attach.sh

#!/bin/bash
geth --preload 'monitorBlocks.js' attach

monitorBlocks.js

// every half second, if the block is different: print new block number and delta since last block number
previousBlockNumber = web3.eth.blockNumber; 
(function () {
	function f() {
	    b = web3.eth.blockNumber;
	    if (b != previousBlockNumber) {
	        console.log("block: " + b + ", diff:" + (b - previousBlockNumber));
	        previousBlockNumber = b;
	    }
	    setTimeout( f, 500 );
	}
	f();
}());