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 1. A higher value is useful on SMP systems or to exploit I/O latency. --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.