A block is a collection of transactions grouped and added to the blockchain simultaneously.

Say you want to get all the transaction IDs in block 84,000, you first use the getblockhash command to get the block hash.

hash=$(bitcoin-cli getblockhash 84000)

Next is to use the getblock command and the hash to retrieve the block data.

data=$(bitcoin-cli getblock $hash)

Then use jq command to parse the data as JSON and retrieve all transaction IDs in the block.

echo $data | jq -r '.tx'

You will get a result of all the tx ids in the block.

[
	"9faa752b6099a8fa58681580540a8450b02304535d97cdd5bcd0617526dd374f",
	"60e7e1aabe64d163e366fa9034ba7bb61759d446b0ea340203562141b6d8570a",
	"7b2dec7f13e9b509d79dc21169ec8f98d1ca78cabe86bf6d1f4b20fddd5e4276",
	"af44af0b3012b7ee6fe15a1b412442c6c5e0e118c9a71a2a9b845586d825891d",
	"d6d99862faa71310ed9208b070ebcbe2e264890582a30b531961b8d696ce30c8",
	"d91b1e32a8ae184a0625f4879fe7a861523cd17422edb69800387ba23efeaa22"
]

The txids (transaction IDs) reference individual transactions included in the block. Each transaction has a unique identifier called a transaction ID (TXID), a double SHA-256 hash of the transaction data.

Each txid uniquely identifies a specific transaction within the block. You can use it to reference and locate a particular transaction when needed.

Nodes on the Bitcoin network can use txids to request specific transactions from other nodes. When syncing or validating the blockchain, nodes can request transaction details using their txids.

The txids are part of the block header, providing cryptographic proof that a specific set of transactions is included in a particular block. The inclusion of the txids in the Merkle tree ensures the integrity of the transactions in the block.

When validating a block, nodes verify that the transactions within the block match the txids included in the block header. This verification ensures that the transactions are correctly included and haven’t been altered.

Subsequent blocks in the blockchain reference the txid of the coinbase transaction from the previous block as part of the input to create a new coinbase transaction. This creates a chain of blocks, where each block refers to the previous block’s coinbase transaction.

Indexing services and explorers use txids to organize and provide easy access to transaction data. Users can search for and retrieve details about specific transactions using their txids.