fix: support pure mode

[phanirithvij]

Previously

$ nix-shell
# prints menu
$ nix-shell --pure
/nix/store/...-devshell-dir/bin/menu: line 4: cat: command not found
bash: cat: command not found
$ nix-shell --pure --run 'exit'
/tmp/nix-shell-704775-1158748079/rc: line 1: rm: command not found

Now

$ nix-shell --pure
# prints menu
[devshell]$
$ nix-shell --pure --run 'exit'
$ echo $?
0

[phanirithvij]

Did this to avoid adding all of coreutils to the naked shell.

[phanirithvij]

builtins.getEnv "IN_NIX_SHELL" doesn't seems to be problematic from my cursory running of nix develop --ignore-environment --pure-eval nix-shell nix-shell --pure etc.

This issue occurs only for nix-shell --pure and I fixed it only for that case.

[phanirithvij]

This cat is fine. because of coreutils added to the path.

[zimbatm]

Do you mind sending a second PR with the changes to lib? Those changes are mostly mechanical and then I can review the meaty changes more easily.

When it comes to with lib;, I'm with you. I used to like with lib; for the convenience, but nowadays I prefer to be explicit always.

The only place where I sometimes still use the keyword is on a very limited scope, like with pkgs; [ curl gnused ].

Sometimes when the repetition of lib. or pkgs. prefix gets too much, I stick a inherit (lib) mkOption mkEnableOption; line at the top.

[phanirithvij]

Opened #360

[zimbatm]

thanks, merged

[zimbatm]

thanks!