nix/doc/manual/source/command-ref/nix-instantiate.md

Name

nix-instantiate - instantiate store derivations from Nix expressions

Synopsis

nix-instantiate [--parse | --eval [--strict] [--raw | --json | --xml] ] [--read-write-mode] [--arg name value] [{--attr| -A} attrPath] [--add-root path] [--expr | -E] files…

nix-instantiate --find-file files…

Description

The command nix-instantiate produces store derivations from (high-level) Nix expressions. It evaluates the Nix expressions in each of files (which defaults to ./default.nix). Each top-level expression should evaluate to a derivation, a list of derivations, or a set of derivations. The paths of the resulting store derivations are printed on standard output.

If files is the character -, then a Nix expression will be read from standard input.

Options

• --add-root path

  See the corresponding option in nix-store.

• --parse

  Just parse the input files, and print their abstract syntax trees on standard output as a Nix expression.

• --eval

  Just parse and evaluate the input files, and print the resulting values on standard output. Store derivations are not serialized and written to the store, but instead just hashed and discarded.

  Warning

  This option produces output which can be parsed as a Nix expression which will produce a different result than the input expression when evaluated. For example, these two Nix expressions print the same result despite having different meaning:

  $ nix-instantiate --eval --expr '{ a = {}; }'
  { a = <CODE>; }
  $ nix-instantiate --eval --expr '{ a = <CODE>; }'
  { a = <CODE>; }
  

  For human-readable output, nix eval (experimental) is more informative:

  $ nix-instantiate --eval --expr 'a: a'
  <LAMBDA>
  $ nix eval --expr 'a: a'
  «lambda @ «string»:1:1»
  

  For machine-readable output, the --xml option produces unambiguous output:

  $ nix-instantiate --eval --xml --expr '{ foo = <CODE>; }'
  <?xml version='1.0' encoding='utf-8'?>
  <expr>
    <attrs>
      <attr column="3" line="1" name="foo">
        <unevaluated />
      </attr>
    </attrs>
  </expr>
  

• --find-file

  Look up the given files in Nix’s search path (as specified by the NIX_PATH environment variable). If found, print the corresponding absolute paths on standard output. For instance, if NIX_PATH is nixpkgs=/home/alice/nixpkgs, then nix-instantiate --find-file nixpkgs/default.nix will print /home/alice/nixpkgs/default.nix.

• --strict

  When used with --eval, recursively evaluate list elements and attributes. Normally, such sub-expressions are left unevaluated (since the Nix language is lazy).

  Warning

  This option can cause non-termination, because lazy data structures can be infinitely large.

• --raw

  When used with --eval, the evaluation result must be a string, which is printed verbatim, without quoting, escaping or trailing newline.

• --json

  When used with --eval, print the resulting value as an JSON representation of the abstract syntax tree rather than as a Nix expression.

• --xml

  When used with --eval, print the resulting value as an XML representation of the abstract syntax tree rather than as a Nix expression. The schema is the same as that used by the toXML built-in.

• --read-write-mode

  When used with --eval, perform evaluation in read/write mode so nix language features that require it will still work (at the cost of needing to do instantiation of every evaluated derivation). If this option is not enabled, there may be uninstantiated store paths in the final output.

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

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

Examples

Instantiate store derivations from a Nix expression, and build them using nix-store:

$ nix-instantiate test.nix (instantiate)
/nix/store/cigxbmvy6dzix98dxxh9b6shg7ar5bvs-perl-BerkeleyDB-0.26.drv

$ nix-store --realise $(nix-instantiate test.nix) (build)
...
/nix/store/qhqk4n8ci095g3sdp93x7rgwyh9rdvgk-perl-BerkeleyDB-0.26 (output path)

$ ls -l /nix/store/qhqk4n8ci095g3sdp93x7rgwyh9rdvgk-perl-BerkeleyDB-0.26
dr-xr-xr-x    2 eelco    users        4096 1970-01-01 01:00 lib
...

You can also give a Nix expression on the command line:

$ nix-instantiate --expr 'with import <nixpkgs> { }; hello'
/nix/store/j8s4zyv75a724q38cb0r87rlczaiag4y-hello-2.8.drv

This is equivalent to:

$ nix-instantiate '<nixpkgs>' --attr hello

Parsing and evaluating Nix expressions:

$ nix-instantiate --parse --expr '1 + 2'
1 + 2

$ nix-instantiate --eval --expr '1 + 2'
3

$ nix-instantiate --eval --xml --expr '1 + 2'
<?xml version='1.0' encoding='utf-8'?>
<expr>
  <int value="3" />
</expr>

The difference between non-strict and strict evaluation:

$ nix-instantiate --eval --xml --expr '{ x = {}; }'
<?xml version='1.0' encoding='utf-8'?>
<expr>
  <attrs>
    <attr column="3" line="1" name="x">
      <unevaluated />
    </attr>
  </attrs>
</expr>

$ nix-instantiate --eval --xml --strict --expr '{ x = {}; }'
<?xml version='1.0' encoding='utf-8'?>
<expr>
  <attrs>
    <attr column="3" line="1" name="x">
      <attrs>
      </attrs>
    </attr>
  </attrs>
</expr>