nix-shell: extend the meaning of $IN_NIX_SHELL

[vcunat]

... to allow detecting --pure, e.g. in NixOS/nixpkgs#15571 (comment)

[vcunat]

I tested the variable really is defined. Someone skilled in perl would better read the change...

[solson]

I'd like to bump this to make sure it doesn't get missed. It seems like a quick thing for the right person to review.

I had to work around the SSL_CERT_FILE problem in multiple contexts and everyone I know who uses nix has had to deal with it, and the sooner this goes through, the sooner that gets solved, right?

The worst case was that the nixos-unstable install CD required setting SSL_CERT_FILE to use the binary cache.

EDIT: I suppose the latter case is somewhat unrelated and should be covered by NixOS/nixpkgs#15579.

[vcunat]

@edolstra: this PR is very simple.

[edolstra]

This breaks Nixpkgs, which has two checks for $IN_NIX_SHELL being equal to 1:

lib/trivial.nix:  inNixShell = builtins.getEnv "IN_NIX_SHELL" == "1";
pkgs/development/tools/pypi2nix/default.nix:    if [ "$IN_NIX_SHELL" != "1" ]; then

[vcunat]

@edolstra: I fixed that up in nixpkgs master NixOS/nixpkgs@d4de02c46. It was trivial; unfortunately the notification for your answer was missed by me somehow.

[domenkozar]

Note that nix-shell has been meanwhile rewritten to C++: #1018

[vcunat]

Still, this PR would be a trivial merge for the meantime before we get perl-free.

[domenkozar]

Pretty sure next Nix release will be perl free, all scripts have been ported ("just" need review): #341

[vcunat]

C++ equivalent implemented: shlevy#2. I'm actually much more comfortable with C(++) than Perl.

[vcunat]

My experience with nix repo is that "just review and merge" tends to take significantly more real time than actual implementation.

[domenkozar]

@vcunat yes, that's because Eelco maintains software that a few thousand people use.

It's also the reason I talked to Shea about the perl port, to get the bus factor down.

We'll improve this, but it's a slow process. I'll help when I can :)

[Ericson2314]

Only this PR, and not the perl-to-C++ one, has the documentation update.

[yurrriq]

I just came across this accidentally. Any chance we can get this updated and merged? Seems like a cool feature.

[vcunat]

The feature is present in nix >= 2.0 via 1bffd83.