [Nix Archive (NAR)][Nix Archive] serialisation:
that is, regular files must have the same contents and permission
(executable or non-executable), and symlinks must have the same
contents.
After completion, or when the command is interrupted, a report on the
achieved savings is printed on standard error.
Use `-vv` or `-vvv` to get some progress indication.
{{#include ./opt-common.md}}
{{#include ../opt-common.md}}
{{#include ../env-common.md}}
## Example
```console
$ nix-store --optimise
hashing files in `/nix/store/qhqx7l2f1kmwihc9bnxs7rc159hsxnf3-gcc-4.1.1'
...
541838819 bytes (516.74 MiB) freed by hard-linking 54143 files;
there are 114486 files with equal contents out of 215894 files in total
```
[Nix Archive]: @docroot@/store/file-system-object/content-address.md#serial-nix-archive

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# Name
`nix-store --print-env` - print the build environment of a derivation
## Synopsis
`nix-store` `--print-env` *drvpath*
## Description
The operation `--print-env` prints out the environment of a derivation
in a format that can be evaluated by a shell. The command line arguments
of the builder are placed in the variable `_args`.
{{#include ./opt-common.md}}
{{#include ../opt-common.md}}
{{#include ../env-common.md}}
## Example
```console
$ nix-store --print-env $(nix-instantiate '<nixpkgs>' -A firefox)
…
export src; src='/nix/store/plpj7qrwcz94z2psh6fchsi7s8yihc7k-firefox-12.0.source.tar.bz2'
export stdenv; stdenv='/nix/store/7c8asx3yfrg5dg1gzhzyq2236zfgibnn-stdenv'
export system; system='x86_64-linux'
export _args; _args='-e /nix/store/9krlzvny65gdc8s7kpb6lkx8cd02c25c-default-builder.sh'
```

