Call nix-shell with path (e.g., to use the latest NixOS/nixpkgs clone)

First of all, pinning this here because I rarely use nix-shell without a shell.nix file, so I keep forgetting the syntax (plus, the syntax of Nix’s command line tools is wildly inconsistent ), and spend extra time finding this tweet:

I shared my screen with a coworker and during an exploratory session I wanted to demonstrate two new operators in Node v14 (?. and ??), so I casually did:

nix-shell -I nixpkgs=channel:nixos-unstable -p nodejs-14_x

Now coworker’s reaction is mindblown.

(Twitter post from Susan Potter)

Rationale

It is faster (as it uses a local Nix expression; in this case, the NixOS/nixpkgs clone), and some of the new packages are only present there (or you’re using a branch other than the channel ones, etc.).

Specify (Nix expression) path for nix-env with -f

The output without the path:

$ nix-env -qaP 'erlang*'
# ...
nixos.erlangR20            erlang-20.3.8.9
nixos.erlangR21            erlang-21.3.8.3
nixos.erlang               erlang-22.1.7
# ...

$ nix-env -f ~/clones/nixpkgs/ -qaP 'erlang*'
# ...
nixos.erlangR20            erlang-20.3.8.9
nixos.erlangR21            erlang-21.3.8.3
nixos.erlang               erlang-22.1.7
# ...
=== >>> erlangR23            erlang-23.0.2  <<<====

Use -I nixpgks=[path] with nix-shell for the same effect

nix-shell does not have a -f switch, and NixOS/nix issue #459 has been raised for just this reason. The current workaround is to use -I.

$ nix-shell -p erlangR23
error: undefined variable 'erlangR23' at (string):1:94
(use '--show-trace' to show detailed location information)

0 [18:28:26] nix-shell -I nixpkgs=~/clones/nixpkgs -p erlangR23

[nix-shell:~/clones/toraritte.github.io]$

Inconsistencies with -I

I assumed that nix-env’s -f and -I thus would be redundant, but the latter doesn’t seem to use the given path (or uses it differently) than nix-shell:

$ nix-env -I nixpkgs=~/clones/nixpkgs -qaP 'erlang*'
# ...
nixos.erlang_odbc_javac    erlang-22.1.7-javac-odbc
nixos.erlang_odbc          erlang-22.1.7-odbc

$ nix-env -f ~/clones/nixpkgs/ -qaP 'erlang*'
# ...
erlang_odbc_javac    erlang-22.3-javac-odbc
erlang_odbc          erlang-22.3-odbc
erlangR23            erlang-23.0.2

From man nix-env:

–file / -f path Specifies the Nix expression (designated below as the active Nix expression) used by the –install, –upgrade, and –query –available operations to obtain derivations. The default is ~/.nix-defexpr.

If the argument 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
include a single top-level directory containing at least a
file named default.nix.

(…)

-I path Add a path to the Nix expression search path. This option may be given multiple times. See the NIX_PATH environment variable for information on the semantics of the Nix search path. Paths added through -I take precedence over NIX_PATH.

The definition of -I in man nix-shell is the same.