nix-super/doc/manual/src/command-ref/opt-common.md

# 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`.