a q s
EXTERNAL uses an external application to retrieve sequences. The ID is passed as an argument to the application, either replacing %s in the command string (if present) or as an additional argument (if there is no %s).
EXTERNAL requires the application to return the sequence on STDOUT. If the application writes to somewhere else, simply wrap it in a script that copies the output to STDOUT.
DB mydb [ #required parameters method: "app" format: "fasta" type: "P" app: "getfromdb" #optional parameters comment: "my own protein database with a custom retrieval program" app: "getfromdb mydatabase \%s" ]
The first app: definition will use the default call 'getfromdb mydb:id'
The alternative app: definition will use the %s format and call 'getfromdb mydatabase id'
Both will pass either the ID or accession from the query, so that USAs mydb-id:x13776 and mydb-acc:x13776 are equivalent.