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# Name
`nix-store --query` - display information about store paths
# Synopsis
`nix-store` {`--query` | `-q`}
{`--outputs` | `--requisites` | `-R` | `--references` | `--referrers` |
`--referrers-closure` | `--deriver` | `-d` | `--valid-derivers` |
`--graph` | `--tree` | `--binding` *name* | `-b` *name* | `--hash` |
`--size` | `--roots`}
[`--use-output`] [`-u`] [`--force-realise`] [`-f`]
*paths…*
# Description
The operation `--query` displays various bits of information about the
store paths . The queries are described below. At most one query can be
specified. The default query is `--outputs`.
The paths *paths* may also be symlinks from outside of the Nix store, to
the Nix store. In that case, the query is applied to the target of the
symlink.
# Common query options
- `--use-output` / `-u`
For each argument to the query that is a [store derivation], apply the
query to the output path of the derivation instead.
- `--force-realise` / `-f`
Realise each argument to the query first (see [`nix-store --realise`](./realise.md)).
[store derivation]: @docroot@/glossary.md#gloss-store-derivation
# Queries
- `--outputs`
Prints out the [output paths] of the store
derivations *paths*. These are the paths that will be produced when
the derivation is built.
[output paths]: @docroot@/glossary.md#gloss-output-path
- `--requisites` / `-R`
Prints out the [closure] of the store path *paths*.
[closure]: @docroot@/glossary.md#gloss-closure
This query has one option:
- `--include-outputs`
Also include the existing output paths of [store derivation]s,
and their closures.
This query can be used to implement various kinds of deployment. A
*source deployment* is obtained by distributing the closure of a
store derivation. A *binary deployment* is obtained by distributing
the closure of an output path. A *cache deployment* (combined
source/binary deployment, including binaries of build-time-only
dependencies) is obtained by distributing the closure of a store
derivation and specifying the option `--include-outputs`.
- `--references`
Prints the set of [references] of the store paths
*paths*, that is, their immediate dependencies. (For *all*
dependencies, use `--requisites`.)
[references]: @docroot@/glossary.md#gloss-reference
- `--referrers`
Prints the set of *referrers* of the store paths *paths*, that is,
the store paths currently existing in the Nix store that refer to
one of *paths*. Note that contrary to the references, the set of
referrers is not constant; it can change as store paths are added or
removed.
- `--referrers-closure`
Prints the closure of the set of store paths *paths* under the
referrers relation; that is, all store paths that directly or
indirectly refer to one of *paths*. These are all the path currently
in the Nix store that are dependent on *paths*.
- `--deriver` / `-d`
Prints the [deriver] that was used to build the store paths *paths*. If
the path has no deriver (e.g., if it is a source file), or if the
deriver is not known (e.g., in the case of a binary-only
deployment), the string `unknown-deriver` is printed.
The returned deriver is not guaranteed to exist in the local store, for
example when *paths* were substituted from a binary cache.
Use `--valid-derivers` instead to obtain valid paths only.
[deriver]: @docroot@/glossary.md#gloss-deriver
- `--valid-derivers`
Prints a set of derivation files (`.drv`) which are supposed produce
said paths when realized. Might print nothing, for example for source paths
or paths substituted from a binary cache.
- `--graph`
Prints the references graph of the store paths *paths* in the format
of the `dot` tool of AT\&T's [Graphviz
package](http://www.graphviz.org/). This can be used to visualise
dependency graphs. To obtain a build-time dependency graph, apply
this to a store derivation. To obtain a runtime dependency graph,
apply it to an output path.
- `--tree`
Prints the references graph of the store paths *paths* as a nested
ASCII tree. References are ordered by descending closure size; this
tends to flatten the tree, making it more readable. The query only
recurses into a store path when it is first encountered; this
prevents a blowup of the tree representation of the graph.
- `--graphml`
Prints the references graph of the store paths *paths* in the
[GraphML](http://graphml.graphdrawing.org/) file format. This can be
used to visualise dependency graphs. To obtain a build-time
dependency graph, apply this to a [store derivation]. To obtain a
runtime dependency graph, apply it to an output path.
- `--binding` *name* / `-b` *name*
Prints the value of the attribute *name* (i.e., environment
variable) of the [store derivation]s *paths*. It is an error for a
derivation to not have the specified attribute.
- `--hash`
Prints the SHA-256 hash of the contents of the store paths *paths*
(that is, the hash of the output of `nix-store --dump` on the given
paths). Since the hash is stored in the Nix database, this is a fast
operation.
- `--size`
Prints the size in bytes of the contents of the store paths *paths*
— to be precise, the size of the output of `nix-store --dump` on
the given paths. Note that the actual disk space required by the
store paths may be higher, especially on filesystems with large
cluster sizes.
- `--roots`
Prints the garbage collector roots that point, directly or
indirectly, at the store paths *paths*.
{{#include ./opt-common.md}}
{{#include ../opt-common.md}}
{{#include ../env-common.md}}
# Examples
Print the closure (runtime dependencies) of the `svn` program in the
current user environment:
```console
$ nix-store --query --requisites $(which svn)
/nix/store/5mbglq5ldqld8sj57273aljwkfvj22mc-subversion-1.1.4
/nix/store/9lz9yc6zgmc0vlqmn2ipcpkjlmbi51vv-glibc-2.3.4
...
```
Print the build-time dependencies of `svn`:
```console
$ nix-store --query --requisites $(nix-store --query --deriver $(which svn))
/nix/store/02iizgn86m42q905rddvg4ja975bk2i4-grep-2.5.1.tar.bz2.drv
/nix/store/07a2bzxmzwz5hp58nf03pahrv2ygwgs3-gcc-wrapper.sh
/nix/store/0ma7c9wsbaxahwwl04gbw3fcd806ski4-glibc-2.3.4.drv
... lots of other paths ...
```
The difference with the previous example is that we ask the closure of
the derivation (`-qd`), not the closure of the output path that contains
`svn`.
Show the build-time dependencies as a tree:
```console
$ nix-store --query --tree $(nix-store --query --deriver $(which svn))
/nix/store/7i5082kfb6yjbqdbiwdhhza0am2xvh6c-subversion-1.1.4.drv
+---/nix/store/d8afh10z72n8l1cr5w42366abiblgn54-builder.sh
+---/nix/store/fmzxmpjx2lh849ph0l36snfj9zdibw67-bash-3.0.drv
| +---/nix/store/570hmhmx3v57605cqg9yfvvyh0nnb8k8-bash
| +---/nix/store/p3srsbd8dx44v2pg6nbnszab5mcwx03v-builder.sh
...
```
Show all paths that depend on the same OpenSSL library as `svn`:
```console
$ nix-store --query --referrers $(nix-store --query --binding openssl $(nix-store --query --deriver $(which svn)))
/nix/store/23ny9l9wixx21632y2wi4p585qhva1q8-sylpheed-1.0.0
/nix/store/5mbglq5ldqld8sj57273aljwkfvj22mc-subversion-1.1.4
/nix/store/dpmvp969yhdqs7lm2r1a3gng7pyq6vy4-subversion-1.1.3
/nix/store/l51240xqsgg8a7yrbqdx1rfzyv6l26fx-lynx-2.8.5
```
Show all paths that directly or indirectly depend on the Glibc (C
library) used by `svn`:
```console
$ nix-store --query --referrers-closure $(ldd $(which svn) | grep /libc.so | awk '{print $3}')
/nix/store/034a6h4vpz9kds5r6kzb9lhh81mscw43-libgnomeprintui-2.8.2
/nix/store/15l3yi0d45prm7a82pcrknxdh6nzmxza-gawk-3.1.4
...
```
Note that `ldd` is a command that prints out the dynamic libraries used
by an ELF executable.
Make a picture of the runtime dependency graph of the current user
environment:
```console
$ nix-store --query --graph ~/.nix-profile | dot -Tps > graph.ps
$ gv graph.ps
```
Show every garbage collector root that points to a store path that
depends on `svn`:
```console
$ nix-store --query --roots $(which svn)
/nix/var/nix/profiles/default-81-link
/nix/var/nix/profiles/default-82-link
/home/eelco/.local/state/nix/profiles/profile-97-link
```

