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RuneRune

Caching

How to skip an expensive task safely. Part of the guides; full syntax in the language guide.

Concept

Section titled “Concept”

Rune is a command runner, not a build system: a task always runs when you ask — it never skips work based on file timestamps. Caching is an explicit, per-task opt-in. When you add [cache(...)], Rune computes a fingerprint over the declared inputs, the task body, the resolved variables, and the executor. If nothing changed and the declared outputs exist, the task is skipped — and the skip is logged (“cached”), never silent.

Syntax

Section titled “Syntax”
[cache(inputs = ["go.mod", "go.sum", "**/*.go"], outputs = ["dist/app"])]
build:
    go build -o dist/app ./...

Clear stored fingerprints with rune --clear-cache.

Runnable example

Section titled “Runnable example”

See examples/caching — run rune build twice; the second run reports the task as cached.

Pitfalls

Section titled “Pitfalls”
  • Missing outputs force a run. If a declared output is absent, the task runs even on a fingerprint match — you can’t “cache” your way out of a deleted artifact.
  • Unresolvable inputs force a run rather than a false skip.
  • Caching is never the default. If you didn’t add [cache], the task runs every time — that’s the point (no .PHONY traps, no stale “up-to-date” guesses).
  • The body is part of the fingerprint. Editing the task’s commands invalidates the cache, as it should.

Next steps

Section titled “Next steps”