nix-store --read-log - print build log


nix-store {--read-log | -l} paths…


The operation --read-log prints the build log of the specified store paths on standard output. The build log is whatever the builder of a derivation wrote to standard output and standard error. If a store path is not a derivation, the deriver of the store path is used.

Build logs are kept in /nix/var/log/nix/drvs. However, there is no guarantee that a build log is available for any particular store path. For instance, if the path was downloaded as a pre-built binary through a substitute, then the log is unavailable.


The following options are allowed for all nix-store operations, but may not always have an effect.

  • --add-root path

    Causes the result of a realisation (--realise and --force-realise) to be registered as a root of the garbage collector. path will be created as a symlink to the resulting store path. In addition, a uniquely named symlink to path will be created in /nix/var/nix/gcroots/auto/. For instance,

    $ nix-store --add-root /home/eelco/bla/result --realise ...
    $ ls -l /nix/var/nix/gcroots/auto
    lrwxrwxrwx    1 ... 2005-03-13 21:10 dn54lcypm8f8... -> /home/eelco/bla/result
    $ ls -l /home/eelco/bla/result
    lrwxrwxrwx    1 ... 2005-03-13 21:10 /home/eelco/bla/result -> /nix/store/1r11343n6qd4...-f-spot-0.0.10

    Thus, when /home/eelco/bla/result is removed, the GC root in the auto directory becomes a dangling symlink and will be ignored by the collector.


    Note that it is not possible to move or rename GC roots, since the symlink in the auto directory will still point to the old location.

    If there are multiple results, then multiple symlinks will be created by sequentially numbering symlinks beyond the first one (e.g., foo, foo-2, foo-3, and so on).

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.


      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 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 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 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 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 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).

  • --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 (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 value builtins.currentSystem for the system 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 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 for some concrete examples.

    In addition to attribute names, you can also specify array indices. For instance, the attribute path 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 --packages convenience flag instead.

  • -I path

    Add an entry to the Nix expression search path. This option may be given multiple times. 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.

Common Environment Variables

Most Nix commands interpret the following environment variables:

    Indicator that tells if the current environment was set up by nix-shell. It can have the values pure or impure.

    A colon-separated list of directories used to look up the location of Nix expressions using paths enclosed in angle brackets (i.e., <path>), e.g. /home/eelco/Dev:/etc/nixos. It can be extended using the -I option.

    If NIX_PATH is not set at all, Nix will fall back to the following list in impure and unrestricted evaluation mode:

    1. $HOME/.nix-defexpr/channels
    2. nixpkgs=/nix/var/nix/profiles/per-user/root/channels/nixpkgs
    3. /nix/var/nix/profiles/per-user/root/channels

    If NIX_PATH is set to an empty string, resolving search paths will always fail. For example, attempting to use <nixpkgs> will produce:

    error: file 'nixpkgs' was not found in the Nix search path
    Normally, the Nix store directory (typically /nix/store) is not allowed to contain any symlink components. This is to prevent “impure” builds. Builders sometimes “canonicalise” paths by resolving all symlink components. Thus, builds on different machines (with /nix/store resolving to different locations) could yield different results. This is generally not a problem, except when builds are deployed to machines where /nix/store resolves differently. If you are sure that you’re not going to do that, you can set NIX_IGNORE_SYMLINK_STORE to 1.

    Note that if you’re symlinking the Nix store so that you can put it on another file system than the root file system, on Linux you’re better off using bind mount points, e.g.,

    $ mkdir /nix
    $ mount -o bind /mnt/otherdisk/nix /nix

    Consult the mount 8 manual page for details.

    Overrides the location of the Nix store (default prefix/store).

    Overrides the location of the Nix static data directory (default prefix/share).

    Overrides the location of the Nix log directory (default prefix/var/log/nix).

    Overrides the location of the Nix state directory (default prefix/var/nix).

    Overrides the location of the system Nix configuration directory (default prefix/etc/nix).

    Applies settings from Nix configuration from the environment. The content is treated as if it was read from a Nix configuration file. Settings are separated by the newline character.

    Overrides the location of the Nix user configuration files to load from.

    The default are the locations according to the XDG Base Directory Specification. See the XDG Base Directories sub-section for details.

    The variable is treated as a list separated by the : token.

    Use the specified directory to store temporary files. In particular, this includes temporary build directories; these can take up substantial amounts of disk space. The default is /tmp.

    This variable should be set to daemon if you want to use the Nix daemon to execute Nix operations. This is necessary in multi-user Nix installations. If the Nix daemon's Unix socket is at some non-standard path, this variable should be set to unix://path/to/socket. Otherwise, it should be left unset.

    If set to 1, Nix will print some evaluation statistics, such as the number of values allocated.

    If set to 1, Nix will print how often functions were called during Nix expression evaluation. This is useful for profiling your Nix expressions.

    If Nix has been configured to use the Boehm garbage collector, this variable sets the initial size of the heap in bytes. It defaults to 384 MiB. Setting it to a low value reduces memory consumption, but will increase runtime due to the overhead of garbage collection.

XDG Base Directories

Nix follows the XDG Base Directory Specification.

For backwards compatibility, Nix commands will follow the standard only when use-xdg-base-directories is enabled. New Nix commands (experimental) conform to the standard by default.

The following environment variables are used to determine locations of various state and configuration files:


$ nix-store --read-log $(which ktorrent)
building /nix/store/dhc73pvzpnzxhdgpimsd9sw39di66ph1-ktorrent-2.2.1
unpacking sources
unpacking source archive /nix/store/p8n1jpqs27mgkjw07pb5269717nzf5f8-ktorrent-2.2.1.tar.gz