Misunderstanding on page /tutorials/first-steps/reproducible-scripts.html

[TNTima]

#!/usr/bin/env nix-shell
#! nix-shell -i bash --pure
#! nix-shell -p bash cacert curl jq python3Packages.xmljson
#! nix-shell -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/2a601aafdc5605a5133a2ca506a34a3a73377247.tar.gz

curl https://github.com/NixOS/nixpkgs/releases.atom | xml2json | jq .

I'm newcomer to nix and I don't understand why nix channel is changed after imports.
Is it okay to post misunderstandings here?

[fricklerhandwerk]

Hi @TNTima, thanks for asking!

Is it okay to post misunderstandings here?

Yes of course, anything that helps us improve tutorials is highly appreciated.

I don't understand why nix channel is changed after imports.

I'm not sure I got your question right. Do you mean the -I nixpkgs=... part? That one is explained in one of the following paragraphs:

The parameter of -I refers to a specific Git commit of the Nixpkgs repository. This ensures that the script will always run with the exact same packages versions, everywhere.

But maybe I just misunderstood your misunderstanding... :)

[TNTima]

Sorry for wait, ping is huge here. What I don't understand why -I line goes before -p line. In my head this means source would be changed after imports already happened.

[fricklerhandwerk]

Ah, so these #! nix-shell lines are somewhat unusual syntax: it's not a series of commands but an accumulation of command-line arguments to the invocation of nix-shell -- their order doesn't matter. You could also write #! /usr/bin/env nix-shell -i bash -p ... -I ... (if your OS supported it) and that would mean the exact same thing.

The explanation is here: https://nix.dev/manual/nix/2.24/command-ref/nix-shell.html#use-as-a--interpreter

The lines starting with #! nix-shell specify nix-shell options (see above). Note that you cannot write #! /usr/bin/env nix-shell -i ... because many operating systems only allow one argument in #! lines.

But both places could use a brief note to clarify that further.