The output index (vout) is a 4-byte unsigned integer that identifies which output from the previous transaction we want to spend.

Let's examine our transaction to understand this better:

In this transaction, we can see the vout value highlighted in orange: 00000000.
When decoded from little-endian format, this represents index 0, meaning this input is spending the first output from the previous transaction.

The vout value tells us which output we're spending from the previous transaction:

• Outputs are zero-indexed (first output is 0, second is 1, etc.)
• Stored as a 4-byte little-endian integer

Outpoints

The combination of a TXID and vout (written as TXID:vout) is called an "outpoint". Every output in the Bitcoin blockchain can be uniquely identified by its outpoint. Here are some notable examples:

• 4a5e1e4baab89f3a32518a88c31bc87f618f76673e2cc77ab2127b7afdeda33b:0 - The first-ever Bitcoin transaction output (genesis block)
• a1075db55d416d3ca199f55b6084e2115b9345e16c5cf302fc80e9d5fbf5d48d:0 - The famous Bitcoin pizza transaction (spending the first output)

Implementation Example

Here's how you might parse a transaction input's vout:

def parse_vout(raw_tx: bytes, offset: int = 0) -> tuple[int, int]:
    """
    Parse an output index from raw transaction bytes
    
    Args:
        raw_tx: Raw transaction bytes
        offset: Starting position in bytes
        
    Returns:
        (vout, new_offset)
    """
    # Read 4 bytes for vout
    vout_bytes = raw_tx[offset:offset + 4]
    
    # Convert to integer (little-endian)
    vout = int.from_bytes(vout_bytes, 'little')
    
    return vout, offset + 4

RPC Commands

• Use gettxout <txid> <n> to view details of an unspent transaction output:

$ bitcoin-cli gettxout "1075db55d416d3ca199f55b6084e2115b9345e16c5cf302fc80e9d5fbf5d48d" 0

{
"bestblock": "00000000000000000003a963aad5832ac54a8a6a2cf4c4856e30e3aa279e42fd",
"confirmations": 750001,
"value": 0.00100000,
"scriptPubKey": {
"asm": "OP_DUP OP_HASH160 62e907b15cbf27d5425399ebf6f0fb50ebb88f1888ac OP_EQUALVERIFY OP_CHECKSIG",
"desc": "pkh([62e907b1]02c6047f9441ed7d6d3045406e95c07cd85c778e4b8cef3ca7abac09b95c709ee5)#8tscl5xj",
"hex": "76a91462e907b15cbf27d5425399ebf6f0fb50ebb88f1888ac",
"type": "pubkeyhash",
"address": "1A1zP1eP5QGefi2DMPTfTL5SLmv7DivfNa"
}
}

• View all unspent transaction outputs (UTXOs) in your wallet with listunspent:

$ bitcoin-cli listunspent

[
{
"txid": "1075db55d416d3ca199f55b6084e2115b9345e16c5cf302fc80e9d5fbf5d48d",
"vout": 0,
"address": "1A1zP1eP5QGefi2DMPTfTL5SLmv7DivfNa",
"label": "",
"amount": 0.00100000,
"confirmations": 750000,
"spendable": true,
"solvable": true,
"desc": "pkh([62e907b1]02c6047f9441ed7d6d3045406e95c07cd85c778e4b8cef3ca7abac09b95c709ee5)#8tscl5xj",
"safe": true
}
]

Usage and Verification in Bitcoin

The vout is crucial for transaction validation and UTXO management. When processing transactions, nodes:

1. Locate and Validate: Use txid to find the previous transaction and verify the vout points to a valid output
2. Check UTXO Status: Confirm the referenced output hasn't been spent already
3. Verify Conditions: Ensure all spending conditions for that output are met