By default, the local copy will be synchronized with the most recent code, which might be an unstable or beta version of bitcoin. Before compiling the code, select a specific version by checking out a release tag. This will synchronize the local copy with a specific snapshot of the code repository identified by a keyword tag. Tags are used by the developers to mark specific releases of the code by version number. First, to find the available tags, we use the git tag command:

The list of tags shows all the released versions of bitcoin. By convention, release candidates, which are intended for testing, have the suffix "rc." Stable releases that can be run on production systems have no suffix. From the preceding list, select the highest version release, which at the time of writing was v0.15.0. To synchronize the local code with this version, use the git checkout command:

You can confirm you have the desired version "checked out" by issuing the command git status:

The source code includes documentation, which can be found in a number of files. Review the main documentation located in README.md in the bitcoin directory by typing **more README.md** at the prompt and using the spacebar to progress to the next page. In this chapter, we will build the command-line bitcoin client, also known as bitcoind on Linux. Review the instructions for compiling the bitcoind command-line client on your platform by typing **more doc/build-unix.md**. Alternative instructions for macOS and Windows can be found in the doc directory, as build-osx.md or build-windows.md, respectively.

Carefully review the build prerequisites, which are in the first part of the build documentation. These are libraries that must be present on your system before you can begin to compile bitcoin. If these prerequisites are missing, the build process will fail with an error. If this happens because you missed a prerequisite, you can install it and then resume the build process from where you left off. Assuming the prerequisites are installed, you start the build process by generating a set of build scripts using the autogen.sh script.

The autogen.sh script creates a set of automatic configuration scripts that will interrogate your system to discover the correct settings and ensure you have all the necessary libraries to compile the code. The most important of these is the configure script that offers a number of different options to customize the build process. Type **./configure --help** to see the various options:

The configure script allows you to enable or disable certain features of bitcoind through the use of the --enable-FEATURE and --disable-FEATURE flags, where FEATURE is replaced by the feature name, as listed in the help output. In this chapter, we will build the bitcoind client with all the default features. We won’t be using the configuration flags, but you should review them to understand what optional features are part of the client. If you are in an academic setting, computer lab restrictions may require you to install applications in your home directory (e.g., using --prefix=$HOME).

Here are some useful options that override the default behavior of the configure script:

Next, run the configure script to automatically discover all the necessary libraries and create a customized build script for your system:

If all went well, the configure command will end by creating the customized build scripts that will allow us to compile bitcoind. If there are any missing libraries or errors, the configure command will terminate with an error instead of creating the build scripts. If an error occurs, it is most likely because of a missing or incompatible library. Review the build documentation again and make sure you install the missing prerequisites. Then run configure again and see if that fixes the error.

Next, you will compile the source code, a process that can take up to an hour to complete, depending on the speed of your CPU and available memory. During the compilation process you should see output every few seconds or every few minutes, or an error if something goes wrong. If an error occurs, or the compilation process is interrupted, it can be resumed any time by typing make again. Type **make** to start compiling the executable application:

On a fast system with more than one CPU, you might want to set the number of parallel compile jobs. For instance, make -j 2 will use two cores if they are available. If all goes well, Bitcoin Core is now compiled. You should run the unit test suite with make check to ensure the linked libraries are not broken in obvious ways. The final step is to install the various executables on your system using the make install command. You may be prompted for your user password, because this step requires administrative privileges:

The default installation of bitcoind puts it in /usr/local/bin. You can confirm that Bitcoin Core is correctly installed by asking the system for the path of the executables, as follows:

Bitcoin Core will look for a configuration file in its data directory on every start. In this section we will examine the various configuration options and set up a configuration file. To locate the configuration file, run bitcoind -printtoconsole in your terminal and look for the first couple of lines.

You can hit Ctrl-C to shut down the node once you determine the location of the config file. Usually the configuration file is inside the .bitcoin data directory under your user’s home directory. Open the configuration file in your preferred editor.

Bitcoin Core offers more than 100 configuration options that modify the behavior of the network node, the storage of the blockchain, and many other aspects of its operation. To see a listing of these options, run bitcoind --help:

Here are some of the most important options that you can set in the configuration file, or as command-line parameters to bitcoind:

Sample configuration of a full-index node shows how you might combine the preceding options, with a fully indexed node, running as an API backend for a bitcoin application.

Sample configuration of a resource-constrained system shows a resource-constrained node running on a smaller server.

Once you’ve edited the configuration file and set the options that best represent your needs, you can test bitcoind with this configuration. Run Bitcoin Core with the option printtoconsole to run in the foreground with output to the console:

You can hit Ctrl-C to interrupt the process once you are satisfied that it is loading the correct settings and running as you expect.

To run Bitcoin Core in the background as a process, start it with the daemon option, as bitcoind -daemon.

To monitor the progress and runtime status of your bitcoin node, use the command bitcoin-cli getblockchaininfo:

This shows a node with a blockchain height of 0 blocks and 83999 headers. The node currently fetches the block headers of the best chain and afterward continues to download the full blocks.

Once you are happy with the configuration options you have selected, you should add bitcoin to the startup scripts in your operating system, so that it runs continuously and restarts when the operating system restarts. You will find a number of example startup scripts for various operating systems in bitcoin’s source directory under contrib/init and a README.md file showing which system uses which script.

Bitcoin Core provides status reports on different modules through the JSON-RPC interface. The most important commands include getblockchaininfo, getmempoolinfo, getnetworkinfo and getwalletinfo.

