nix-debugging

Nix Debugging

Common Errors and Fixes

Infinite Recursion

error: infinite recursion encountered

Causes:

• Using rec { } where an attribute references itself circularly
• Overlay using final where prev is needed (or vice versa)
• NixOS module that sets an option it also reads without mkIf

Debug: Add builtins.trace calls to narrow down which attribute triggers it. In overlays, ensure you use prev.pkg for the package being modified and final.dep for dependencies.

Hash Mismatch

error: hash mismatch in fixed-output derivation
  specified: sha256-AAAA...
  got:       sha256-BBBB...

Fix: Replace the hash with the correct one from the error. Or use lib.fakeHash / "" during development to get the correct hash from the error.

Attribute Not Found

error: attribute 'foo' missing

Debug:

# Check if it exists
nix eval nixpkgs#foo --apply 'x: builtins.typeOf x'

# List available attributes
nix eval nixpkgs#lib --apply builtins.attrNames

Common cause: typo in package name, or the package was renamed/removed in a nixpkgs update.

Collision Between Packages

error: collision between '/nix/store/...-foo/bin/bar' and '/nix/store/...-baz/bin/bar'

Fix: One of the packages provides the same file. Use lib.hiPrio to prefer one, or remove the conflicting package:

home.packages = [
  (lib.hiPrio pkgs.foo)  # This one wins
  pkgs.baz
];

IFD (Import From Derivation)

error: cannot build during evaluation (import from derivation)

IFD happens when evaluation requires building something first. Common with generated Nix expressions. Fix by:

• Pre-generating the Nix file and committing it
• Using builtins.fetchurl instead of derivation-based fetchers during eval
• Allowing IFD with --allow-import-from-derivation (not recommended for CI)

Unfree Package

error: Package 'foo' has an unfree license ('unfree')

Fix:

# In flake or configuration
nixpkgs.config.allowUnfree = true;

# Or per-package
nixpkgs.config.allowUnfreePredicate = pkg:
  builtins.elem (lib.getName pkg) [ "foo" ];

# CLI
NIXPKGS_ALLOW_UNFREE=1 nix build --impure

Debugging Tools

builtins.trace

Print during evaluation:

let
  x = builtins.trace "evaluating x" (1 + 1);
  y = builtins.trace "x is ${toString x}" (x + 1);
in y

lib.traceVal x prints and returns x. lib.traceValSeq x forces deep evaluation before printing.

nix log

Show build logs for failed (or successful) builds:

nix log nixpkgs#hello          # Log from last build
nix log /nix/store/...-hello   # Log for specific store path

nix why-depends

Trace why one package depends on another:

nix why-depends nixpkgs#myapp nixpkgs#gcc

Useful for understanding closure size and unexpected dependencies.

nix path-info

Inspect store paths:

nix path-info -rsSh nixpkgs#hello   # Show closure size
nix path-info --json nixpkgs#hello  # JSON output

nix eval

Evaluate expressions without building:

nix eval nixpkgs#hello.version                    # "2.12.1"
nix eval nixpkgs#hello.meta.license.shortName     # "gpl3Plus"
nix eval --expr 'builtins.attrNames (import <nixpkgs> {})'  # List all packages

nix repl

Interactive evaluation:

nix repl -f '<nixpkgs>'
# or
nix repl --expr 'import <nixpkgs> {}'

nix-repl> hello.version
"2.12.1"
nix-repl> lib.attrNames (lib.filterAttrs (n: v: lib.isDerivation v) python3Packages)

:lf . loads the current flake in the repl.

Build Debugging

# Build with verbose output
nix build -L nixpkgs#hello

# Keep failed build directory for inspection
nix build --keep-failed nixpkgs#hello
# Failed build dir is printed: /tmp/nix-build-hello-xxx

# Override a phase interactively
nix develop nixpkgs#hello
# Then run phases manually:
unpackPhase
configurePhase
buildPhase

Garbage Collection

nix-collect-garbage         # Remove unreferenced store paths
nix-collect-garbage -d      # Also delete old profiles/generations
nix store gc                # New CLI equivalent
nix store optimise          # Deduplicate store (hardlinks)

# Check store integrity
nix store verify --all

# Show what would be deleted
nix-collect-garbage --print-dead

Performance

• Evaluation slow? Check for IFD, large builtins.readDir on big directories, or deep recursive imports
• Build slow? Check if substitutes (binary cache) are configured: nix config show | grep substituters
• Large closures? Use nix path-info -rsSh and nix why-depends to find unexpected runtime dependencies