At this point, the transaction is created, but not yet transmitted to the network. To send our transaction, we need to sign it using the  bitcoin-cli signrawtransaction command. We sign the transaction using our private key (related to the receiving address) to prove to the network our ownership of the output, and therefore our authority to spend the held bitcoins.

The first step will be to extract the private key associated with the first address used to receive the bitcoins from the faucet:

electrum --testnet listaddresses | electrum --testnet  getprivatekeys -

Notice the presence of a dash at the end of the command. It will be replaced by the values returned from the pipe. As a result, you'll get a list of private keys. Copy the first one without the p2pkh prefix, as follows:

Next, we need to get scriptPubKey from the output we are willing to spend. For that, firstly, we have to retrieve the transaction from the blockchain, using electrum  gettransaction --testnet "0791521362528725683caedf998006cf68b1cd817be1694ef0daca265d9b4252".

Secondly, we use the resultant raw form to get scriptPubKey, as follows:

electrum deserialize --testnet 0200000001915bf222c2e4e6ff36760168904ae102a0e968d83b3c575077d5475aa94dd9bf010000006b483045022100b129bc0fb5631aa668c48bb7a8fef0c81fec131d2f68ba430cd7cd9de0bd971b02203dabbf054790e31b4fd1b9a333881cd480c19b38a229e70f886dbb88ee4673f1012103bcf53d63d2fa14ee04d9ebb9170dfa7987298689c7e6ceb765c1d3ccd7f9ad01feffffff02d618b24a000000001976a914b9172e192d2805ea52fa975847eea0657e38fef888ac80778e06000000001976a914edcce89f510bf95606ec6a79cb28a745c039e22088ac63b31400

Unlike before, here we are loading and deserializing the received transaction from the faucet. We will get the outputs created in this transaction, as follows:

The part surrounded in red is scriptPubKey of the unspent transaction output.

A scriptPubKey can be seen in the outputs; it represents the conditions that are set for spending the outputs. The new owner can sign using the private key associated with the address receiving the output to fulfil the conditions of scriptPubKey.

The network checks whether the digital signature is valid, and if so makes it an input for the new transaction. The cryptographic parts—scriptSig and scriptPubKey—are particularly complex, and will be discussed in the next chapter.

Copy scriptPubKey from the output, and pass it along the other options to the signrawtransaction command, as follows:

signrawtransaction "Raw hexstring" ( [{"txid":"id","vout":n,"scriptPubKey":"hex","redeemScript":"hex"},..] ["privatekey",..])

The second argument is a JSON array of the previous transaction outputs we are consuming, and the third argument is the private key belonging to the address that received the output. The result will be similar to the following output:

After succeeding in signing the raw transaction, it is time to send the signed transaction to the testnet.