nix3-repl(1)

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…

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

•   --debugger
    start an interactive environment if evaluation fails

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.

•   --eval-store store-url
    The Nix store to use for evaluations.

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

•   --include / -I path
    Add path to the list of locations used to look up <...> file names.

•   --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.

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

•   --recreate-lock-file
    Recreate the flake’s lock file from scratch.

•   --update-input input-path
    Update a specific flake input (ignoring its previous entry in the lock file).

Options that change the interpretation of installables:

•   --derivation
    Operate on the store derivation rather than its outputs.

•   --expr 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.