Followup question: I am running a bitcoin full-node and it took a month to sync the blockchain. I did not create the full-node with a bitcoin wallet. I stumbled upon this 'importaddress' command (https://bitcoincore.org/en/doc/0.16.1/rpc/wallet/importaddress/) and I was wondering if I could somehow run the command without a bitcoin wallet or find a way to attach a bitcoin wallet and rescan the blockchain without having to restart the entire downloading of the blockchain again. Is it possible to attach a wallet without re-downloading the entire blockchain?
A wallet is not related to whether the blockchain has to be downloaded. The blockchain is not stored in a wallet. Rescanning does not redownload the blockchain unless you are using a pruned node.
importaddress, you must have a wallet. It cannot work without a wallet file. If you don't have a wallet file for some reason, starting Bitcoin Core normally (i.e. without any wallet disabling options) will have it automatically create a wallet for you. If you have a wallet and just want to import the address into another one, you can create one using the
createwallet command. If you have another wallet file, you can move it to the Bitcoin data directory and load the wallet using the
However loading or creating a wallet is not what you really want to do. You really don't want to import an address into your wallet; you really want to check if that address actually has the amount of Bitcoin that you see listed on a website. To do that, you can just use the
scantxoutset command which was introduced in Bitcoin Core 0.17. It does not need a wallet and works way faster than a rescan. See it's help text (by using
help scantxoutset) for how to use it.
scantxoutset does is it scans the UTXO set for any UTXOs that match a given pattern. This will tell you what UTXOs are associated with an address or output script. From there you can sum their values and calculate the total available balance. This will give you the information you want to see without needing a wallet and without scanning the entire blockchain.