Question: Is there a command I can give the Monero daemon to calculate the total of all coinbase transactions so far?

How do block explorers calculate the Monero coin supply and how can I verify their accuracy?

Answer: 
There is no such command. However, it'd be easy to add. Feel free to add a request for this on <https://github.com/monero-project/bitmonero/issues>
You can print the total coin supply with the command `print_coinbase_tx_sum`

However, this assumes that there were no issues with transaction construction. In this case, the supply can be audited afterwards to see if there were any errors. Here is the information for a [vulnerability](https://getmonero.org/2017/05/17/disclosure-of-a-major-bug-in-cryptonote-based-currencies.html) that was found with Monero and patched before exploitation:

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> In Monero we've discovered and patched a critical bug that affects all CryptoNote-based cryptocurrencies, and allows for the creation of an unlimited number of coins in a way that is undetectable to an observer unless they know about the fatal flaw and can search for it.
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> We patched it quite some time ago, and confirmed that the Monero blockchain had NEVER been exploited using this, but until the hard fork that we had a few weeks ago we were unsure as to whether or not the entire network had updated.
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