--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. 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. --build-output / -B Causes the output written by build actions to standard output and standard error to be echoed to standard error, regardless of verbosity level. By default, it is only echoed at a verbosity level of at least 4 (Debug), and is suppressed at lower levels. Note that it is always written to a log file in prefix/nix/var/log/nix. --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.