nix-output-monitor

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

While your build runs nom will draw something like this at the bottom of your build log:

Status

This was an experimental fun project, which proofed to be useful to quite a lot of people. 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) support for v2 output may come in the future.

Installing

• nixpkgs: nom is in nixpkgs. Just install pkgs.nix-output-monitor in the usual way. You might want to install it from nixos-unstable to get the newest version.
• 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.

Running

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

Explanation

Legend

Nom tries to convey informations via symbols and colors

• ▶, yellow: running builds
• ✔, green: completed builds
• ⏳︎︎︎, blue: planned builds
• ⬇, cyan: downloads often in the form completed/total
• ⬆, magenta: uploads
• ⚠, red: failed builds
• ∅: a moving average over past builds of this derivation
• ⏱︎: running time
• ∑: 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". Also different terminals might work differently well. I recommend: pkgs.foot.

How to Read the Dependency Graph

• nom will sort all builds into a tree.
• Children of a node are direct dependencies.
• nom will try to show you the most relevant part of the dependency tree, roughly aiming to fill a third of your terminal
• No build will be printed twice in the tree, it will only be shown for the lower most dependency.
• Everytime nom decides to not show all direct dependencies of a build (and for root nodes), it will print a & and a summary over the build state of all dependencies.
• Use the colors from above to read the summary

Example Runs

An example remote build:

An example failing remote build:

An example really large build (haskell-language-server):

An example running sudo nixos-rebuild switch:

Implementation

Right now nom uses four 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 $XDG_CACHE_HOME/nix-output-monitor/build-reports.csv.

Limitations

• 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!

• nix-build does not show info when a download or upload is finished, so we currently cannot differentiate between started and completed downloads.

• remote builds will sometimes be shown as running even when they are actually still waiting for uploads or downloads.

• 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.