With gpg --verify, how do I distinguish valid signatures and valid signatures from trusted keys programmatically?

Question

I want to write a script that runs gpg --verify to check a signature.

The gpg command returns 1 for invalid signatures and 0 for valid ones. But it gives different output for keys that are in my trustdb and keys that aren't:

gpg: WARNING: This key is not certified with a trusted signature!
gpg:          There is no indication that the signature belongs to the owner.

I want to run gpg in such a way that this warning is treated as an error, and without resorting to grepping the output. Is that possible?

By the way: It's fine if the solution requires me to use gpgme, but I haven't found anything related to the trustdb that looks remotely relevant in gpgme's documentation yet.

Answer 1

This is not a particularly elegant solution, but it may be usable.

check_sig(){
    local LC_ALL=C output
    output=$(gpg --verify -- "$1" 2>&1) || return 1
    ! grep -Fqx 'gpg: WARNING: This key is not certified with a trusted signature!' <<<"$output"
}