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# Name
`nix-store --read-log` - print build log
# Synopsis
`nix-store` {`--read-log` | `-l`} *paths…*
# Description
The operation `--read-log` prints the build log of the specified store
paths on standard output. The build log is whatever the builder of a
derivation wrote to standard output and standard error. If a store path
is not a derivation, the deriver of the store path is used.
Build logs are kept in `/nix/var/log/nix/drvs`. However, there is no
guarantee that a build log is available for any particular store path.
For instance, if the path was downloaded as a pre-built binary through a
substitute, then the log is unavailable.
{{#include ./opt-common.md}}
{{#include ../opt-common.md}}
{{#include ../env-common.md}}
# Example
```console
$ nix-store --read-log $(which ktorrent)
building /nix/store/dhc73pvzpnzxhdgpimsd9sw39di66ph1-ktorrent-2.2.1
unpacking sources
unpacking source archive /nix/store/p8n1jpqs27mgkjw07pb5269717nzf5f8-ktorrent-2.2.1.tar.gz
ktorrent-2.2.1/
ktorrent-2.2.1/NEWS
...
```

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# Name
`nix-store --realise` - build or fetch store objects
# Synopsis
`nix-store` {`--realise` | `-r`} *paths…* [`--dry-run`]
# Description
Each of *paths* is processed as follows:
- If the path leads to a [store derivation]:
1. If it is not [valid], substitute the store derivation file itself.
2. Realise its [output paths]:
- Try to fetch from [substituters] the [store objects] associated with the output paths in the store derivation's [closure].
- With [content-addressed derivations] (experimental):
Determine the output paths to realise by querying content-addressed realisation entries in the [Nix database].
- For any store paths that cannot be substituted, produce the required store objects:
1. Realise all outputs of the derivation's dependencies
2. Run the derivation's [`builder`](@docroot@/language/derivations.md#attr-builder) executable
<!-- TODO: Link to build process page #8888 -->
- Otherwise, and if the path is not already valid: Try to fetch the associated [store objects] in the path's [closure] from [substituters].
If no substitutes are available and no store derivation is given, realisation fails.
[store paths]: @docroot@/store/store-path.md
[valid]: @docroot@/glossary.md#gloss-validity
[store derivation]: @docroot@/glossary.md#gloss-store-derivation
[output paths]: @docroot@/glossary.md#gloss-output-path
[store objects]: @docroot@/store/store-object.md
[closure]: @docroot@/glossary.md#gloss-closure
[substituters]: @docroot@/command-ref/conf-file.md#conf-substituters
[content-addressed derivations]: @docroot@/development/experimental-features.md#xp-feature-ca-derivations
[Nix database]: @docroot@/glossary.md#gloss-nix-database
The resulting paths are printed on standard output.
For non-derivation arguments, the argument itself is printed.
{{#include ../status-build-failure.md}}
# Options
- `--dry-run`
Print on standard error a description of what packages would be
built or downloaded, without actually performing the operation.
- `--ignore-unknown`
If a non-derivation path does not have a substitute, then silently
ignore it.
- `--check`
This option allows you to check whether a derivation is
deterministic. It rebuilds the specified derivation and checks
whether the result is bitwise-identical with the existing outputs,
printing an error if that’s not the case. The outputs of the
specified derivation must already exist. When used with `-K`, if an
output path is not identical to the corresponding output from the
previous build, the new output path is left in
`/nix/store/name.check.`
{{#include ./opt-common.md}}
{{#include ../opt-common.md}}
{{#include ../env-common.md}}
# Examples
This operation is typically used to build [store derivation]s produced by
[`nix-instantiate`](@docroot@/command-ref/nix-instantiate.md):
```console
$ nix-store --realise $(nix-instantiate ./test.nix)
/nix/store/31axcgrlbfsxzmfff1gyj1bf62hvkby2-aterm-2.3.1
```
This is essentially what [`nix-build`](@docroot@/command-ref/nix-build.md) does.
To test whether a previously-built derivation is deterministic:
```console
$ nix-build '<nixpkgs>' --attr hello --check -K
```
Use [`nix-store --read-log`](./read-log.md) to show the stderr and stdout of a build:
```console
$ nix-store --read-log $(nix-instantiate ./test.nix)
```

