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cli program, that likes json logs and it helps you to like them too
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logfrog likes json logs and it helps you to like them too

logfrog is a cli program that processes json logs line by line from stdin. Those logs typically come from loggers like logrus, zap, apex or others, that let you write logs as json objects.

use cases

docker-compose logs

docker-compose logs --no-color -f | logfrog --docker-compose


docker logs some-container 2>&1 | logfrog

json log files

tail -f path-to-file.json | logfrog

js filtering

logfrog lets you transform and filter json logs with a javascript function named filter that must be defined in a .js file that is passed with the ar --js-filter

tail -f path-to-file.json | logfrog --js-filter path/to/filter.js
  • the js file is executed with the otto vm

  • it has to contain a filter function like the one below

  • the file will be reevaluated, when it changes

  • this is highly EXPERIMENTAL and we would love to hear back from you

// filter function must be named filter
// @param service:string only filled with logfrog --docker-compose
// @param logEntry:{msg?:string;level?:string;time?:string, ...}
// @return logEntry | null when null is returned this entry is filtered out
function filter(service, logEntry) {
  // let us look at the service
  switch (service.substr(0, service.length - 2)) {
    case "elasticsearch":
      // very minimal log entries for elastic search
      return { level: logEntry.level, msg: logEntry.message };
  // log entry manipulation
  // some date formatting
  logEntry.msg = logEntry.msg.substr(0, 256);
  logEntry.time = new Date(logEntry.time).toLocaleString();
  // trimming a stack
  if (logEntry.stack) {
    logEntry.stack = logEntry.stack.substr(0, 300) + " ...";
  // go crazy and have fun ;)
  return logEntry;

standard fields

This is an initial set of fields, please let us know what we should add.

  • msg <- msg, message, Message
  • level <- level, Level
  • time <- time, timestamp, Timestamp


  • add hombrew support
  • map more fields
  • stern mode like docker-compose
  • maybe add a web frontend ?!
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