How to look up an already-stored bag in the storage service

This guide explains how to look up a bag which has been stored in the storage service.

You need to know:

The space and external identifier of the bag
The API URL for your storage service instance
The token URL for your storage service instance
A client ID and secret for the storage service
An upload bucket for the storage service

To look up a bag in the storage service:

Fetch an access token for the OAuth2 credentials grant:

curl -X POST "$TOKEN_URL" \
  --data grant_type=client_credentials \
  --data client_id="$CLIENT_ID" \
  --data client_secret="$CLIENT_SECRET"

This will return a response like:

{"access_token":"eyJraWQi...","expires_in":3600,"token_type":"Bearer"}

Remember the access_token.

Make a GET request to the /bags API, passing the space and external identiifer in the path:
```
curl "$API_URL/bags/$SPACE/$EXTERNAL_IDENTIFIER" \
  --header "Authorization: $ACCESS_TOKEN"
```
This will return a JSON storage manifest (if the bag exists), or a 404 error if not.