< Object
Included Modules

Simple logging utility.

Author:NAKAMURA, Hiroshi <>
Documentation:NAKAMURA, Hiroshi and Gavin Sinclair
License:You can redistribute it and/or modify it under the same terms of Ruby‘s license; either the dual license version in 2003, or any later version.
Revision:$Id: logger.rb 11708 2007-02-12 23:01:19Z shyouhei $


The Logger class provides a simple but sophisticated logging utility that anyone can use because it‘s included in the Ruby 1.8.x standard library.

The HOWTOs below give a code-based overview of Logger‘s usage, but the basic concept is as follows. You create a Logger object (output to a file or elsewhere), and use it to log messages. The messages will have varying levels (info, error, etc), reflecting their varying importance. The levels, and their meanings, are:

FATAL:an unhandleable error that results in a program crash
ERROR:a handleable error condition
WARN:a warning
INFO:generic (useful) information about system operation
DEBUG:low-level information for developers

So each message has a level, and the Logger itself has a level, which acts as a filter, so you can control the amount of information emitted from the logger without having to remove actual messages.

For instance, in a production system, you may have your logger(s) set to INFO (or WARN if you don‘t want the log files growing large with repetitive information). When you are developing it, though, you probably want to know about the program‘s internal state, and would set them to DEBUG.


A simple example demonstrates the above explanation:

  log =
  log.level = Logger::WARN

  log.debug("Created logger")"Program started")
  log.warn("Nothing to do!")

    File.each_line(path) do |line|
      unless line =~ /^(\w+) = (.*)$/
        log.error("Line in wrong format: #{line}")
  rescue => err
    log.fatal("Caught exception; exiting")

Because the Logger‘s level is set to WARN, only the warning, error, and fatal messages are recorded. The debug and info messages are silently discarded.


There are several interesting features that Logger provides, like auto-rolling of log files, setting the format of log messages, and specifying a program name in conjunction with the message. The next section shows you how to achieve these things.


How to create a logger

The options below give you various choices, in more or less increasing complexity.

  1. Create a logger which logs messages to STDERR/STDOUT.
      logger =
      logger =
  2. Create a logger for the file which has the specified name.
      logger ='logfile.log')
  3. Create a logger for the specified file.
      file ='foo.log', File::WRONLY | File::APPEND)
      # To create new (and to remove old) logfile, add File::CREAT like;
      #   file = open('foo.log', File::WRONLY | File::APPEND | File::CREAT)
      logger =
  4. Create a logger which ages logfile once it reaches a certain size. Leave 10 "old log files" and each file is about 1,024,000 bytes.
      logger ='foo.log', 10, 1024000)
  5. Create a logger which ages logfile daily/weekly/monthly.
      logger ='foo.log', 'daily')
      logger ='foo.log', 'weekly')
      logger ='foo.log', 'monthly')

How to log a message

Notice the different methods (fatal, error, info) being used to log messages of various levels. Other methods in this family are warn and debug. add is used below to log a message of an arbitrary (perhaps dynamic) level.

  1. Message in block.
      logger.fatal { "Argument 'foo' not given." }
  2. Message as a string.
      logger.error "Argument #{ @foo } mismatch."
  3. With progname.'initialize') { "Initializing..." }
  4. With severity.
      logger.add(Logger::FATAL) { 'Fatal error!' }

How to close a logger


Setting severity threshold

  1. Original interface.
      logger.sev_threshold = Logger::WARN
  2. Log4r (somewhat) compatible interface.
      logger.level = Logger::INFO


Log messages are rendered in the output stream in a certain format. The default format and a sample are shown below:

Log format:

  SeverityID, [Date Time mSec #pid] SeverityLabel -- ProgName: message

Log sample:

  I, [Wed Mar 03 02:34:24 JST 1999 895701 #19074]  INFO -- Main: info.

You may change the date and time format in this manner:

  logger.datetime_format = "%Y-%m-%d %H:%M:%S"
        # e.g. "2004-01-03 00:54:26"

There is currently no supported way to change the overall format, but you may have some luck hacking the Format constant.

