Skip to main content
Doppels manifests use {{ ... }} template expressions to wire inputs into step environments and pass step outputs into your Capability’s declared returns. Expressions let you write manifests that are fully self-contained and deterministic — the same inputs always flow through the same paths to the same outputs, with no side channels or implicit state.

​
Syntax

There are exactly two expression forms: Expressions appear inside double-curly-brace delimiters and must be placed within quoted YAML strings. For example:

​
Where expressions are allowed

Expressions are valid in exactly three locations within a shell Recipe:

​
1. env values in steps

Use expressions to inject inputs (or prior step results) into a step’s environment before the script runs.

​
2. Declarative produces.file paths

Use expressions to construct artifact file paths that incorporate input values or step results.

​
3. returns values

Use expressions to map step results to the outputs declared in the Capability contract.

​
Where expressions are NOT allowed

​
Inside run.script

You must never place {{ }} expressions directly inside a step’s run.script block. Instead, inject the value through env: and reference it as a normal shell variable. Wrong — do not do this:
Correct:
This restriction exists to prevent shell injection: if a user provides a malicious input value, an expression embedded directly in the script would execute arbitrary code. Routing values through environment variables eliminates that risk.

​
Anywhere outside the three permitted fields

Expressions are not evaluated in metadata fields, requires, defaults, procedure, evidence, or any other part of a manifest. Use them only in env values, declarative produces.file paths, and returns values.

​
Complete wiring example

The example below shows all three permitted uses in a single step, along with the corresponding returns section.

​
Type coercion in returns

When a step’s produces.env captures an exported shell variable, the value arrives as plain text. The returns block converts that text to the type declared in the Capability’s outputs. The rules are strict — there is no lenient coercion and no whitespace trimming: If a captured value does not satisfy the target type’s rule, the run fails with a type conversion error. Export your shell variables in the exact format the Capability expects.