How do I find the txid and vout values for an input I want to spend?

Question

I was in the http://coinb.in/multisig/#newTransaction and wanted to make a input but it needed 2 values.

1. Txid, How do I get that?
2. TxIn/vout, Whats that and how do I get that?

Answer 1

If you want to send some coins, you have to have been given some coins before.

When someone sends coins to you, they make a new transaction. This transaction has a TxID (which is the double SHA256 hash of the raw transaction bytes) that everyone uses to refer to this transaction. A transaction can have multiple outputs, and each outputs allocates some of the previously unspent transaction outputs (UTXO) that were used as inputs in the new transaction.

So, really this question comes down to: what are bitcoins? Bitcoins are a pair of TxID and an index that uniquely specify an entry in the blockchain.

The developer documentation (https://bitcoin.org/en/developer-documentation) goes into great detail if you want to learn more.

As far as how do you actually get your hands on these values, you just have to have someone send you coins to your address (or mine them), and then you could just look your address up on a blockchain explorer and find any unspent outputs it has. From there, you can just use the web interface to see the txid and the output index. For example, I just grabbed a random transaction off from blockchain.info at https://blockchain.info/tx/86e06fed6f597dc212e72556a84660d84e9536e081719624a0a30a685238d0d7. You can see the txid is 86e06fed6f597dc212e72556a84660d84e9536e081719624a0a30a685238d0d7 and that there are two outputs (indices 0 and 1), the first of which has been spent, and the second one which has not been spent (at the time of writing).