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. --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 Sets the maximum number of build jobs that Nix will perform in parallel to the specified number. The default is specified by the build-max-jobs configuration setting, which itself defaults to 1. A higher value is useful on SMP systems or to exploit I/O latency. --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 build-max-silent-time configuration setting. 0 means no time-out. --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 nstallation 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). --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. --log-type type This option determines how the output written to standard error is formatted. Nix’s diagnostic messages are typically nested. For instance, when tracing Nix expression evaluation (nix-env -vvvvv, messages from subexpressions are nested inside their parent expressions. Nix builder output is also often nested. For instance, the Nix Packages generic builder nests the various build tasks (unpack, configure, compile, etc.), and the GNU Make in stdenv-linux has been patched to provide nesting for recursive Make invocations. type can be one of the following: pretty Pretty-print the output, indicating different nesting levels using spaces. This is the default. escapes Indicate nesting using escape codes that can be interpreted by the nix-log2xml tool in the Nix source distribution. The resulting XML file can be fed into the log2html.xsl stylesheet to create an HTML file that can be browsed interactively, using Javascript to expand and collapse parts of the output. flat Remove all nesting. --arg name value This option is accepted by nix-env, nix-instantiate 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 (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 file pkgs/top-level/all-packages.nix in Nixpkgs is actually a function: { # The system (e.g., `i686-linux') for which to build the packages. system ? __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 __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 In nix-env, nix-instantiate and nix-build, --attr allows you to select an attribute from the top-level Nix expression being evaluated. 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 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.