Debugging Nix Expressions

I think this is still undocumented, but running any nix command with the -L -L flag will print the logs to stdout.

Each derivation of a flake comes with a nix shell. That is:

nix develop .#packages.x86_64-linux.mypackage --ignore-env

nix develop .#packages.x86_64-linux.mypackage --ignore-env

will drop you into what is nix nix env in your local directory without the sandboxing effects.

You can then run each of the phases (note: this is still impure):

$configurePhase
$buildPhase
$checkPhase

$configurePhase
$buildPhase
$checkPhase

This can be useful for initially debugging nix expressions, and is straight out of the nix manual

As mentioned in this awesome blog post, breakpointHook breakpointHook and cntr cntr are useful for inspecting build outputs.

In particular, on Linux, inserting breakpointHook breakpointHook to the *inputs *inputs for any phase will run the phase to completion or error, then provide a command for dropping into a shell containing the artifacts of the phase.

This is of course useful for debugging the buildPhase buildPhase , but also can be used in other phases. For example, to inspect checkPhase checkPhase output, inserting breakpointHook breakpointHook into the checkInputs checkInputs will pause the build at the end of the checkPhase. This isn't documented, but is quite useful.

One subtlety: how does cntr fit into this? It turns out there's another undocumented hook called breakpointHookCntr breakpointHookCntr which will use cntr cntr to drop into the derivation and allow for inspection with dev tooling that is already on the PATH (though, it will have to be tooling that is in the nix store). Note that the given cntr cntr command will have to be run as root (I'm not sure how to give the CAP_SYS_CHROOT CAP_SYS_CHROOT capability to a normal user).