yaml-schema is a tool to validate YAML files against a YAML schema.

The YAML Schema specification is based on JSON Schema (https://json-schema.org/), but expressed as YAML.

yaml-schema is both a Rust library and an executable.

Given a schema.yaml file containing:

type: object
properties:
  foo:
    type: string
  bar:
    type: number

And a valid.yaml file containing:

foo: "I'm a string"
bar: 42

Then when you issue the command

ys -f schema.yaml valid.yaml

Then the command should succeed with exit code 0

On the other hand, when given an invalid.yaml file containing:

foo: 42
bar: "I'm a string"

Then the command

ys -f schema.yaml invalid.yaml

Should fail with exit code 1

Pass --json to emit structured errors instead of plain text. Use it with the same options as usual.

Successful validation (exit code 0): stdout is empty.

ys --json -f schema.yaml valid.yaml

(no output on stdout)

Validation failures (exit code 1): stdout is a single JSON array of objects, one per error. Each object has:

Using the same schema.yaml / invalid.yaml scenario as above, with foo and bar violating their types:

ys --json -f schema.yaml invalid.yaml

stdout (pretty-printed; the tool emits compact JSON on one line):

[
  {
    "col": 5,
    "error": "Expected a string, but got: 42 (int)",
    "index": 5,
    "line": 1,
    "path": "foo"
  },
  {
    "col": 5,
    "error": "Expected a number, but got: \"I'm a string\" (string)",
    "index": 13,
    "line": 2,
    "path": "bar"
  }
]

Other failures (exit code 1): schema load errors, missing arguments, YAML parse errors, and similar issues print a single JSON object on stderr: {"error":"<message>"}.

If the schema file cannot be read:

ys --json -f /path/to/missing-schema.yaml valid.yaml

stderr:

{"error":"Failed to read YAML schema file /path/to/missing-schema.yaml: No such file or directory (os error 2)"}

The exact error text depends on the failure (OS messages, parse errors, etc.).

Validation errors are written to stdout; non-validation errors use stderr, so callers can distinguish validation results from tooling or I/O failures.

yaml-schema uses Cucumber to specify and test features:

• CLI usage
• Basic features
• String validation
• Numeric types
• Const
• Enums
• Object types
• Arrays
• Composition
• Unevaluated properties/items

See the features folder for all examples.

Currently, yaml-schema requires Git, Rust and Cargo to build and install: https://doc.rust-lang.org/cargo/

To install the stable release from crates.io:

cargo install yaml-schema

That should build and install the executable at $HOME/.cargo/bin/ys (which should be in your PATH)

Alternatively, one can install from latest source:

cargo install --git https://github.com/yaml-schema/yaml-schema

Running ys without any options or arguments should display the help:

A tool for validating YAML against a schema

Usage: ys [OPTIONS] [FILE] [COMMAND]

Commands:
  version  Display the ys version
  help     Print this message or the help of the given subcommand(s)

Arguments:
  [FILE]  The YAML file to validate

Options:
  -f, --schema <SCHEMAS>  The schema to validate against
      --fail-fast         Specify this flag to exit (1) as soon as any error is encountered
  -h, --help              Print help
  -V, --version           Print version

yaml-schema is self-validating. That is, running

cargo run -- -f yaml-schema.yaml yaml-schema.yaml

should always succeed.