Provided by: nix-bin_2.18.1+dfsg-1ubuntu5_amd64 
Name
nix-store --gc - run garbage collection
Synopsis
nix-store --gc [--print-roots | --print-live | --print-dead] [--max-freed bytes]
Description
Without additional flags, the operation --gc performs a garbage collection on the Nix
store. That is, all paths in the Nix store not reachable via file system references from a
set of “roots”, are deleted.
The following suboperations may be specified:
• --print-roots
This operation prints on standard output the set of roots used by the garbage
collector.
• --print-live
This operation prints on standard output the set of “live” store paths, which are all
the store paths reachable from the roots. Live paths should never be deleted, since
that would break consistency — it would become possible that applications are installed
that reference things that are no longer present in the store.
• --print-dead
This operation prints out on standard output the set of “dead” store paths, which is
just the opposite of the set of live paths: any path in the store that is not live
(with respect to the roots) is dead.
By default, all unreachable paths are deleted. The following options control what gets
deleted and in what order:
• --max-freed bytes
Keep deleting paths until at least bytes bytes have been deleted, then stop. The
argument bytes can be followed by the multiplicative suffix K, M, G or T, denoting KiB,
MiB, GiB or TiB units.
The behaviour of the collector is also influenced by the keep-outputs and keep-derivations
settings in the Nix configuration file.
By default, the collector prints the total number of freed bytes when it finishes (or when
it is interrupted). With --print-dead, it prints the number of bytes that would be freed.
Options
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.
Warning
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.
Warning
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 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 --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:
• IN_NIX_SHELL
Indicator that tells if the current environment was set up by nix-shell. It can have
the values pure or impure.
• NIX_PATH
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
• NIX_IGNORE_SYMLINK_STORE
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.
• NIX_STORE_DIR
Overrides the location of the Nix store (default prefix/store).
• NIX_DATA_DIR
Overrides the location of the Nix static data directory (default prefix/share).
• NIX_LOG_DIR
Overrides the location of the Nix log directory (default prefix/var/log/nix).
• NIX_STATE_DIR
Overrides the location of the Nix state directory (default prefix/var/nix).
• NIX_CONF_DIR
Overrides the location of the system Nix configuration directory (default
prefix/etc/nix).
• NIX_CONFIG
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.
• NIX_USER_CONF_FILES
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.
• TMPDIR
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.
• NIX_REMOTE
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.
• NIX_SHOW_STATS
If set to 1, Nix will print some evaluation statistics, such as the number of values
allocated.
• NIX_COUNT_CALLS
If set to 1, Nix will print how often functions were called during Nix expression
evaluation. This is useful for profiling your Nix expressions.
• GC_INITIAL_HEAP_SIZE
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:
• [XDG_CONFIG_HOME]{#env-XDGCONFIGHOME} (default ~/.config)
• [XDG_STATE_HOME]{#env-XDGSTATEHOME} (default ~/.local/state)
• [XDG_CACHE_HOME]{#env-XDGCACHEHOME} (default ~/.cache)
Examples
To delete all unreachable paths, just do:
$ nix-store --gc
deleting `/nix/store/kq82idx6g0nyzsp2s14gfsc38npai7lf-cairo-1.0.4.tar.gz.drv'
...
8825586 bytes freed (8.42 MiB)
To delete at least 100 MiBs of unreachable paths:
$ nix-store --gc --max-freed $((100 * 1024 * 1024))
nix-store –gc(1)