How to use the command lnav (with examples)

lnav is an advanced log file viewer that allows users to analyze logs with little to no setup. It provides a user-friendly interface for viewing log files and offers various features, such as log file validation and remote log file access.

Use case 1: View logs of a program, specifying log files, directories or URLs

Code:

lnav path/to/log_or_directory|url

Motivation: This use case is useful when you want to view the logs of a specific program or application. By specifying the path to a log file, a directory containing log files, or even a URL to a log file, you can quickly analyze and troubleshoot any issues.

Explanation: The lnav command followed by the path to a log file, directory, or URL will open the lnav viewer and display the contents of the specified logs. It accepts both local file paths and remote URLs for log file viewing.

Example output:

[INFO] Loading log file: path/to/log_file.log
[INFO] Viewing logs for log_file.log
[TIMESTAMP] [LOG LEVEL] [LOG MESSAGE]
2022-01-01T12:34:56Z INFO Application started successfully
2022-01-01T12:36:12Z ERROR Invalid input entered
2022-01-01T12:38:45Z DEBUG Processing data...

Use case 2: View logs of a specific remote host

Code:

lnav ssh user@host1.example.com:/var/log/syslog.log

Motivation: In a distributed environment, it’s often necessary to view logs from remote hosts. This use case allows you to SSH into a remote host and view its log files without having to manually transfer the files to your local machine.

Explanation: The lnav command followed by ssh user@host:/path/to/log_file establishes a passwordless SSH connection to the specified remote host and displays the logs of the specified log file (syslog.log in this case).

Example output:

[INFO] Connecting to remote host at host1.example.com...
[INFO] Loading log file: /var/log/syslog.log
[INFO] Viewing logs for syslog.log on host1.example.com
[TIMESTAMP] [LOG LEVEL] [LOG MESSAGE]
2022-01-01T12:34:56Z INFO System started
2022-01-01T12:36:12Z WARNING Disk space low
2022-01-01T12:38:45Z ERROR Network connectivity lost

Use case 3: Validate the format of log files against the configuration and report any errors

Code:

lnav -C path/to/log_directory

Motivation: Log files often follow specific formats and configurations. Validating the format of log files against their corresponding configurations can help identify formatting errors, missing fields, or inconsistencies. This use case is useful when troubleshooting log ingestion pipelines or ensuring log file integrity.

Explanation: The lnav command followed by the -C option and the path to a directory containing log files will validate the format of the log files against their configurations. Any errors or inconsistencies will be reported, allowing you to address them promptly.

Example output:

[INFO] Validating log files in path/to/log_directory...
[ERROR] Invalid log entry in access.log: Missing timestamp field
[ERROR] Invalid log entry in error.log: Invalid log level: "INFOR"
[ERROR] Invalid log entry in debug.log: Missing log message field

Conclusion:

lnav is a powerful tool for viewing and analyzing log files. By understanding and utilizing its various command-line options, you can efficiently navigate through log files, troubleshoot issues, and ensure log file integrity. Whether you need to view local logs, access remote logs, or validate log file formats, lnav provides an intuitive interface for log analysis.