Classes & Modules


Name   Description
ProgName = "#{$1}/#{$2}"
SEV_LABEL = %w(DEBUG INFO WARN ERROR FATAL ANY) Severity label for logging. (max 5 char)
VERSION = "1.2.6"


Name Visibility R/W Description
formatter public RW Logging formatter. formatter#call is invoked with 4 arguments; severity, time, progname and msg for each log. Bear in mind that time is a Time and msg is an Object that user passed and it could not be a String. It is expected to return a logdev#write-able Object. Default formatter is used when no formatter is set.
level public RW Logging severity threshold (e.g. Logger::INFO).
progname public RW Logging program name.


Method Alias Description
level → sev_threshold
level= → sev_threshold=



Visibility Signature
public new (logdev, shift_age = 0, shift_size = 1048576)


Visibility Signature
public << (msg)
public add (severity, message = nil, progname = nil) {|| ...}
public close ()
public datetime_format ()
public datetime_format= (datetime_format)
public debug (progname = nil, &block)
public debug? ()
public error (progname = nil, &block)
public error? ()
public fatal (progname = nil, &block)
public fatal? ()
public info (progname = nil, &block)
public info? ()
public log (severity, message = nil, progname = nil, &block)
public unknown (progname = nil, &block)
public warn (progname = nil, &block)
public warn? ()

Class Method Detail

new(logdev, shift_age = 0, shift_size = 1048576)

Synopsis, shift_age = 7, shift_size = 1048576), shift_age = 'weekly')


logdev:The log device. This is a filename (String) or IO object (typically STDOUT, STDERR, or an open file).
shift_age:Number of old log files to keep, or frequency of rotation (daily, weekly or monthly).
shift_size:Maximum logfile size (only applies when shift_age is a number).


Create an instance.

Instance Method Detail


Dump given message to the log device without any formatting. If no log device exists, return nil.

add(severity, message = nil, progname = nil) {|| ...}


  Logger#add(severity, message = nil, progname = nil) { ... }


severity:Severity. Constants are defined in Logger namespace: DEBUG, INFO, WARN, ERROR, FATAL, or UNKNOWN.
message:The log message. A String or Exception.
progname:Program name string. Can be omitted. Treated as a message if no message and block are given.
block:Can be omitted. Called to get a message string if message is nil.


true if successful, false otherwise.

When the given severity is not high enough (for this particular logger), log no message, and return true.


Log a message if the given severity is high enough. This is the generic logging method. Users will be more inclined to use debug, info, warn, error, and fatal.

Message format: message can be any object, but it has to be converted to a String in order to log it. Generally, inspect is used if the given object is not a String. A special case is an Exception object, which will be printed in detail, including message, class, and backtrace. See msg2str for the implementation if required.


  • Logfile is not locked.
  • Append open does not need to lock file.
  • But on the OS which supports multi I/O, records possibly be mixed.


Close the logging device.



Logging date-time format (string passed to strftime).

debug(progname = nil, &block)

Log a DEBUG message.

See info for more information.


Returns true iff the current severity level allows for the printing of DEBUG messages.

error(progname = nil, &block)

Log an ERROR message.

See info for more information.


Returns true iff the current severity level allows for the printing of ERROR messages.

fatal(progname = nil, &block)

Log a FATAL message.

See info for more information.


Returns true iff the current severity level allows for the printing of FATAL messages.

info(progname = nil, &block)

Log an INFO message.

The message can come either from the progname argument or the block. If both are provided, then the block is used as the message, and progname is used as the program name.

Examples"MainApp") { "Received connection from #{ip}" }
  # ... "Waiting for input from user"
  # ... { "User typed #{input}" }

You‘ll probably stick to the second form above, unless you want to provide a program name (which you can do with Logger#progname= as well).


See add.


Returns true iff the current severity level allows for the printing of INFO messages.

log(severity, message = nil, progname = nil, &block)

Alias for add

unknown(progname = nil, &block)

Log an UNKNOWN message. This will be printed no matter what the logger level.

See info for more information.

warn(progname = nil, &block)

Log a WARN message.

See info for more information.


Returns true iff the current severity level allows for the printing of WARN messages.