nix-shell(1)

Name

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. Note that the shell used to run commands is obtained from NIX_BUILD_SHELL (#env-NIX_BUILD_SHELL) / <nixpkgs> from NIX_PATH, and therefore not affected by --pure.

•   --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.

{{#include ./opt-common.md}}

Environment variables

•   NIX_BUILD_SHELL (#env-NIX_BUILD_SHELL)

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

        Note

        The shell obtained using this method may not necessarily be the same as any shells requested in path.

        **Example

        Despite --pure, this invocation will not result in a fully reproducible shell environment:

        #!/usr/bin/env -S nix-shell --pure
        let
          pkgs = import (fetchTarball "https://github.com/NixOS/nixpkgs/archive/854fdc68881791812eddd33b2fed94b954979a8e.tar.gz") {};
        in
        pkgs.mkShell {
          buildInputs = pkgs.bashInteractive;
        }

{{#include ./env-common.md}}

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 python3 --packages python3 python3Packages.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 = $token->get_attr("href");
        print "$href\n" 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 ]; } ""

The script’s file name is passed as the first argument to the interpreter specified by the -i flag.

Aside from the very first line, which is a directive to the operating system, the additional #! nix-shell lines do not need to be at the beginning of the file. This allows wrapping them in block comments for languages where # does not start a comment, such as ECMAScript, Erlang, PHP, or Ruby.