JavaScript Console

Geth responds to instructions encoded as JSON objects as defined in the JSON-RPC-API. A Geth user can send these instructions directly, for example over HTTP using tools like Curl. The code snippet below shows a request for an account balance sent to a local Geth node with the HTTP port 8545 exposed.

curl --data '{"jsonrpc":"2.0","method":"eth_getBalance", "params": ["0x9b1d35635cc34752ca54713bb99d38614f63c955", "latest"], "id":2}' -H "Content-Type: application/json" localhost:8545

This returns a result which is also a JSON object, with values expressed as hexadecimal strings, for example:

This is a low level and rather error-prone way to interact with Geth. Most developers prefer to use convenience libraries that abstract away some of the more tedious and awkward tasks such as converting values from hexadecimal strings into numbers, or converting between denominations of ether (Wei, Gwei, etc). One such library is Web3.js. The purpose of Geth's Javascript console is to provide a built-in environment to use a subset of the Web3.js libraries to interact with a Geth node.

Starting the console

There are two ways to start an interactive session using Geth console. The first is to provide the console command when Geth is started up. This starts the node and runs the console in the same terminal. It is therefore convenient to suppress the logs from the node to prevent them from obscuring the console. If the logs are not needed, they can be redirected to the dev/null path, effectively muting them. Alternatively, if the logs are required they can be redirected to a text file. The level of detail provided in the logs can be adjusted by providing a value between 1-6 to the --verbosity flag as in the example below:

# to mute logs
geth <other flags> console 2> /dev/null

# to save logs to file
geth <other flags> console --verbosity 3 2> geth-logs.log

Alternatively, a Javascript console can be attached to an existing Geth instance (i.e. one that is running in another terminal or remotely). In this case, geth attach can be used to open a Javascript console connected to the Geth node. It is also necessary to define the method used to connect the console to the node. Geth supports websockets, HTTP or local IPC. To use HTTP or Websockets, these must be enabled at the node by providing the following flags at startup:

# enable websockets
geth <other flags> --ws

# enable http
geth <other flags> --http

The commands above use default HTTP/WS endpoints and only enables the default JSON-RPC libraries. To update the Websockets or HTTP endpoints used, or to add support for additional libraries, the .addr .port and .api flags can be used as follows:

# define a custom http address, custom http port and enable libraries
geth <other commands> --http --http.addr 192.60.52.21 --http.port 8552 --http.api eth,web3,admin

# define a custom Websockets address and enable libraries
geth <other commands> --ws --ws.addr 192.60.52.21 --ws.port 8552 --ws.api eth,web3,admin

It is important to note that by default some functionality, including account unlocking is forbidden when HTTP or Websockets access is enabled. This is because an attacker that manages to access the node via the externally-exposed HTTP/WS port then control the unlocked account. This is not a hypothetical risk: there are bots that continually scan for http-enabled Ethereum nodes to attack

The Javascript console can also be connected to a Geth node using IPC. When Geth is started, a geth.ipc file is automatically generated and saved to the data directory. This file, or a custom path to a specific ipc file can be passed to geth attach as follows:

geth attach datadir/geth.ipc

Once started, the console looks like this:

Interactive use

Once the console has been started, it can be used to interact with Geth. The console supports Javascript and the full Geth JSON-RPC API. For example, to check the balance of the first account already existing in the keystore:

eth.getBalance(eth.accounts[0]);

To send a transaction (without global account unlocking):

eth.sendTransaction({
  to: eth.accounts[0],
  to: eth.accounts[1],
  value: web3.toWei(0.5, 'ether')
});

It is also possible to load pre-written Javascript files into the console by passing the --preload flag when starting the console. This is useful for setting up complex contract objects or loading frequently-used functions.

geth console --preload "/my/scripts/folder/utils.js"

Once the interactive session is over, the console can be closed down by typing exit or CTRL-D.

Remember that interactions that touch accounts need approval in Clef - either manually or by writing a custom ruleset.

Non-interactive Use: Script Mode

It is also possible to execute JavaScript code non-interactively by passing the --exec and a JSON-RPC-API endpoint to geth attach or geth console. The result is displayed directly in the terminal rather than in an interactive Javascript console.

For example, to display the accounts in the keystore:

geth attach --exec eth.accounts

geth attach --exec eth.blockNumber

The same syntax can be used to execute a local script file with more complex statements on a remote node over http, for example:

geth attach http://geth.example.org:8545 --exec 'loadScript("/tmp/checkbalances.js")'

geth attach http://geth.example.org:8545 --jspath "/tmp" --exec 'loadScript("checkbalances.js")'

The --jspath flag is used to set a library directory for the Javascript scripts. Any parameters passed to loadScript() that do not explicitly define an absolute path will be interpreted relative to the jspath directory.

Timers

In addition to the full functionality of JS (as per ECMA5), the Ethereum Javascript Runtime Environment (JSRE) is augmented with various timers. It implements setInterval, clearInterval, setTimeout, clearTimeout which some users will be familiar with from browser windows. It also provides implementation for admin.sleep(seconds) and a block based timer, admin.sleepBlocks(n) which sleeps till the number of new blocks added is equal to or greater than n.

Caveats

Geth's console is built using the GoJa JS Virtual Machine which is compatible with ECMAScript 5.1. This does not support promises or async functions. Web3js depends upon the bignumber.js library. This is auto-loaded into the console.