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# Name
`nix --repair-path` - re-download path from substituter
# Synopsis
`nix-store` `--repair-path` *paths…*
# Description
The operation `--repair-path` attempts to “repair” the specified paths
by redownloading them using the available substituters. If no
substitutes are available, then repair is not possible.
> **Warning**
>
> During repair, there is a very small time window during which the old
> path (if it exists) is moved out of the way and replaced with the new
> path. If repair is interrupted in between, then the system may be left
> in a broken state (e.g., if the path contains a critical system
> component like the GNU C Library).
# Example
```console
$ nix-store --verify-path /nix/store/dj7a81wsm1ijwwpkks3725661h3263p5-glibc-2.13
path `/nix/store/dj7a81wsm1ijwwpkks3725661h3263p5-glibc-2.13' was modified!
expected hash `2db57715ae90b7e31ff1f2ecb8c12ec1cc43da920efcbe3b22763f36a1861588',
got `481c5aa5483ebc97c20457bb8bca24deea56550d3985cda0027f67fe54b808e4'
$ nix-store --repair-path /nix/store/dj7a81wsm1ijwwpkks3725661h3263p5-glibc-2.13
fetching path `/nix/store/d7a81wsm1ijwwpkks3725661h3263p5-glibc-2.13'...
…
```

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# Name
`nix-store --restore` - extract a Nix archive
## Synopsis
`nix-store` `--restore` *path*
## Description
The operation `--restore` unpacks a [Nix Archive (NAR)][Nix Archive] to *path*, which must
not already exist. The archive is read from standard input.
[Nix Archive]: @docroot@/store/file-system-object/content-address.md#serial-nix-archive
{{#include ./opt-common.md}}
{{#include ../opt-common.md}}
{{#include ../env-common.md}}

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# Name
`nix-store --serve` - serve local Nix store over SSH
# Synopsis
`nix-store` `--serve` [`--write`]
# Description
The operation `--serve` provides access to the Nix store over stdin and
stdout, and is intended to be used as a means of providing Nix store
access to a restricted ssh user.
The following flags are available:
- `--write`
Allow the connected client to request the realization of
derivations. In effect, this can be used to make the host act as a
remote builder.
{{#include ./opt-common.md}}
{{#include ../opt-common.md}}
{{#include ../env-common.md}}
# Examples
To turn a host into a build server, the `authorized_keys` file can be
used to provide build access to a given SSH public key:
```console
$ cat <<EOF >>/root/.ssh/authorized_keys
command="nice -n20 nix-store --serve --write" ssh-rsa AAAAB3NzaC1yc2EAAAA...
EOF
```

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# Name
`nix-store --verify-path` - check path contents against Nix database
## Synopsis
`nix-store` `--verify-path` *paths…*
## Description
The operation `--verify-path` compares the contents of the given store
paths to their cryptographic hashes stored in Nix’s database. For every
changed path, it prints a warning message. The exit status is 0 if no
path has changed, and 1 otherwise.
{{#include ./opt-common.md}}
{{#include ../opt-common.md}}
{{#include ../env-common.md}}
## Example
To verify the integrity of the `svn` command and all its dependencies:
```console
$ nix-store --verify-path $(nix-store --query --requisites $(which svn))
```

