# Check
The `check` subcommand validates the pipeline structure without executing it.
## CLI usage
```sh
$ my_app check
Pipeline is valid.
```
If validation fails:
```sh
$ my_app check
Error: Pipeline validation failed:
- Node 'process' requires dataset 0x7f8a..., which is produced by a later node
```
## What it validates
`check` calls `StepInfo::check()` on the pipeline, which verifies:
- **Sequential ordering** — no node reads a dataset produced by a later node
- **No duplicate outputs** — each dataset is produced by at most one node
- **Params are read-only** — no node writes to a `Param`
- **Pipeline contracts** — `Pipeline` declared inputs/outputs match children
See [Check](../pipelines/check.md) for the full list of `CheckError` variants.
## Programmatic use
You can call `check()` directly on any `StepInfo`:
```rust,ignore
let steps = pipeline(&catalog, ¶ms);
match steps.check() {
Ok(()) => println!("Valid!"),
Err(e) => eprintln!("Error: {e}"),
}
```
Or via `App::dispatch`, which calls `check()` when the command is `Command::Check`:
```rust,ignore
App::new(catalog, params)
.with_command(Command::Check)
.dispatch(pipeline)?;
```