JSON Field Grep in Node.JS

jgrep.js

#!/usr/bin/env node

var stdin = process.stdin,
    fields = process.argv.slice(2),
    buf = [];
    
if (fields.length === 0){
  console.error('Usage: jgrep "some.fields in.json" < STDIN');
  process.exit(1);
}

/**
 * Safely resoves an object using dot notation
 * 
 * @param {Object} obj The object to resolve
 * @param {String} selector The selector in dot notation to resolve (eg. 'foo.bar.baz' )
 */
function resolve(obj, selector){
  var parts = selector.split('.'),
      part = parts.shift();
  
  if(obj === undefined){
    return obj;
  }
  
  if (parts.length === 0){
    return obj[part];
  } else {
    return resolve(obj[part], parts.join('.'));
  }
}

/**
 * Builds the object structure to match the selector
 *
 * @param {Object} obj The object to build on
 * @param {String} selector The selector in dot notation to resolve (eg. 'foo.bar.baz' )
 */
function build(obj, selector){
  var parts = selector.split('.'),
      part = parts.shift();
  
  obj[part] = obj[part] || {};
  
  if (parts.length > 0){
    build(obj[part], parts.join('.'));
  }
}

/**
 * Returns only the fields specified
 * 
 * @param {String} data The stringified JSON data
 * @param {Array} fields An array of fields to return, in dot notation.
 */
function jGrep(data){
  try{
    data = JSON.parse(data);
    var i = 0, flen = fields.length,
        ret = {}, field, retfield, farr, lastfield;
    
    for (; i < flen; i += 1){
      field = fields[i];      
      build(ret, field);      
      farr = field.split('.');
      lastfield = farr.pop();
      
      if (farr.length){
        retfield = resolve(ret, farr.join('.'));  
      } else {
        retfield = ret;
      }
      
      retfield[lastfield] = resolve(data, field);
    }
    
    console.log(JSON.stringify(ret));
  } catch(e){
    return undefined;
  }
}


/**
 * Flushes the buffer to jGrep as we have a full line
 */
function flush(){
  var data = buf.join('');
  buf = [];
  jGrep(data);
}

/**
 * buffers data and flushes when appropriate
 */
function buffer(data){
  if (data.indexOf('\n') > -1){
    data = data.split('\n');
    buf.push(data.shift());
    flush();
    buffer(data.join('\n'));
  } else {
    buf.push(data);
  }
}


/**
 * receives data from stdin
 */
stdin.on('data', function(data){
  buffer(data.toString());
});

stdin.resume();

// vim: ft=javascript:ts=2:sw=2

takes stdin and greps outputs only the fields specified, each line is treated as its own json document. chmod +x it to use it as a binary

Usage:

tail -f /tmp/mylog.json | ./jgrep.js 'field.one' 'field.two' 'some.other.field' 'yay'