Pipe your nix-build output through the nix-output-monitor (aka nom) to get additional information while building.

Packaging status


This is an experimental fun project. The purpose of it is to write something fun and useful in Haskell. You are free and very welcome to contribute feedback, issues or PRs, but I do not commit to maintain this project over a long time period. This program relies on the output of the v1 nix commands (i.e. nix-build and not nix build) and will hopefully one day be obsolete.

Best case scenario: This could serve as inspiration as to how to improve nix output in the future.


  • nixpkgs: nom is in nixpkgs/nixos unstable just install pkgs.nix-output-monitor in the usual way.
  • cabal: Install cabal-install and run cabal install in the checked out repo.
  • nix: or run nix-build or nix-env or include the default.nix of this repo in your nixos config.


The Easy Way

When installed from nixpkgs you can replace every call to nix-build with the bundled wrapper script nom-build.

The Flexible Way

Once you have installed nix-output-monitor to your path, run any nix command (nixos-rebuild,nix-build,home-manager switch, not nix build.) and pipe stderr and stdout into nom.

nix-build |& nom

Don‘t forget to redirect stderr, too. That's what the &, does.

Preserving Colored Text

To preserve the color of the redirected text you can use the unbuffer command from the expect package. (The nom-build wrapper does this for you.)

unbuffer nix-build |& nom

Icons / Legend

running = "▶" -- yellow, running builds
done = "✔" -- green, completed builds
todo = "⏳" -- blue, planned builds/downloads
down = "⬇" -- green, downloads (running and finished sadly in one number)
up = "⬆" -- green, uploads
warning = "⚠" -- failed build
goal = "🏁" -- the final build of the running command
average = "∅" -- a moving average over past builds of this derivation
bigsum = "∑" -- a summary over all packages and hosts

If you can‘t see all icons you maybe need another terminal font. I recommend any font from pkgs.nerdfonts e.g. "JetBrainsMono Nerd Font".

Example Run

With some local builds and downloads:


With more builds on a remote server:



The program consists of three steps in a loop, powered by Haskell lazyness.

  1. parsing (If you have suggestion for improving my parser combinators, I am all ears!)
  2. updating state (with IO to look into the the nix store.)
  3. printing

Right now it uses three sources of information:

  1. The parsed nix-build output
  2. it checks if build results exist in the nix-store
  3. it querys .drv files for information about the out output path.
  4. It caches build times in ~/.cache/nix-output-monitor/build-reports.csv.


This will fail in unexpected and expected ways. nix-output-monitor receives most it's information from parsing nix-build output. The parser might be to strict or to loose for use cases I didn‘t think of. Then the numbers displayed will be off!

Terminal clearing and reprinting is brittle. It might fail with your terminal or terminal width. But at this point I‘ve invested some effort to make it usable. This program also makes assumptions like your nix-store is at "/nix/store" or that every derivation has an output at "out".

Luckily I don‘t think this program screws up anything more than your terminal.


The biggest strength of nom are ovbiously the colorful unicode symbols! (If you don‘t agree I accept PRs to making that configurable.)

nom ignores any output it doesn‘t recognize and will always print out everything it receives. So you can never loose information (besides coloring of nix output).

nom does not assume that you run exactly one nix-build. If you run e.g. a script running multiple builds it will aggregate the information of all of them.