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# Name
`nix-store --verify` - check Nix database for consistency
# Synopsis
`nix-store` `--verify` [`--check-contents`] [`--repair`]
# Description
The operation `--verify` verifies the internal consistency of the Nix
database, and the consistency between the Nix database and the Nix
store. Any inconsistencies encountered are automatically repaired.
Inconsistencies are generally the result of the Nix store or database
being modified by non-Nix tools, or of bugs in Nix itself.
This operation has the following options:
- `--check-contents`
Checks that the contents of every valid store path has not been
altered by computing a SHA-256 hash of the contents and comparing it
with the hash stored in the Nix database at build time. Paths that
have been modified are printed out. For large stores,
`--check-contents` is obviously quite slow.
- `--repair`
If any valid path is missing from the store, or (if
`--check-contents` is given) the contents of a valid path has been
modified, then try to repair the path by redownloading it. See
`nix-store --repair-path` for details.
{{#include ./opt-common.md}}
{{#include ../opt-common.md}}
{{#include ../env-common.md}}

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<!-- Some of the options documented here are hardcopied from
src/libcmd/common-eval-args.cc
-->
# Common Options
Most Nix commands accept the following command-line options:
- <span id="opt-help">[`--help`](#opt-help)</span>
Prints out a summary of the command syntax and exits.
- <span id="opt-version">[`--version`](#opt-version)</span>
Prints out the Nix version number on standard output and exits.
- <span id="opt-verbose">[`--verbose`](#opt-verbose)</span> / `-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 printed on standard error, never on standard output.
This option may be specified repeatedly.
Currently, the following verbosity levels exist:
- `0` “Errors only”
Only print messages explaining why the Nix invocation failed.
- `1` “Informational”
Print *useful* messages about what Nix is doing.
This is the default.
- `2` “Talkative”
Print more informational messages.
- `3` “Chatty”
Print even more informational messages.
- `4` “Debug”
Print debug information.
- `5` “Vomit”
Print vast amounts of debug information.
- <span id="opt-quiet">[`--quiet`](#opt-quiet)</span>
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.
- <span id="opt-log-format">[`--log-format`](#opt-log-format)</span> *format*
This option can be used to change the output of the log format, with *format* being one of:
- `raw`
This is the raw format, as outputted by nix-build.
- `internal-json`
Outputs the logs in a structured manner.
> **Warning**
>
> While the schema itself is relatively stable, the format of
> the error-messages (namely of the `msg`-field) can change
> between releases.
- `bar`
Only display a progress bar during the builds.
- `bar-with-logs`
Display the raw logs, with the progress bar at the bottom.
- <span id="opt-no-build-output">[`--no-build-output`](#opt-no-build-output)</span> / `-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`.
- <span id="opt-max-jobs">[`--max-jobs`](#opt-max-jobs)</span> / `-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` configuration setting, which itself defaults to `1`.
A higher value is useful on SMP systems or to exploit I/O latency.
Setting it to `0` disallows building on the local machine, which is useful when you want builds to happen only on remote builders.
- <span id="opt-cores">[`--cores`](#opt-cores)</span>
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 instance, in Nixpkgs, if the derivation attribute `enableParallelBuilding` is set to `true`, the builder passes the `-jN` flag to GNU Make.
It defaults to the value of the `cores` configuration setting, if set, or `1` otherwise.
The value `0` means that the builder should use all available CPU cores in the system.
- <span id="opt-max-silent-time">[`--max-silent-time`](#opt-max-silent-time)</span>
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.
- <span id="opt-timeout">[`--timeout`](#opt-timeout)</span>
Sets the maximum number of seconds that a builder can run.
The default is specified by the `timeout` configuration setting.
`0` means no timeout.
- <span id="opt-keep-going">[`--keep-going`](#opt-keep-going)</span> / `-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 itself.
Without this option, Nix stops if any build fails (except for builds of substitutes), possibly killing builds in progress (in case of parallel or distributed builds).
- <span id="opt-keep-failed">[`--keep-failed`](#opt-keep-failed)</span> / `-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.
- <span id="opt-fallback">[`--fallback`](#opt-fallback)</span>
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.
The most common scenario in which this is useful is when we have registered substitutes in order to perform binary distribution from, say, a network repository.
If the repository is down, the realisation of the derivation will fail.
When this option is specified, Nix will build the derivation instead.
