How to parse logs with grok in Logstash

The grok filter plugin uses patterns to parse unstructured logs into structured fields. By extracting fields like timestamps, IP addresses, or usernames, logs become more searchable and analyzable.

Built-in grok patterns simplify matching common log formats. Custom patterns can handle unique or proprietary log structures.

Accurate grok parsing leads to richer, more meaningful data stored in Elasticsearch, enhancing querying and visualization in Kibana.

1. Edit the pipeline configuration file to add a grok filter.

   $ sudo nano /etc/logstash/conf.d/grok_example.conf
   (no direct output)

   Choose an indicative filename for this pipeline configuration.

2. Define an input reading from a log file or Filebeat.
3. In the filter section, use the grok plugin with a matching pattern.

   Use a predefined pattern like %{COMBINEDAPACHELOG} for Apache logs.

4. Test the configuration.

   $ sudo /usr/share/logstash/bin/logstash --path.config /etc/logstash/conf.d --config.test_and_exit
   Configuration OK

   Fix any pattern syntax errors before applying to production.

5. Restart Logstash to apply changes.

   $ sudo systemctl restart logstash
   (no output)

6. Verify that parsed fields appear in Elasticsearch documents.

   Grok-transformed logs are more easily queried and visualized.