NOTE: This is a draft in progress, so that I can get some feedback from early reviewers. It is not yet ready for learning.
This chapter lays out three ways to send funds via Bitcoin's cli interface. §4.1 described how to do so with a simple command, and §4.4 detailed how to use a more dangerous raw transaction. This final section splits the difference by showing how to make raw transactions simpler and safer.
The methodology for automated raw transactions is simple: you create a raw transaction, but you use the fundrawtransaction command to ask the bitcoind to run the calculations for you.
In order to use this command, you'll need to ensure that your ~/.bitcoin/bitcoin.conf file contains rational variables for calculating transaction fees. Please see §4.1: Sending Coins The Easy Way for more information on this.
For very conservative numbers, we suggested adding the following to the bitcoin.conf:
mintxfee=0.0001
txconfirmtarget=6
To keep the tutorial moving along (and more generally to move money fast) we suggested the following:
mintxfee=0.001
txconfirmtarget=1
To use fundrawtransaction you first need to create a bare-bones raw transaction that lists no inputs and no change address. You'll just list your recipient and how much you want to send them:
$ unfinishedtx=$(bitcoin-cli -named createrawtransaction inputs='''[]''' outputs='''{ "'$recipient'": 1.0 }''')
You then tell bitcoin-cli to fund that bare-bones transaction:
$ bitcoin-cli -named fundrawtransaction hexstring=$unfinishedtx
{
"hex": "020000000169847669938c6a66ef790b87ebb6233059609bee4601476c5948db1a4defc9690100000000feffffff02a8e30f05000000001976a914a6f0ee37c44947f4137d56e4aab12f27ad50369188ac00e1f505000000001976a914e7c1345fc8f87c68170b3aa798a956c2fe6a9eff88ac00000000",
"changepos": 0,
"fee": 0.00022600
}
That provides a lot of useful information, but once you're confident with how it works, you'll want to use JQ to save your hex to a variable, as usual:
$ rawtxhex3=$(bitcoin-cli -named fundrawtransaction hexstring=$unfinishedtx | jq -r '.hex')
It seems like magic, so the first few times you use fundrawtransaction, you'll probably want to verify it.
Running decoderawtransaction will show that the raw transaction is now laid out correctly, using one or more of your UTXOs and sending excess funds back to a change address:
$ bitcoin-cli -named decoderawtransaction hexstring=$rawtxhex3
{
"txid": "2e34603b7449d29412fb7b0d184085d4d839d965f2bba361749c20d9dbae3d0b",
"hash": "2e34603b7449d29412fb7b0d184085d4d839d965f2bba361749c20d9dbae3d0b",
"size": 119,
"vsize": 119,
"version": 2,
"locktime": 0,
"vin": [
{
"txid": "69c9ef4d1adb48596c470146ee9b60593023b6eb870b79ef666a8c9369768469",
"vout": 1,
"scriptSig": {
"asm": "",
"hex": ""
},
"sequence": 4294967294
}
],
"vout": [
{
"value": 0.84927400,
"n": 0,
"scriptPubKey": {
"asm": "OP_DUP OP_HASH160 123cd8796558d195e52137ce3800e5f8120ee46f OP_EQUALVERIFY OP_CHECKSIG",
"hex": "76a914123cd8796558d195e52137ce3800e5f8120ee46f88ac",
"reqSigs": 1,
"type": "pubkeyhash",
"addresses": [
"mhBPM8hU2PHjDTUvwa3SC7pqv8ExkK6mH8"
]
}
},
{
"value": 1.00000000,
"n": 1,
"scriptPubKey": {
"asm": "OP_DUP OP_HASH160 e7c1345fc8f87c68170b3aa798a956c2fe6a9eff OP_EQUALVERIFY OP_CHECKSIG",
"hex": "76a914e7c1345fc8f87c68170b3aa798a956c2fe6a9eff88ac",
"reqSigs": 1,
"type": "pubkeyhash",
"addresses": [
"n2eMqTT929pb1RDNuqEnxdaLau1rxy3efi"
]
}
}
]
}
We saw the fee in the more extensive output, before we saved the hex to a variable with JQ, but you can verify it with the btctxfee JQ alias:
$ btctxfee $rawtxhex3
.00023
Finally, you can use validateaddress to see that the generated change address really belongs to you:
$ bitcoin-cli -named validateaddress address=mhBPM8hU2PHjDTUvwa3SC7pqv8ExkK6mH8
{
"isvalid": true,
"address": "mhBPM8hU2PHjDTUvwa3SC7pqv8ExkK6mH8",
"scriptPubKey": "76a914123cd8796558d195e52137ce3800e5f8120ee46f88ac",
"ismine": true,
"iswatchonly": false,
"isscript": false,
"pubkey": "029045eaa55d283526c723e6d5495d9b3f077b545563f86465aafcd9bfdd50359e",Y
"iscompressed": true,
"timestamp": 1489170694,
"hdkeypath": "m/0'/0'/11'",
"hdmasterkeyid": "144a68bde927a1fed7c2b71ad9010b0201819be5"
}
Note the ismine results.
At this point you can sign and send the transaction as usual.
$ signedtx3=$(bitcoin-cli -named signrawtransaction hexstring=$rawtxhex3 | jq -r '.hex')
$ bitcoin-cli -named sendrawtransaction hexstring=$signedtx3
In several minutes, you'll have your change back:
$ bitcoin-cli listunspent
[
{
"txid": "37698ad6e7f62df07c2fbc549339aa680a7fa18328d7ad14ecb72b21c505cbc6",
"vout": 0,
"address": "mhBPM8hU2PHjDTUvwa3SC7pqv8ExkK6mH8",
"scriptPubKey": "76a914123cd8796558d195e52137ce3800e5f8120ee46f88ac",
"amount": 0.84927400,
"confirmations": 1,
"spendable": true,
"solvable": true
}
]
If you must send funds with raw transactions then fundrawtransaction gives you a nice alternative where fees, inputs, and outputs are calculated for you, so you don't accidentally lose a bunch of money.
The advantages. It provides a nice balance. If you're sending funds by hand and sendtoaddress doesn't offer enough control for whatever reason, you can get some of the advantages of raw transactions without the dangers. This methodology should be used whenever possible if you're sending raw transactions by hand.
The disadvantages. It's a hodge-podge. Though there are a few additional options for the fundrawtransaction command that weren't mentioned here, your control is still limited. You'd probably never want to use this method if you were writing a program where the whole goal is to know exactly what's going on.
Advance through "bitcoin-cli" with Chapter Five: Controlling Bitcoin Transactions.