Bitcoin’s getblockchaininfo RPC command was introduced earlier. The getnetworkinfo command displays basic information about the status of the bitcoin network node. Use bitcoin-cli to run it:

The data is returned in JavaScript Object Notation (JSON), a format that can easily be "consumed" by all programming languages but is also quite human-readable. Among this data we see the version numbers for the bitcoin software client (150000) and bitcoin protocol (70015). We see the current number of connections (8) and various information about the bitcoin network and the settings related to this client.

Commands: getrawtransaction, decoderawtransaction

In [cup_of_coffee], Alice bought a cup of coffee from Bob’s Cafe. Her transaction was recorded on the blockchain with transaction ID (txid) 0627052b6f28912f2703066a912ea577f2ce4da4caa5a5fbd8a57286c345c2f2. Let’s use the API to retrieve and examine that transaction by passing the transaction ID as a parameter:

The command getrawtransaction returns a serialized transaction in hexadecimal notation. To decode that, we use the decoderawtransaction command, passing the hex data as a parameter. You can copy the hex returned by getrawtransaction and paste it as a parameter to decoderawtransaction:

The transaction decode shows all the components of this transaction, including the transaction inputs and outputs. In this case we see that the transaction that credited our new address with 15 millibits used one input and generated two outputs. The input to this transaction was the output from a previously confirmed transaction (shown as the vin txid starting with 7957a35fe). The two outputs correspond to the 15 millibit credit and an output with change back to the sender.

We can further explore the blockchain by examining the previous transaction referenced by its txid in this transaction using the same commands (e.g., getrawtransaction). Jumping from transaction to transaction we can follow a chain of transactions back as the coins are transmitted from owner address to owner address.

Commands: getblock, getblockhash

Exploring blocks is similar to exploring transactions. However, blocks can be referenced either by the block height or by the block hash. First, let’s find a block by its height. In [cup_of_coffee], we saw that Alice’s transaction was included in block 277316.

We use the getblockhash command, which takes the block height as the parameter and returns the block hash for that block:

Now that we know which block Alice’s transaction was included in, we can query that block. We use the getblock command with the block hash as the parameter:

The block contains 419 transactions and the 64th transaction listed (0627052b...) is Alice’s coffee payment. The height entry tells us this is the 277316th block in the blockchain.

The bitcoin-cli helper is very useful for exploring the Bitcoin Core API and testing functions. But the whole point of an application programming interface is to access functions programmatically. In this section we will demonstrate accessing Bitcoin Core from another program.

Bitcoin Core’s API is a JSON-RPC interface. JSON stands for JavaScript Object Notation and it is a very convenient way to present data that both humans and programs can easily read. RPC stands for Remote Procedure Call, which means that we are calling procedures (functions) that are remote (on the Bitcoin Core node) via a network protocol. In this case, the network protocol is HTTP, or HTTPS (for encrypted connections).

When we used the bitcoin-cli command to get help on a command, it showed us an example of using curl, the versatile command-line HTTP client to construct one of these JSON-RPC calls:

This command shows that curl submits an HTTP request to the local host (127.0.0.1), connecting to the default bitcoin port (8332), and submitting a jsonrpc request for the getblockchaininfo method using text/plain encoding.

You might notice that curl will ask for credentials to be sent along with the request. Bitcoin Core will create a random password on each start and place it in the data directory under the name .cookie. The bitcoin-cli helper can read this password file given the data directory. Similarly, you can copy the password and pass it to curl (or any higher level Bitcoin Core RPC wrappers). Alternatively, you can create a static password with the helper script provided in ./share/rpcauth/rpcauth.py in Bitcoin Core’s source directory.

If you’re implementing a JSON-RPC call in your own program, you can use a generic HTTP library to construct the call, similar to what is shown in the preceding curl example.

However, there are libraries in most every programming language that "wrap" the Bitcoin Core API in a way that makes this a lot simpler. We will use the python-bitcoinlib library to simplify API access. Remember, this requires you to have a running Bitcoin Core instance, which will be used to make JSON-RPC calls.

The Python script in Running getblockchaininfo via Bitcoin Core’s JSON-RPC API makes a simple getblockchaininfo call and prints the block parameter from the data returned by Bitcoin Core.

Running it gives us the following result:

It tells us that our local Bitcoin Core node has 394075 blocks in its blockchain. Not a spectacular result, but it demonstrates the basic use of the library as a simplified interface to Bitcoin Core’s JSON-RPC API.

Next, let’s use the getrawtransaction and decodetransaction calls to retrieve the details of Alice’s coffee payment. In Retrieving a transaction and iterating its outputs, we retrieve Alice’s transaction and list the transaction’s outputs. For each output, we show the recipient address and value. As a reminder, Alice’s transaction had one output paying Bob’s Cafe and one output for change back to Alice.

Running this code, we get:

Both of the preceding examples are rather simple. You don’t really need a program to run them; you could just as easily use the bitcoin-cli helper. The next example, however, requires several hundred RPC calls and more clearly demonstrates the use of a programmatic interface.

In Retrieving a block and adding all the transaction outputs, we first retrieve block 277316, then retrieve each of the 419 transactions within by reference to each transaction ID. Next, we iterate through each of the transaction’s outputs and add up the value.

Running this code, we get:

Our example code calculates that the total value transacted in this block is 10,322.07722534 BTC (including 25 BTC reward and 0.0909 BTC in fees). Compare that to the amount reported by a block explorer site by searching for the block hash or height. Some block explorers report the total value excluding the reward and excluding the fees. See if you can spot the difference.

Many more libraries exist in a variety of other programming languages and more are created all the time.