# Prepare a dataset

`rosetta_port` turns bags into a LeRobot dataset. Every option is on the
[rosetta_port](../reference/rosetta-port.md) page.

## Port

```bash
ros2 run rosetta rosetta_port \
    --raw-dir datasets/bags \
    --contract robot.yaml \
    --repo-id my_org/my_dataset \
    --root datasets/lerobot
```

The dataset lands in `datasets/lerobot/my_org/my_dataset`. Each bag
directory under `--raw-dir` becomes one episode. The porter runs the same
alignment code as live inference, so a dataset frame is what the robot would
see at runtime.

## Check the result

```bash
ls datasets/lerobot/my_org/my_dataset/meta/
```

`info.json` lists the features and their shapes. `rosetta_contract.yaml` is
the contract the porter used.

## Revise and port again

Edit the contract and run the same command with a new `--repo-id`. A new
key, a different `fps`, a different alignment or a different image size
doesn't need a new recording.

## Port in parallel

Split the bags into shards. Each process takes every nth bag.

```bash
for i in 0 1 2 3; do
  ros2 run rosetta rosetta_port --raw-dir datasets/bags --contract robot.yaml \
      --repo-id my_org/my_dataset --root datasets/lerobot \
      --num-shards 4 --shard-index $i &
done
wait
```

## Push to the Hugging Face Hub

```bash
ros2 run rosetta rosetta_port ... --repo-id <hf_user>/<name> --push-to-hub
```

The upload is private unless you add `--hub-public`.

## If an episode fails

The porter logs `FAILED` for that bag and moves on. An observation topic
missing from the bag, or present with no messages, fails the episode. A
missing action, reward or signal topic doesn't: the key is zero-filled with a
warning. Check the bag with `ros2 bag info` against the topic names in the
contract.