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Finally figured out how to set up my systems a couple months ago, and have been using them happily ever since, but realized that it's time to update stuff. My NixOS setup involves declarative package management using Home Manager, but completely forgot the basics and wasn't sure if Home Manager would complicate things as the repo only mentions the home-manager tool which I don't have.

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TL;DR

  1. Are you on the latest channel?
    (See how to check and set below.)

  2. sudo nix-channel --update

  3. a. (declarative/NixOS) sudo nixos-rebuild switch
    b.                   (ad-hoc) nix-env -u '*'
    c.     (Home Manager) home-manager switch (I think)

These steps should work regardless of using NixOS or not.

NOTE-1 (flakes): It's a fairly good bet that none of this will work if you are using flakes.

NOTE-2 (NixOS + Home Manager): If nixos-rebuild switch fails with cryptic error messages, look at your Home Manager config. At least, in the past 5 years, whenever the error wasn't clear (e.g., nix.maxJobs deprecated in favor of nix.settings.max-jobs), it was always something about Home Manager.

Step 0. Check and set the latest channel

Step 1.'s nix-channel --update will only pull the changes in the channel that is currently set! To check the current one you are subscribed to, issue

sudo nix-channel --list

The latest NixOS manual's Chapter 4. Upgrading NixOS will always show the name and the link of the latest channel. The Nix Channel Status page is also a very good resource.

For example, I was subscribed to channel 20.09 so I needed to update it to 21.11 by doing

$ sudo nix-channel --list
nixos https://nixos.org/channels/nixos-20.09

$ sudo nix-channel --remove nixos
$ sudo nix-channel --add https://nixos.org/channels/nixos-21.11 nixos

More info:

  • The Nix manual's 4.4 Channels section is a short intro to Nix channels.

  • The NixOS wiki's Nix channels article goes more into the details and has a good summary on the basic commands. (Personally, I found this one more useful.)

Step 1. Update the channel

This is required both for declarative and "ad-hoc" [sic] setups3:

sudo nix-channel --update

Even though the Nix manual has a 4.1 Basic Package Management section, it is best documented in the NixOS manual (see Chapter 7. Package Management). (Albeit, I didn't find it straightforward figuring out the correct order of steps from these; the biggest help was this NixOS discourse thread.)

Home Manager does not seem to complicate this process. (At least, I'm using it on NixOS, and nixos-rebuild switch always takes care of everything with my setup.)

TIP FOR NIXOS USERS
The nixos-rebuild switch --upgrade combines Step 1. and Step 2. as it "is equivalent to the more verbose nix-channel --update nixos; nixos-rebuild switch".

Step 2. Update the packages

The two adjectives, "ad-hoc" [sic] and "declarative", are introduced in the NixOS manual to differentiate the two modes of Nix package management3:

  • "ad-hoc": Managing packages with nix-env.1

  • "declarative": In the NixOS manual, this refers to declaring packages as a list in NixOS' configuration.nix, but it can be done in many other ways2.

The commands:

a. To update all packages declared in NixOS' configuration.nix, use:

sudo nixos-rebuild switch

b. To update all packages installed with nix-env:

nix-env -u '*'

c. To update all packages installed using Home Manager:

home-manager switch

(I think. Again, I'm using Home Manager in NixOS, and never had to touch any Home Manager commands. See Home-manager equivalent of “apt upgrade”)


Footnotes

[1]: As far as I know, the NixOS manual is the only official document that uses the term "ad-hoc [sic] package management" and only to mean package management using nix-env.

(Therefore I find the term misleading, because using packages via nix-shell -p commands or custom shell.nix-es are also ad hoc forms of package management. Also, nix-env can be messy.)

[2]: For example with Home Manager, using shell.nix-es, flakes. (It is probably incorrect to list flakes, as it is a mechanism that can be used with Home Manager and nix-shell Nix expressions...)

[3]: Not sure if there is a consensus what "ad-hoc", "declarative", and "imperative" package management means in Nix; footnote 1 has my thoughts about "ad-hoc", some say that using channels is always an imperative form of package management, so "declarative" seems to be overloaded...

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