nix-shell

Start an interactive shell based on a Nix expression.

Synopsis

nix-shell [--arg name value] [--argstr name value] [{--attr | -A} attrPath] [--command cmd] [--run cmd] [--exclude regexp] [--pure] [--keep name] {{--packages | -p} {packages | expressions} … | [path]}

Disambiguation

This man page describes the command nix-shell, which is distinct from nix shell. For documentation on the latter, run nix shell --help or see man nix3-shell.

Description

The command nix-shell will build the dependencies of the specified derivation, but not the derivation itself. It will then start an interactive shell in which all environment variables defined by the derivation path have been set to their corresponding values, and the script $stdenv/setup has been sourced. This is useful for reproducing the environment of a derivation for development.

If path is not given, nix-shell defaults to shell.nix if it exists, and default.nix otherwise.

If path starts with http:// or https://, it is interpreted as the URL of a tarball that will be downloaded and unpacked to a temporary location. The tarball must include a single top-level directory containing at least a file named default.nix.

If the derivation defines the variable shellHook, it will be run after $stdenv/setup has been sourced. Since this hook is not executed by regular Nix builds, it allows you to perform initialisation specific to nix-shell. For example, the derivation attribute

shellHook =
  ''
    echo "Hello shell"
    export SOME_API_TOKEN="$(cat ~/.config/some-app/api-token)"
  '';

will cause nix-shell to print Hello shell and set the SOME_API_TOKEN environment variable to a user-configured value.

Options

All options not listed here are passed to nix-store --realise, except for --arg and --attr / -A which are passed to nix-instantiate.

• --command cmd In the environment of the derivation, run the shell command cmd. This command is executed in an interactive shell. (Use --run to use a non-interactive shell instead.) However, a call to exit is implicitly added to the command, so the shell will exit after running the command. To prevent this, add return at the end; e.g. --command "echo Hello; return" will print Hello and then drop you into the interactive shell. This can be useful for doing any additional initialisation.

• --run cmd Like --command, but executes the command in a non-interactive shell. This means (among other things) that if you hit Ctrl-C while the command is running, the shell exits.

• --exclude regexp Do not build any dependencies whose store path matches the regular expression regexp. This option may be specified multiple times.

• --pure If this flag is specified, the environment is almost entirely cleared before the interactive shell is started, so you get an environment that more closely corresponds to the “real” Nix build. A few variables, in particular HOME, USER and DISPLAY, are retained.

• --packages / -p packages… Set up an environment in which the specified packages are present. The command line arguments are interpreted as attribute names inside the Nix Packages collection. Thus, nix-shell --packages libjpeg openjdk will start a shell in which the packages denoted by the attribute names libjpeg and openjdk are present.

• -i interpreter The chained script interpreter to be invoked by nix-shell. Only applicable in #!-scripts (described below).

• --keep name When a --pure shell is started, keep the listed environment variables.

Common Options

Most commands in Lix accept the following command-line options:

• --help

Prints out a summary of the command syntax and exits.

• --version

Prints out the Lix version number on standard output and exits.

• --verbose / -v

Increases the level of verbosity of diagnostic messages printed on standard error. For each Lix 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 Lix invocation failed.

• 1 “Informational”

  Print useful messages about what Lix 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 Lix 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 Lix 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, Lix will still build the other inputs, but not the derivation itself. Without this option, Lix 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 Lix 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, Lix 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 Lix database. Most Lix operations do need database access, so those operations will fail.

Warning

FIXME(Lix): sometimes you want --store dummy instead, because this option sometimes doesn't work. Document why this is.

• --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 Lix configuration option name to value. This overrides settings in the Lix configuration file (see nix.conf(5)).

• --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.

Note

See man nix.conf for overriding configuration settings with command line flags.

Environment variables

• NIX_BUILD_SHELL Shell used to start the interactive environment. Defaults to the bash found in <nixpkgs>, falling back to the bash found in PATH if not found.

Common Environment Variables

Most commands in Lix 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, Lix 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 Lix static data directory (default prefix/share).

• NIX_LOG_DIR Overrides the location of the Lix log directory (default prefix/var/log/nix).

• NIX_STATE_DIR Overrides the location of the Lix state directory (default prefix/var/nix).

• NIX_CONF_DIR Overrides the location of the system Lix configuration directory (default prefix/etc/nix).

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

• NIX_USER_CONF_FILES Overrides the location of the Lix 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 Lix daemon to execute Nix operations. This is necessary in multi-user Nix installations. If the Lix 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, Lix will print some evaluation statistics, such as the number of values allocated.

• NIX_COUNT_CALLS If set to 1, Lix 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

Lix follows the XDG Base Directory Specification.

For backwards compatibility, commands in Lix 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 (default ~/.config)
• XDG_STATE_HOME (default ~/.local/state)
• XDG_CACHE_HOME (default ~/.cache)

Examples

To build the dependencies of the package Pan, and start an interactive shell in which to build it:

$ nix-shell '<nixpkgs>' --attr pan
[nix-shell]$ eval ${unpackPhase:-unpackPhase}
[nix-shell]$ cd $sourceRoot
[nix-shell]$ eval ${patchPhase:-patchPhase}
[nix-shell]$ eval ${configurePhase:-configurePhase}
[nix-shell]$ eval ${buildPhase:-buildPhase}
[nix-shell]$ ./pan/gui/pan

The reason we use form eval ${configurePhase:-configurePhase} here is because those packages that override these phases do so by exporting the overridden values in the environment variable of the same name. Here bash is being told to either evaluate the contents of 'configurePhase', if it exists as a variable, otherwise evaluate the configurePhase function.

