Warning
This program is experimental and its interface is subject to change.

Name

nix repl - start an interactive environment for evaluating Nix expressions

Synopsis

nix repl [option...] installables...

Note: this command's interface is based heavily around installables, which you may want to read about first (nix --help).

Examples

  • Display all special commands within the REPL:

    # nix repl
nix-repl> :?

  • Evaluate some simple Nix expressions:

    # nix repl

nix-repl> 1 + 2
3

nix-repl> map (x: x * 2) [1 2 3]
[ 2 4 6 ]

  • Interact with Nixpkgs in the REPL:

    # nix repl --file example.nix
Loading Installable ''...
Added 3 variables.

# nix repl --expr '{a={b=3;c=4;};}'
Loading Installable ''...
Added 1 variables.

# nix repl --expr '{a={b=3;c=4;};}' a
Loading Installable ''...
Added 1 variables.

# nix repl --extra-experimental-features 'flakes repl-flake' nixpkgs
Loading Installable 'flake:nixpkgs#'...
Added 5 variables.

nix-repl> legacyPackages.x86_64-linux.emacs.name
"emacs-27.1"

nix-repl> legacyPackages.x86_64-linux.emacs.name
"emacs-27.1"

nix-repl> :q

# nix repl --expr 'import <nixpkgs>{}'

Loading Installable ''...
Added 12439 variables.

nix-repl> emacs.name
"emacs-27.1"

nix-repl> emacs.drvPath
"/nix/store/lp0sjrhgg03y2n0l10n70rg0k7hhyz0l-emacs-27.1.drv"

nix-repl> drv = runCommand "hello" { buildInputs = [ hello ]; } "hello; hello > $out"

nix-repl> :b drv
this derivation produced the following outputs:
  out -> /nix/store/0njwbgwmkwls0w5dv9mpc1pq5fj39q0l-hello

nix-repl> builtins.readFile drv
"Hello, world!\n"

nix-repl> :log drv
Hello, world!

Description

This command provides an interactive environment for evaluating Nix expressions. (REPL stands for 'read–eval–print loop'.)

On startup, it loads the Nix expressions named files and adds them into the lexical scope. You can load addition files using the :l <filename> command, or reload all files using :r.

Options

  • --stdin Read installables from the standard input. No default installable applied.

Common evaluation options:

  • --arg name expr Pass the value expr as the argument name to Nix functions.

  • --argstr name string Pass the string string as the argument name to Nix functions.

  • --debugger Start an interactive environment if evaluation fails.

  • --eval-store store-url The URL of the Nix store to use for evaluation, i.e. to store derivations (.drv files) and inputs referenced by them.

  • --impure Allow access to mutable paths and repositories.

  • --include / -I path Add path to the Nix search path. The Nix search path is initialized from the colon-separated NIX_PATH environment variable, and is used to look up the location of Nix expressions using paths enclosed in angle brackets (i.e., <nixpkgs>).

    For instance, passing

    -I /home/eelco/Dev
-I /etc/nixos

    will cause Lix to look for paths relative to /home/eelco/Dev and /etc/nixos, in that order. This is equivalent to setting the NIX_PATH environment variable to

    /home/eelco/Dev:/etc/nixos

    It is also possible to match paths against a prefix. For example, passing

    -I nixpkgs=/home/eelco/Dev/nixpkgs-branch
-I /etc/nixos

    will cause Lix to search for <nixpkgs/path> in /home/eelco/Dev/nixpkgs-branch/path and /etc/nixos/nixpkgs/path.

    If a path in the Nix search 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 consist of a single top-level directory. For example, passing

    -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/master.tar.gz

    tells Lix to download and use the current contents of the master branch in the nixpkgs repository.

    The URLs of the tarballs from the official nixos.org channels (see the manual page for nix-channel) can be abbreviated as channel:<channel-name>. For instance, the following two flags are equivalent:

    -I nixpkgs=channel:nixos-21.05
-I nixpkgs=https://nixos.org/channels/nixos-21.05/nixexprs.tar.xz

    You can also fetch source trees using flake URLs and add them to the search path. For instance,

    -I nixpkgs=flake:nixpkgs

    specifies that the prefix nixpkgs shall refer to the source tree downloaded from the nixpkgs entry in the flake registry. Similarly,

    -I nixpkgs=flake:github:NixOS/nixpkgs/nixos-22.05

    makes <nixpkgs> refer to a particular branch of the NixOS/nixpkgs repository on GitHub.

  • --override-flake original-ref resolved-ref Override the flake registries, redirecting original-ref to resolved-ref.

Common flake-related options:

  • --commit-lock-file Commit changes to the flake's lock file.

  • --inputs-from flake-url Use the inputs of the specified flake as registry entries.

  • --no-registries Don't allow lookups in the flake registries. This option is deprecated; use --no-use-registries.

  • --no-update-lock-file Do not allow any updates to the flake's lock file.

  • --no-write-lock-file Do not write the flake's newly generated lock file.

  • --output-lock-file flake-lock-path Write the given lock file instead of flake.lock within the top-level flake.

  • --override-input input-path flake-url Override a specific flake input (e.g. dwarffs/nixpkgs). This implies --no-write-lock-file.

  • --reference-lock-file flake-lock-path Read the given lock file instead of flake.lock within the top-level flake.

Logging-related options:

  • --debug Set the logging verbosity level to 'debug'.

  • --log-format format Set the format of log output; one of raw, internal-json, bar, bar-with-logs, multiline or multiline-with-logs.

  • --print-build-logs / -L Print full build logs on standard error.

  • --quiet Decrease the logging verbosity level.

  • --verbose / -v Increase the logging verbosity level.

Miscellaneous global options:

  • --help Show usage information.

  • --offline Disable substituters and consider all previously downloaded files up-to-date.

  • --option name value Set the Lix configuration setting name to value (overriding nix.conf).

  • --refresh Consider all previously downloaded files out-of-date.

  • --repair During evaluation, rewrite missing or corrupted files in the Nix store. During building, rebuild missing or corrupted store paths.

  • --version Show version information.

Options that change the interpretation of installables:

  • --expr / -E expr Interpret installables as attribute paths relative to the Nix expression expr.

  • --file / -f file Interpret installables as attribute paths relative to the Nix expression stored in file. If file is the character -, then a Nix expression will be read from standard input. Implies --impure.

Note

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