Common Options
Most Nix commands accept the following command-line options:
-
Prints out a summary of the command syntax and exits.
-
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.
-
-
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
formatThis 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.
-
-
--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
numberSets 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 themax-jobs
configuration setting, which itself defaults to1
. 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. -
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 attributeenableParallelBuilding
is set totrue
, the builder passes the-jN
flag to GNU Make. It defaults to the value of thecores
configuration setting, if set, or1
otherwise. The value0
means that the builder should use all available CPU cores in the system. -
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. -
Sets the maximum number of seconds that a builder can run. The default is specified by the
timeout
configuration setting.0
means no timeout. -
--keep-going
/-k
Keep going 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. -
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).
-
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 valueThis option is accepted by
nix-env
,nix-instantiate
,nix-shell
andnix-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 (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 --install --attr pkgname
), the function will be called automatically using the valuebuiltins.currentSystem
for thesystem
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.) -
--argstr
name valueThis 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
attrPathSelect an attribute from the top-level Nix expression being evaluated. (
nix-env
,nix-instantiate
,nix-build
andnix-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 pathxorg.xorgserver
would cause the expressione.xorg.xorgserver
to be used. Seenix-env --install
for some concrete examples.In addition to attribute names, you can also specify array indices. For instance, the attribute path
foo.3.bar
selects thebar
attribute of the fourth element of the array in thefoo
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
andnix-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 thenix-shell --packages
convenience flag instead. -
-I
pathAdd an entry to the Nix expression search path. This option may be given multiple times. Paths added through
-I
take precedence overNIX_PATH
. -
--option
name valueSet the Nix configuration option name to value. This overrides settings in the Nix configuration file (see nix.conf5).
-
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
for overriding configuration settings with command line flags.