To clear the environment first, and do some additional automatic initialisation of the interactive shell:

$ nix-shell '<nixpkgs>' --attr pan --pure 
    --command 'export NIX_DEBUG=1; export NIX_CORES=8; return'

Nix expressions can also be given on the command line using the -E and -p flags. For instance, the following starts a shell containing the packages sqlite and libX11:

$ nix-shell --expr 'with import <nixpkgs> { }; runCommand "dummy" { buildInputs = [ sqlite xorg.libX11 ]; } ""'

A shorter way to do the same is:

$ nix-shell --packages sqlite xorg.libX11
[nix-shell]$ echo $NIX_LDFLAGS
… -L/nix/store/j1zg5v…-sqlite-3.8.0.2/lib -L/nix/store/0gmcz9…-libX11-1.6.1/lib …

Note that -p accepts multiple full nix expressions that are valid in the buildInputs = [ ... ] shown above, not only package names. So the following is also legal:

$ nix-shell --packages sqlite 'git.override { withManual = false; }'

The -p flag looks up Nixpkgs in the Nix search path. You can override it by passing -I or setting NIX_PATH. For example, the following gives you a shell containing the Pan package from a specific revision of Nixpkgs:

$ nix-shell --packages pan -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/8a3eea054838b55aca962c3fbde9c83c102b8bf2.tar.gz

[nix-shell:~]$ pan --version
Pan 0.139

Use as a #!-interpreter

You can use nix-shell as a script interpreter to allow scripts written in arbitrary languages to obtain their own dependencies via Nix. This is done by starting the script with the following lines:

#! /usr/bin/env nix-shell
#! nix-shell -i real-interpreter --packages packages

where real-interpreter is the “real” script interpreter that will be invoked by nix-shell after it has obtained the dependencies and initialised the environment, and packages are the attribute names of the dependencies in Nixpkgs.

The lines starting with nix-shell specify nix-shell options (see above). Note that you cannot write /usr/bin/env nix-shell -i ... because many operating systems only allow one argument in #! lines.

For example, here is a Python script that depends on Python and the prettytable package:

#! /usr/bin/env nix-shell
#! nix-shell -i python --packages python pythonPackages.prettytable

import prettytable

## Print a simple table.
t = prettytable.PrettyTable(["N", "N^2"])
for n in range(1, 10): t.add_row([n, n * n])
print t

Similarly, the following is a Perl script that specifies that it requires Perl and the HTML::TokeParser::Simple and LWP packages:

#! /usr/bin/env nix-shell
#! nix-shell -i perl --packages perl perlPackages.HTMLTokeParserSimple perlPackages.LWP

use HTML::TokeParser::Simple;

## Fetch nixos.org and print all hrefs.
my $p = HTML::TokeParser::Simple->new(url => 'http://nixos.org/');

while (my $token = $p->get_tag("a")) {
    my $href = $token->get_attr("href");
    print "$hrefn" if $href;
}

Sometimes you need to pass a simple Nix expression to customize a package like Terraform:

#! /usr/bin/env nix-shell
#! nix-shell -i bash --packages 'terraform.withPlugins (plugins: [ plugins.openstack ])'

terraform apply

Note

You must use single or double quotes (', ") when passing a simple Nix expression in a nix-shell shebang.

Finally, using the merging of multiple nix-shell shebangs the following Haskell script uses a specific branch of Nixpkgs/NixOS (the 20.03 stable branch):

#! /usr/bin/env nix-shell
#! nix-shell -i runghc --packages 'haskellPackages.ghcWithPackages (ps: [ps.download-curl ps.tagsoup])'
#! nix-shell -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/nixos-20.03.tar.gz

import Network.Curl.Download
import Text.HTML.TagSoup
import Data.Either
import Data.ByteString.Char8 (unpack)

-- Fetch nixos.org and print all hrefs.
main = do
  resp <- openURI "https://nixos.org/"
  let tags = filter (isTagOpenName "a") $ parseTags $ unpack $ fromRight undefined resp
  let tags' = map (fromAttrib "href") tags
  mapM_ putStrLn $ filter (/= "") tags'

If you want to be even more precise, you can specify a specific revision of Nixpkgs:

#! nix-shell -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/0672315759b3e15e2121365f067c1c8c56bb4722.tar.gz

The examples above all used -p to get dependencies from Nixpkgs. You can also use a Nix expression to build your own dependencies. For example, the Python example could have been written as:

#! /usr/bin/env nix-shell
#! nix-shell deps.nix -i python

where the file deps.nix in the same directory as the #!-script contains:

with import <nixpkgs> {};

runCommand "dummy" { buildInputs = [ python pythonPackages.prettytable ]; } ""