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

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# Common Environment Variables
Most Nix commands interpret the following environment variables:
- `IN_NIX_SHELL`
Indicator that tells if the current environment was set up by
`nix-shell`. Since Nix 2.0 the values are `"pure"` and `"impure"`
- `NIX_PATH`
A colon-separated list of directories used to look up Nix
expressions enclosed in angle brackets (i.e., `<path>`). For
instance, the value
/home/eelco/Dev:/etc/nixos
will cause Nix to look for paths relative to `/home/eelco/Dev` and
`/etc/nixos`, in this order. It is also possible to match paths
against a prefix. For example, the value
nixpkgs=/home/eelco/Dev/nixpkgs-branch:/etc/nixos
will cause Nix to search for `<nixpkgs/path>` in
`/home/eelco/Dev/nixpkgs-branch/path` and `/etc/nixos/nixpkgs/path`.
If a path in the Nix search 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
consist of a single top-level directory. For example, setting
`NIX_PATH` to
nixpkgs=https://github.com/NixOS/nixpkgs/archive/nixos-15.09.tar.gz
tells Nix to download the latest revision in the Nixpkgs/NixOS 15.09
channel.
A following shorthand can be used to refer to the official channels:
nixpkgs=channel:nixos-15.09
The search path can be extended using the `-I` option, which takes
precedence over `NIX_PATH`.
- `NIX_IGNORE_SYMLINK_STORE`
Normally, the Nix store directory (typically `/nix/store`) is not
allowed to contain any symlink components. This is to prevent
“impure” builds. Builders sometimes “canonicalise” paths by
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.,
$ mkdir /nix
$ mount -o bind /mnt/otherdisk/nix /nix
Consult the mount 8 manual page for details.
- `NIX_STORE_DIR`
Overrides the location of the Nix store (default `prefix/store`).
- `NIX_DATA_DIR`
Overrides the location of the Nix static data directory (default
`prefix/share`).
- `NIX_LOG_DIR`
Overrides the location of the Nix log directory (default
`prefix/var/log/nix`).
- `NIX_STATE_DIR`
Overrides the location of the Nix state directory (default
`prefix/var/nix`).
- `NIX_CONF_DIR`
Overrides the location of the system Nix configuration directory
(default `prefix/etc/nix`).
- `NIX_USER_CONF_FILES`
Overrides the location of the user Nix configuration files to load
from (defaults to the XDG spec locations). The variable is treated
as a list separated by the `:` token.
- `TMPDIR`
Use the specified directory to store temporary files. In particular,
this includes temporary build directories; these can take up
substantial amounts of disk space. The default is `/tmp`.
- `NIX_REMOTE`
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](#ssec-multi-user). 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.
- `NIX_SHOW_STATS`
If set to `1`, Nix will print some evaluation statistics, such as
the number of values allocated.
- `NIX_COUNT_CALLS`
If set to `1`, Nix will print how often functions were called during
Nix expression evaluation. This is useful for profiling your Nix
expressions.
- `GC_INITIAL_HEAP_SIZE`
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.

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# Common Options
Most Nix commands accept the following command-line options:
- `--help`
Prints out a summary of the command syntax and exits.
- `--version`
Prints out the Nix version number on standard output and exits.
- `--verbose` / `-v`
Increases the level of verbosity of diagnostic messages printed on
standard error. For each Nix operation, the information printed on
standard output is well-defined; any diagnostic information is
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.
- `--quiet`
Decreases the level of verbosity of diagnostic messages printed on
standard error. This is the inverse option to `-v` / `--verbose`.
This option may be specified repeatedly. See the previous verbosity
levels list.
- `--log-format` format
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. NOTE: the json schema
is not guarantees to be stable 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.
- `--no-build-output` / `-Q`
By default, output written by builders to standard output and
standard error is echoed to the Nix command's standard error. This
option suppresses this behaviour. Note that the builder's standard
output and error are always written to a log file in
`prefix/nix/var/log/nix`.
- `--max-jobs` / `-j` number
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`](#conf-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.
- `--cores`
Sets the value of the `NIX_BUILD_CORES` environment variable in the
invocation of builders. Builders can use this variable at their
discretion to control the maximum amount of parallelism. For
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`](#conf-cores) configuration setting, if set, or `1`
otherwise. The value `0` means that the builder should use all
available CPU cores in the system.
- `--max-silent-time`
Sets the maximum number of seconds that a builder can go without
producing any data on standard output or standard error. The default
is specified by the [`max-silent-time`](#conf-max-silent-time)
configuration setting. `0` means no time-out.
- `--timeout`
Sets the maximum number of seconds that a builder can run. The
default is specified by the [`timeout`](#conf-timeout) configuration
setting. `0` means no timeout.
- `--keep-going` / `-k`
Keep going in case of failed builds, to the greatest extent
possible. That is, if building an input of some derivation fails,
Nix will still build the other inputs, but not the derivation
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).
- `--keep-failed` / `-K`
Specifies that in case of a build failure, the temporary directory
(usually in `/tmp`) in which the build takes place should not be
deleted. The path of the build directory is printed as an
informational message.
- `--fallback`
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).
- `--no-build-hook`
Disables the build hook mechanism. This allows to ignore remote
builders if they are setup on the machine.
It's useful in cases where the bandwidth between the client and the
remote builder is too low. In that case it can take more time to
upload the sources to the remote builder and fetch back the result
than to do the computation locally.
- `--readonly-mode`
When this option is used, no attempt is made to open the Nix
database. Most Nix operations do need database access, so those
operations will fail.
- `--arg` name value
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](#ss-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:
{ # 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 -i
pkgname`), the function will be called automatically using the value
[`builtins.currentSystem`](#builtin-currentSystem) for the `system`
argument. You can override this using `--arg`, e.g., `nix-env -i
pkgname --arg system
\"i686-freebsd\"`. (Note that since the argument is a Nix string
literal, you have to escape the quotes.)
- `--argstr` name value
This option is like `--arg`, only the value is not a Nix expression
but a string. So instead of `--arg system \"i686-linux\"` (the outer
quotes are to keep the shell happy) you can say `--argstr system
i686-linux`.
- `--attr` / `-A` attrPath
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`](#refsec-nix-env-install-examples) 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.
- `--expr` / `-E`
Interpret the command line arguments as a list of Nix expressions to
be parsed and evaluated, rather than as a list of file names of Nix
expressions. (`nix-instantiate`, `nix-build` and `nix-shell` only.)
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 -p` convenience flag
instead.
- `-I` path
Add a path to the Nix expression search path. This option may be
given multiple times. See the NIX\_PATH\</literal\> environment
variable for information on the semantics of the Nix search path.
Paths added through `-I` take precedence over `NIX_PATH`.
- `--option` name value
Set the Nix configuration option name to value. This overrides
settings in the Nix configuration file (see nix.conf5).
- `--repair`
Fix corrupted or missing store paths by redownloading or rebuilding
them. Note that this is slow because it requires computing a
cryptographic hash of the contents of every path in the closure of
the build. Also note the warning under `nix-store --repair-path`.