How to find the commit a Nix channel points to?

Question

When trying to revive old projects, sometimes I have to fish around for older Nixpkgs commits to get things started.

For example, most of my shell.nix files start like this,

{pkgs ? import <nixpkgs> {} }:

pkgs.mkShell {
  # ...
}

that would import all expressions from the commit where the current channel points to, but after some time, there have been so many changes that my project won't build anymore; then I will have to find a commit that still works, and go from there.

$ nix-shell --arg pkgs \
> 'import (fetchTarball "https://github.com/NixOS/nixpkgs/archive/0c0fe6d85b92c4e992e314bd6f9943413af9a309.tar.gz") {}'

It would probably be the best if I could just save the working commit from the channel at the time, and have something solid to fall back to when needed later.

Answer 1

To expand on the answer provided in the NixOS discourse thread How to see what commit is my channel on?:

Note: Before trying any of these methods, make sure that you are issuing commands with the right user!
No harm done either way, but if you are on NixOS and manage things declaratively (as root when rebuilding the system) then you might get a different commit hash than what you need. (Especially if you ever issued nix-channel --update without sudo, which will set up a channel for your user profile as well.)

Method 0

According to the NixOS wiki's Nix channels entry, "a channel is a name for the latest "verified" git commits in Nixpkgs". That is, at any given time, a channel points to a specific commit in the Nixpkgs git repository on Github; each Nix channel is actually a git branch in the repo:

$ nix-channel --list
nixos https://nixos.org/channels/nixos-20.09
-----                            ----------- 
(name)                          (branch-name)
#                                     |
#                                     V
#              https://github.com/NixOS/nixpkgs/tree/<branch-name>
#         i.e. https://github.com/NixOS/nixpkgs/tree/nixos-20.09

So if you just executed nix-channel --update before nix-shell, and it works, just look up the last commit in the channel branch in the Nixpkgs repo.

Method 1

"Chapter 12. Channels" of the Nix manual mentions that nix-channel --update "makes the union of each channel’s Nix expressions available by default to nix-env operations (via the symlink ~/.nix-defexpr/channels)".

To see where the ~/.nix-defexpr/channels symlink points to, use readlink -f to follow the symlink chain and combine it with ls to get straight to the point:

$ ls -l $(readlink -f ~/.nix-defexpr/channels)
total 6432
dr-xr-xr-x    2 root root      4096 Jan  1  1970 ./
drwxrwxr-t 8191 root nixbld 6569984 Feb  9 15:51 ../
lrwxrwxrwx    1 root root        78 Jan  1  1970 nixos -> /nix/store/k737c631q19n54fhjmnf68frg5dar14w-nixos-20.09.3009.8e78c2cfbae/nixos/
lrwxrwxrwx    1 root root        60 Jan  1  1970 manifest.nix -> /nix/store/a5wl1fri2sasnsb1i5zscni5h7kjg7d6-env-manifest.nix

My channel's name is nixos, and it points to

/nix/store/k7..4w-nixos-20.09.3009.8e78c2cfbae/nixos/
                                   -----------
                                        ^
                                        |
                                   channel-commit

and the commit hash is right after the MAJOR.MINOR.PATCH version number.

---

Aside: To construct the tarball URL for fetchTarball in the question, use the following template:

https://github.com/<user>/<repo>/archive/<full-or-abbr-commit-hash>.tar.gz

For example:

https://github.com/NixOS/nixpkgs/archive/8e78c2cfbae.tar.gz

Alternatively, click the green "Code" button, and copy the URL of the "Download ZIP" link (and change the zip extension to tar.gz).

---

Fun fact: if you did nix-channel --update before method 1, then URLs https://github.com/NixOS/nixpkgs/tree/<branch-name> and https://github.com/NixOS/nixpkgs/tree/<channel-commit> will point to the same place in the Nixpkgs repo.

Answer 2

>= nixos 20.09

nix-shell -p nix-info --run "nix-info -m"

• system: "x86_64-linux"
• host os: Linux 5.9.15, NixOS, 20.09.2344.a3a3dda3bac (Nightingale) # channel-commit
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.3.9
• channels(root): "nixos-20.09.2344.a3a3dda3bac" # channel-commit
• channels(user): "nixos-20.09.2152.e34208e1003" # channel-commit
• nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos

---

check the ending e.g. a3a3dda3bac in the NixOS history like release 20.09

---

I would not use pkgs ? import <nixpkgs> {} for developing environments which are not tied to the current system you are running on

• instead import e.g. in let area via a variable

for developing environments:

# nix-shell shell_test.nix --argstr commitNixpkgs "0c0fe6d85b92c4e992e314bd6f9943413af9a309 --show-trace

{pkgs ? import <nixpkgs> {},  commitNixpkgs ? "0c0fe6d85b92c4e992e314bd6f9943413af9a309" ; ref ? "refs/heads/nixpkgs-unstable", rev ? "502845c3e31ef3de0e424f3fcb09217df2ce6df6" , ...}:

let 
    nixPkgs = import (fetchTarball "https://github.com/NixOS/nixpkgs/archive/${commitNixpkgs}.tar.gz") {config= { allowUnfree = true ; } ;} ; 
    # https://channels.nix.gsc.io/
    # git ls-remote https://github.com/nixos/nixpkgs | grep unstable
 
    # or via fetchGit 
    nixPkgs_fG = import (builtins.fetchGit {
       name = "nixpkgs-unstable";
       url = "https://github.com/nixos/nixpkgs-channels.git";
       ref = ${ref};        #"refs/heads/nixpkgs-unstable";
       rev = ${rev};        #"502845c3e31ef3de0e424f3fcb09217df2ce6df6"; 
   }) {config= { allowUnfree = true ; } ;  } ;
   # nix-shell -p nix-prefetch-git --run ' nix-prefetch-git https://github.com/nixos/nixpkgs.git refs/heads/nixos-unstable '

in 
  ...    

Answer 3

To get the full Git revision, run cat /nix/var/nix/profiles/per-user/root/channels/nixpkgs/.git-revision; echo (there's no newline at EOF, so we need to add one to avoid the prompt overwriting it). If that path doesn't exist you're probably using a different channel name, and need to specify it. Don't use a glob unless you're absolutely sure there's only ever one channel.

Then run nix-prefetch-url --type sha256 --unpack "https://github.com/nixos/nixpkgs/archive/$(cat /nix/var/nix/profiles/per-user/root/channels/nixpkgs/.git-revision).tar.gz" to get the SHA-256 hash.