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] fileish…

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 fileish is the character -, then a Nix expression will be read from standard input. Otherwise, each fileish is interpreted the same as with nix-build. See that section for complete details (nix-build --help), but in summary, a path argument may be one of:

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. The output format of the AST depends on the current internal representation and may change in the future.

Tooling can use the stderr and exit code of --parse to check any Nix code for correctness, but should not rely on stdout without careful versioning. Note that --parse also checks for unbound variables. In cases where this is undesired, with {}; can be prepended to the program to transform all such parse errors into eval errors.
--eval
Just parse and evaluate the input files, and print the resulting values on standard output. No instantiation of store derivations takes place.
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 result must be coercible to a string, i.e., something that can be converted using ${...}.

Integers will always generate an error when output via --raw, regardless of coerce-integers being enabled, to avoid ambiguity.

The output is printed exactly as-is, with no quotes, escaping, or trailing newline.
--json
When used with --eval, print the resulting value as an JSON representation of the resulting value rather than as a Nix expression.

The conversion behaviour, if --strict is passed, is the same as builtins.toJSON.
--xml
When used with --eval, print the resulting value as an XML representation of the resulting value rather than as a Nix expression. The schema is the same as that used by builtins.toXML.
--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.

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>