# 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*]} # 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 ```nix shellHook = '' echo "Hello shell" ''; ``` will cause `nix-shell` to print `Hello shell`. # 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 (depending on your Bash installation) `/etc/bashrc` is still sourced, so any variables set there will affect the interactive shell. - `--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 -p 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. The following common options are supported: # Environment variables - `NIX_BUILD_SHELL` Shell used to start the interactive environment. Defaults to the `bash` found in `PATH`. # Examples To build the dependencies of the package Pan, and start an interactive shell in which to build it: ```console $ nix-shell '' -A pan [nix-shell]$ unpackPhase [nix-shell]$ cd pan-* [nix-shell]$ configurePhase [nix-shell]$ buildPhase [nix-shell]$ ./pan/gui/pan ``` To clear the environment first, and do some additional automatic initialisation of the interactive shell: ```console $ nix-shell '' -A 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`: ```console $ nix-shell -E 'with import { }; runCommand "dummy" { buildInputs = [ sqlite xorg.libX11 ]; } ""' ``` A shorter way to do the same is: ```console $ nix-shell -p 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: ```console $ nix-shell -p 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: ```console $ nix-shell -p 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: ```bash #! /usr/bin/env nix-shell #! nix-shell -i real-interpreter -p 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: ```python #! /usr/bin/env nix-shell #! nix-shell -i python -p 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: ```perl #! /usr/bin/env nix-shell #! nix-shell -i perl -p 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: ```bash #! /usr/bin/env nix-shell #! nix-shell -i bash -p "terraform.withPlugins (plugins: [ plugins.openstack ])" terraform apply ``` > **Note** > > You must use 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 18.03 stable branch): ```haskell #! /usr/bin/env nix-shell #! nix-shell -i runghc -p "haskellPackages.ghcWithPackages (ps: [ps.HTTP ps.tagsoup])" #! nix-shell -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/nixos-18.03.tar.gz import Network.HTTP import Text.HTML.TagSoup -- Fetch nixos.org and print all hrefs. main = do resp <- Network.HTTP.simpleHTTP (getRequest "http://nixos.org/") body <- getResponseBody resp let tags = filter (isTagOpenName "a") $ parseTags body 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: ```python #! /usr/bin/env nix-shell #! nix-shell deps.nix -i python ``` where the file `deps.nix` in the same directory as the `#!`-script contains: ```nix with import {}; runCommand "dummy" { buildInputs = [ python pythonPackages.prettytable ]; } "" ```