Thus, installation from binaries falls back on installation from source.
This option is not the default since it is generally not desirable for a transient failure in obtaining the substitutes to lead to a full build from source (with the related consumption of resources).
- <span id="opt-readonly-mode">[`--readonly-mode`](#opt-readonly-mode)</span>
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.
- <span id="opt-arg">[`--arg`](#opt-arg)</span> *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](@docroot@/language/syntax.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 function with an argument named *name*, it will call it with value *value*.
For instance, the top-level `default.nix` in Nixpkgs is actually a function:
```nix
{ # The system (e.g., `i686-linux') for which to build the packages.
system ? builtins.currentSystem
...
}: ...
```
So if you call this Nix expression (e.g., when you do `nix-env --install --attr pkgname`), the function will be called automatically using the value [`builtins.currentSystem`](@docroot@/language/builtins.md) for the `system` argument.
You can override this using `--arg`, e.g., `nix-env --install --attr pkgname --arg system \"i686-freebsd\"`.
(Note that since the argument is a Nix string literal, you have to escape the quotes.)
- <span id="opt-arg-from-file">[`--arg-from-file`](#opt-arg-from-file)</span> *name* *path*
Pass the contents of file *path* as the argument *name* to Nix functions.
- <span id="opt-arg-from-stdin">[`--arg-from-stdin`](#opt-arg-from-stdin)</span> *name*
Pass the contents of stdin as the argument *name* to Nix functions.
- <span id="opt-argstr">[`--argstr`](#opt-argstr)</span> *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`.
- <span id="opt-attr">[`--attr`](#opt-attr)</span> / `-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 of attribute names separated by dots.
For instance, given a top-level Nix expression *e*, the attribute path `xorg.xorgserver` would cause the expression `e.xorg.xorgserver` to be used.
See [`nix-env --install`](@docroot@/command-ref/nix-env/install.md) for some concrete examples.
In addition to attribute names, you can also specify array indices.
For instance, the attribute path `foo.3.bar` selects the `bar`
attribute of the fourth element of the array in the `foo` attribute
of the top-level expression.
- <span id="opt-eval-store">[`--eval-store`](#opt-eval-store)</span> *store-url*
The [URL to the Nix store](@docroot@/store/types/index.md#store-url-format) to use for evaluation, i.e. where to store derivations (`.drv` files) and inputs referenced by them.
- <span id="opt-expr">[`--expr`](#opt-expr)</span> / `-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.)
For `nix-shell`, this option is commonly used to give you a shell in which you can build the packages returned by the expression.
If you want to get a shell which contain the *built* packages ready for use, give your expression to the `nix-shell --packages ` convenience flag instead.
- <span id="opt-I">[`-I` / `--include`](#opt-I)</span> *path*
Add an entry to the list of search paths used to resolve [lookup paths](@docroot@/language/constructs/lookup-path.md).
This option may be given multiple times.
Paths added through `-I` take precedence over the [`nix-path` configuration setting](@docroot@/command-ref/conf-file.md#conf-nix-path) and the [`NIX_PATH` environment variable](@docroot@/command-ref/env-common.md#env-NIX_PATH).
- <span id="opt-impure">[`--impure`](#opt-impure)</span>
Allow access to mutable paths and repositories.
- <span id="opt-option">[`--option`](#opt-option)</span> *name* *value*
Set the Nix configuration option *name* to *value*.
This overrides settings in the Nix configuration file (see nix.conf5).
- <span id="opt-repair">[`--repair`](#opt-repair)</span>
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 the build.
Also note the warning under `nix-store --repair-path`.
> **Note**
>
> See [`man nix.conf`](@docroot@/command-ref/conf-file.md#command-line-flags) for overriding configuration settings with command line flags.

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# Special exit codes for build failure
1xx status codes are used when requested builds failed.
The following codes are in use:
- `100` Generic build failure
The builder process returned with a non-zero exit code.
- `101` Build timeout
The build was aborted because it did not complete within the specified `timeout`.
- `102` Hash mismatch
The build output was rejected because it does not match the
[`outputHash` attribute of the derivation](@docroot@/language/advanced-attributes.md).
- `104` Not deterministic
The build succeeded in check mode but the resulting output is not binary reproducible.
With the `--keep-going` flag it's possible for multiple failures to occur.
In this case the 1xx status codes are or combined using
[bitwise OR](https://en.wikipedia.org/wiki/Bitwise_operation#OR).
```
0b1100100
^^^^
|||`- timeout
||`-- output hash mismatch
|`--- build failure
`---- not deterministic
```

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# Utilities
This section lists utilities that you can use when you work with Nix.