Logger

The Logger class provides a flexible logging utility for Ruby applications. It allows you to record messages at different severity levels and direct output to various destinations like files, standard output, or custom IO objects.

Overview

Logger is built into Ruby’s standard library and offers:

• Severity levels: DEBUG, INFO, WARN, ERROR, FATAL
• Multiple outputs: Files, STDOUT, STDERR, or any IO object
• Log rotation: Automatic rotation by size or time
• Formatting: Customizable log message formats

Creating a Logger

require 'logger'

# Log to a file
logger = Logger.new('application.log')

# Log to standard output
logger = Logger.new(STDOUT)

# Log to standard error (default)
logger = Logger.new(STDERR)

# Log to a custom IO object
logger = Logger.new(File.open('app.log', 'a'))

Logger.new Parameters

Parameter	Type	Default	Description
logdev	String or IO	Required	File path or IO object
shift_age	Integer	0	Number of old log files to keep
shift_size	Integer	1048576	Max size before rotation (bytes)
level	Integer	DEBUG	Minimum log level

Logging Severity Levels

Logger defines five severity levels:

require 'logger'

logger = Logger.new(STDOUT)

logger.debug("Debug message - detailed diagnostic info")
logger.info("Info message - general operational events")
logger.warn("Warning message - something might be wrong")
logger.error("Error message - something failed")
logger.fatal("Fatal message - critical failure")

Level Methods

Method	Level	Description
debug	0	Detailed diagnostic information
info	1	General informational messages
warn	2	Warning messages
error	3	Error messages
fatal	4	Fatal/critical messages

You can also use the numeric level directly:

logger.level = Logger::DEBUG   # 0
logger.level = Logger::INFO    # 1
logger.level = Logger::WARN    # 2
logger.level = Logger::ERROR   # 3
logger.level = Logger::FATAL   # 4

To disable logging entirely:

logger.level = Logger::UNKNOWN  # or any value > FATAL

Setting the Log Level

Control which messages get logged:

logger = Logger.new(STDOUT)

# Only log warnings and errors
logger.level = Logger::WARN

logger.debug("This won't show")  # Skipped
logger.info("This won't show")   # Skipped
logger.warn("Warning shown")      # => "W -- : Warning shown"
logger.error("Error shown")      # => "E -- : Error shown"

Log Formatting

Customize how log messages appear:

logger = Logger.new(STDOUT)

# Default format
# "D, [.ms]  # -- : message"

# Custom format
logger.formatter = proc do |severity, datetime, progname, msg|
  "[#{datetime.strftime('%Y-%m-%d %H:%M:%S')}] #{severity.ljust(5)} - #{msg}\n"
end

logger.info("Application started")
# => [2026-03-13 12:00:00] INFO  - Application started

Formatter Parameters

Parameter	Description
severity	Level name (DEBUG, INFO, etc.)
datetime	Time object of the log entry
progname	Program name (from progname= method)
msg	The actual message

Log Rotation

Automatically rotate logs to prevent disk space issues:

# Rotate when file exceeds 1MB, keep 5 old files
logger = Logger.new('app.log', 5, 1048576)

# Rotate daily, keep 7 days of logs
logger = Logger.new('app.log', 7)

Rotation Parameters

Parameter	Description
shift_age	Number of old files to keep, or frequency (daily, weekly)
shift_size	Maximum size in bytes before rotation

# Weekly rotation
logger = Logger.new('weekly.log', 'weekly')

# Size-based rotation (10MB max)
logger = Logger.new('large.log', 3, 10 * 1024 * 1024)

Practical Examples

Application Logging

require 'logger'

class Application
  def initialize
    @logger = Logger.new('app.log')
    @logger.level = Logger::INFO
    @logger.progname = 'MyApp'
  end
  
  def run
    @logger.info("Application starting")
    # ... application code ...
    @logger.info("Application finished")
  rescue => e
    @logger.error("Application failed: #{e.message}")
  end
end

Multiple Loggers

require 'logger'

# Debug log - detailed
debug_log = Logger.new('debug.log')
debug_log.level = Logger::DEBUG

# Production log - warnings and above
prod_log = Logger.new('production.log')
prod_log.level = Logger::WARN

Rails-Like Logging

require 'logger'

class CustomLogger < Logger
  def format_message(level, datetime, progname, msg)
    "#{level} [#{datetime}]: #{msg}\n"
  end
end

logger = CustomLogger.new('custom.log')
logger.info("Custom format message")

Logging with Block Syntax

logger = Logger.new(STDOUT)

# Only evaluates block if level is enabled
logger.debug { "Expensive debug: #{expensive_computation}" }
logger.info { "User #{current_user.id} logged in" }

Logger Constants

Constant	Value	Description
DEBUG	0	Debug level
INFO	1	Info level
WARN	2	Warning level
ERROR	3	Error level
FATAL	4	Fatal level
UNKNOWN	5	Unknown/severe level

Ruby Version History

• Ruby 1.8: Introduced the Logger class
• Ruby 2.1: Improved thread safety
• Ruby 2.5: Added support for log rotation by time period

See Also

• File — file operations for log storage
• Dir — directory operations
• YAML — serialize log data