Nix and NixOS Version Reference

Updated

September 12, 2026

A structured reference for Boring-NixOS readers tracking which Nix CLI features and NixOS system options became available in each release. Coverage spans Nix 2.18 – 2.35 and NixOS 23.05 – 26.05. This book’s examples target Nix 2.35 and NixOS 26.05 “Yarara”. Earlier rows are history so you can read old flakes.

Each section contains a table of reader-relevant changes followed by one short runnable example that exercises the most impactful addition in that release. All examples assume the infrastructure desk scenario: a shared flake that builds team tooling and drives a set of NixOS workstations.

Nix CLI Releases

Nix 2.18

Area	Change
`nix flake metadata`	Prints flake description, inputs, and their resolved URLs; last-modified timestamp now shown per input.
`--log-format`	Accepts `raw`, `internal-json`, `bar`, `bar-with-logs`; useful for CI pipelines that consume structured output.
Evaluator	`builtins.fetchTree` no longer fetches tarballs eagerly when the hash is already in the store.
Error messages	Multi-line errors now include a source excerpt with a caret pointing at the offending token.

Key feature — nix flake metadata

Save as show-meta.sh:

# show-meta.sh
nix flake metadata \
  --log-format bar \
  github:NixOS/nixpkgs/nixos-26.05

Run:

bash show-meta.sh

Output:

Resolved URL:  github:NixOS/nixpkgs/nixos-26.05
Locked URL:    github:NixOS/nixpkgs/a1b2c3d4e5f6...
Description:   Nix Packages collection & NixOS
Path:          /nix/store/xxxxxxxx-source
Last modified: 2024-11-30 12:00:00
Inputs:
└───nixpkgs: (this flake)

Nix 2.19

Area	Change
`nix store gc`	Replaces `nix-collect-garbage` progressively; supports `--max-freed <bytes>` to cap space reclaimed per run.
`nix store gc --dry-run`	Reports paths that would be deleted without touching the store.
`nix profile`	`nix profile diff-closures` added; shows package diff between two profile generations.
Daemon	Store lock contention reduced for concurrent multi-user builds.

Key feature — nix store gc --dry-run

Save as gc-preview.sh:

# gc-preview.sh
# Preview how much the desk workstation store can shrink
# before committing to deletion.
nix store gc --dry-run --max-freed $((10 * 1024 * 1024 * 1024))

Run:

bash gc-preview.sh

Output:

Would delete: /nix/store/aaa...-old-toolchain-2023
Would delete: /nix/store/bbb...-devshell-2023-11
...
Would free 7.4 GiB (limit was 10.0 GiB)

Nix 2.20

Area	Change
`nix flake archive`	Copies all flake inputs into a binary cache or a local store, enabling fully offline evaluation afterwards.
`nix flake archive --to`	Accepts `file:///path`, `s3://bucket`, or `ssh://host` as destination.
`nix copy`	`--substitute-on-destination` flag added; destination fetches missing paths from its own substituters before failing.
Build sandbox	Linux sandbox now mounts `/proc` as `tmpfs` by default, improving isolation.

Key feature — nix flake archive to a local mirror

Save as archive-inputs.sh:

# archive-inputs.sh
# Snapshot all flake inputs into a local binary cache so the
# desk's air-gapped build box can evaluate without internet.
nix flake archive \
  --to file:///srv/nix-cache \
  /home/ops/desk-infra

Run:

bash archive-inputs.sh

Output:

archiving input 'nixpkgs'
  /nix/store/xxxxxxxx-source → file:///srv/nix-cache
archiving input 'home-manager'
  /nix/store/yyyyyyyy-source → file:///srv/nix-cache
done — 2 inputs archived

Nix 2.21

Area	Change
`nix eval --apply`	Applies a Nix function to the evaluated result before printing; enables one-liner transformations.
`nix eval --json`	Now handles `null` and `true`/`false` correctly in all output formatters.
`nix flake update`	`--commit-lock-file` flag commits the updated `flake.lock` with a machine-generated commit message.
Repl	`:reload` re-reads the current file without restarting the session.

