Example: `--expr 'import <nixpkgs> {}' hello`

When the option `--expr` *expression* \[*attrpath*...\] is given, installables are interpreted as the value of the of the Nix expression.
If attribute paths are provided, commands will operate on the corresponding values accessible at these paths.
The Nix expression, or any selected attribute, must evaluate to a derivation.

You may need to specify `--impure` if the expression references impure inputs (such as `<nixpkgs>`).

## Derivation output selection

Derivations can have multiple outputs, each corresponding to a
different store path. For instance, a package can have a `bin` output
that contains programs, and a `dev` output that provides development
artifacts like C/C++ header files. The outputs on which `nix` commands
operate are determined as follows:

* You can explicitly specify the desired outputs using the syntax *installable*`^`*output1*`,`*...*`,`*outputN* — that is, a caret followed immediately by a comma-separated list of derivation outputs to select.
  For installables specified as [Flake output attributes](#flake-output-attribute) or [Store paths](#store-path), the output is specified in the same argument:

  For example, you can obtain the `dev` and `static` outputs of the `glibc` package:

  ```console
  # nix build 'nixpkgs#glibc^dev,static'
  # ls ./result-dev/include/ ./result-static/lib/
  …
  ```

  and likewise, using a store path to a "drv" file to specify the derivation:

  ```console
  # nix build '/nix/store/gzaflydcr6sb3567hap9q6srzx8ggdgg-glibc-2.33-78.drv^dev,static'
  …
  ```

  For `--expr` and `-f`/`--file`, the derivation output is specified as part of the attribute path:

  ```console
  $ nix build -f '<nixpkgs>' 'glibc^dev,static'
  $ nix build --impure --expr 'import <nixpkgs> { }' 'glibc^dev,static'
  ```

  This syntax is the same even if the actual attribute path is empty:

  ```console
  $ nix build --impure --expr 'let pkgs = import <nixpkgs> { }; in pkgs.glibc' '^dev,static'
  ```

* You can also specify that *all* outputs should be used using the
  syntax *installable*`^*`. For example, the following shows the size
  of all outputs of the `glibc` package in the binary cache:

  ```console
  # nix path-info --closure-size --eval-store auto --store https://cache.nixos.org 'nixpkgs#glibc^*'
  /nix/store/g02b1lpbddhymmcjb923kf0l7s9nww58-glibc-2.33-123                 33208200
  /nix/store/851dp95qqiisjifi639r0zzg5l465ny4-glibc-2.33-123-bin             36142896
  /nix/store/kdgs3q6r7xdff1p7a9hnjr43xw2404z7-glibc-2.33-123-debug          155787312
  /nix/store/n4xa8h6pbmqmwnq0mmsz08l38abb06zc-glibc-2.33-123-static          42488328
  /nix/store/q6580lr01jpcsqs4r5arlh4ki2c1m9rv-glibc-2.33-123-dev             44200560