Key feature — nix eval --apply

Save as list-packages.sh:

# list-packages.sh
# Print just the names of packages exposed by the desk flake
# without writing a separate script.
nix eval \
  /home/ops/desk-infra#packages.x86_64-linux \
  --apply 'pkgs: builtins.attrNames pkgs' \
  --json

Run:

bash list-packages.sh

Output:

["desk-tools","lint-runner","release-packager"]

Nix 2.22

Area	Change
`nix build --rebuild`	Rebuilds a derivation even if its output is in the store, then compares the new result to the cached one — a fast reproducibility check.
`nix copy --from` / `--to`	Both flags marked stable; no longer experimental.
`nix flake check`	Reports unfree license violations when `--allow-unfree` is absent.
Evaluator	`builtins.readFileType` added: returns `"regular"`, `"directory"`, `"symlink"`, or `"unknown"`.

Key feature — nix build --rebuild

Save as verify-repro.sh:

# verify-repro.sh
# Rebuild the desk tool and confirm the output hash is stable.
# A mismatch here means a non-deterministic build is lurking.
nix build \
  /home/ops/desk-infra#packages.x86_64-linux.desk-tools \
  --rebuild

Run:

bash verify-repro.sh

Output (reproducible):

warning: performing a check build
[3/0/1 built] building desk-tools-1.4.0...
/nix/store/abc123...-desk-tools-1.4.0 (deterministic ✓)

Output (non-reproducible):

warning: performing a check build
error: derivation '/nix/store/...-desk-tools-1.4.0.drv' may not
       be deterministic: output '/nix/store/abc123...' differs
       from '/nix/store/def456...'

Nix 2.23

Area	Change
Lazy trees	Flake inputs are fetched only when their subtree is actually evaluated; large monorepo inputs no longer block evaluation.
`nix fmt`	Declared stable; runs the formatter specified in `flake.nix` `formatter` output.
`nix repl` `:doc`	Prints the docstring attached to any Nix function or built-in.
`nix repl` `:log`	Shows the build log for a derivation path without leaving the REPL.
`nix flake update`	Accepts multiple named inputs: `nix flake update nixpkgs home-manager`.

Key feature — nix fmt enforced in CI

Save as fmt-check.sh:

# fmt-check.sh
# Fail CI if any Nix file in the desk flake needs reformatting.
nix fmt /home/ops/desk-infra -- --check

Run:

bash fmt-check.sh

Output (clean):

All Nix files already formatted.

Output (needs formatting):

Would reformat: /home/ops/desk-infra/modules/workstation.nix
error: 1 file would be reformatted

Nix 2.24

Area	Change
`nix path-info --json`	Output is now a structured JSON array with `path`, `narHash`, `narSize`, `references`, `registrationTime`, and `deriver` fields.
`nix flake check`	Checks `nixosConfigurations`, `darwinConfigurations`, and `homeConfigurations` outputs for module evaluation errors.
`nix store diff-closures`	Declared stable; compares the full runtime closure of two store paths and reports added/removed/changed paths.
`nix build --print-build-logs`	Default changed to `true` in interactive terminals.

Key feature — nix store diff-closures

Save as diff-release.sh:

# diff-release.sh
# Compare the desk-tools closure between the last release and
# HEAD so the team knows exactly what changed before shipping.
OLD=$(nix build \
  "/home/ops/desk-infra?ref=v1.3.0#packages.x86_64-linux.desk-tools" \
  --no-link --print-out-paths)

NEW=$(nix build \
  /home/ops/desk-infra#packages.x86_64-linux.desk-tools \
  --no-link --print-out-paths)

nix store diff-closures "$OLD" "$NEW"

Run:

bash diff-release.sh

Output:

desk-tools: 1.3.0 → 1.4.0
openssl: 3.2.1 → 3.2.2 (+12 KiB)
python3: 3.11.8 → 3.12.3 (+1.1 MiB)
curl: (unchanged)
added: zlib 1.3.1