Description

The Logger class provides a simple but sophisticated logging utility that you can use to output messages.

The messages have associated levels, such as INFO or ERROR that indicate their importance. You can then give the Logger a level, and only messages at that level or higher will be printed.

The levels are:

UNKNOWN

An unknown message that should always be logged.

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.

For instance, in a production system, you may have your Logger set to INFO or even WARN. When you are developing the system, however, you probably want to know about the program's internal state, and would set the Logger to DEBUG.

Note: Logger does not escape or sanitize any messages passed to it. Developers should be aware of when potentially malicious data (user-input) is passed to Logger, and manually escape the untrusted data:

logger.info("User-input: #{input.dump}")
logger.info("User-input: %p" % input)

You can use formatter= for escaping all data.

original_formatter = Logger::Formatter.new
logger.formatter = proc { |severity, datetime, progname, msg|
  original_formatter.call(severity, datetime, progname, msg.dump)
}
logger.info(input)

Example

This creates a Logger that outputs to the standard output stream, with a level of WARN:

require 'logger'

logger = Logger.new(STDOUT)
logger.level = Logger::WARN

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

path = "a_non_existent_file"

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

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.

Features

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.

HOWTOs

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.new(STDERR)
    logger = Logger.new(STDOUT)
    
  2. Create a logger for the file which has the specified name.

    logger = Logger.new('logfile.log')
    
  3. Create a logger for the specified file.

    file = File.open('foo.log', File::WRONLY | File::APPEND)
    # To create new (and to remove old) logfile, add File::CREAT like:
    # file = File.open('foo.log', File::WRONLY | File::APPEND | File::CREAT)
    logger = Logger.new(file)
    
  4. Create a logger which ages the logfile once it reaches a certain size. Leave 10 “old” log files where each file is about 1,024,000 bytes.

    logger = Logger.new('foo.log', 10, 1024000)
    
  5. Create a logger which ages the logfile daily/weekly/monthly.

    logger = Logger.new('foo.log', 'daily')
    logger = Logger.new('foo.log', 'weekly')
    logger = Logger.new('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 a block.

    logger.fatal { "Argument 'foo' not given." }
    
  2. Message as a string.

    logger.error "Argument #{@foo} mismatch."
    
  3. With progname.

    logger.info('initialize') { "Initializing..." }
    
  4. With severity.

    logger.add(Logger::FATAL) { 'Fatal error!' }
    

The block form allows you to create potentially complex log messages, but to delay their evaluation until and unless the message is logged. For example, if we have the following:

logger.debug { "This is a " + potentially + " expensive operation" }

If the logger's level is INFO or higher, no debug messages will be logged, and the entire block will not even be evaluated. Compare to this:

logger.debug("This is a " + potentially + " expensive operation")

Here, the string concatenation is done every time, even if the log level is not set to show the debug message.

How to close a logger

logger.close

Setting severity threshold

  1. Original interface.

    logger.sev_threshold = Logger::WARN
    
  2. Log4r (somewhat) compatible interface.

    logger.level = Logger::INFO
    
    # DEBUG < INFO < WARN < ERROR < FATAL < UNKNOWN
    
  3. Symbol or String (case insensitive)

    logger.level = :info
    logger.level = 'INFO'
    
    # :debug < :info < :warn < :error < :fatal < :unknown
    

Format

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

Log format:

SeverityID, [DateTime #pid] SeverityLabel -- ProgName: message

Log sample:

I, [1999-03-03T02:34:24.895701 #19074]  INFO -- Main: info.

You may change the date and time format via datetime_format=.

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

Or, you may change the overall format via the formatter= method.

logger.formatter = proc do |severity, datetime, progname, msg|
  "#{datetime}: #{msg}\n"
end
# e.g. "2005-09-22 08:51:08 +0900: hello world"
Namespace
Methods
#
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Included Modules
Constants
VERSION = "1.2.7"
 
ProgName = "#{name}/#{rev}".freeze
 
SEV_LABEL = %w(DEBUG INFO WARN ERROR FATAL ANY).each(&:freeze).freeze
 

Severity label for logging (max 5 chars).

Attributes
[RW] formatter

Logging formatter, as a Proc that will take four arguments and return the formatted message. The arguments are:

severity

The Severity of the log message.

time

A Time instance representing when the message was logged.

progname

The progname configured, or passed to the logger method.

msg

The Object the user passed to the log message; not necessarily a String.

The block should return an Object that can be written to the logging device via write. The default formatter is used when no formatter is set.

[R] level

Logging severity threshold (e.g. Logger::INFO).

[RW] progname

Program name to include in log messages.

[R] sev_threshold

Logging severity threshold (e.g. Logger::INFO).

Class Public methods
Logger.new(logdev, shift_age = 7, shift_size = 1048576)
Logger.new(logdev, shift_age = 'weekly')

Args

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).

Description

Create an instance.

# File lib/logger.rb, line 346
def initialize(logdev, shift_age = 0, shift_size = 1048576)
  @progname = nil
  @level = DEBUG
  @default_formatter = Formatter.new
  @formatter = nil
  @logdev = nil
  if logdev
    @logdev = LogDevice.new(logdev, :shift_age => shift_age,
      :shift_size => shift_size)
  end
end
Instance Public methods
<<(msg)

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

# File lib/logger.rb, line 443
def <<(msg)
  unless @logdev.nil?
    @logdev.write(msg)
  end
end
Logger#add(severity, message = nil, progname = nil) { ... }

Args

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.

Return

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

Description

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.

Bugs

  • Logfile is not locked.

  • Append open does not need to lock file.

  • If the OS supports multi I/O, records possibly may be mixed.

Also aliased as: log
# File lib/logger.rb, line 419
def add(severity, message = nil, progname = nil)
  severity ||= UNKNOWN
  if @logdev.nil? or severity < @level
    return true
  end
  progname ||= @progname
  if message.nil?
    if block_given?
      message = yield
    else
      message = progname
      progname = @progname
    end
  end
  @logdev.write(
    format_message(format_severity(severity), Time.now, progname, message))
  true
end
close()

Close the logging device.

# File lib/logger.rb, line 532
def close
  @logdev.close if @logdev
end
datetime_format()

Returns the date format being used. See datetime_format=

# File lib/logger.rb, line 285
def datetime_format
  @default_formatter.datetime_format
end
datetime_format=(datetime_format)

Set date-time format.

datetime_format

A string suitable for passing to strftime.

# File lib/logger.rb, line 280
def datetime_format=(datetime_format)
  @default_formatter.datetime_format = datetime_format
end
debug(progname = nil, &block)

Log a DEBUG message.

See info for more information.

# File lib/logger.rb, line 454
def debug(progname = nil, &block)
  add(DEBUG, nil, progname, &block)
end
debug?()

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

# File lib/logger.rb, line 308
def debug?; @level <= DEBUG; end
error(progname = nil, &block)

Log an ERROR message.

See info for more information.

# File lib/logger.rb, line 506
def error(progname = nil, &block)
  add(ERROR, nil, progname, &block)
end
error?()

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

# File lib/logger.rb, line 320
def error?; @level <= ERROR; end
fatal(progname = nil, &block)

Log a FATAL message.

See info for more information.

# File lib/logger.rb, line 515
def fatal(progname = nil, &block)
  add(FATAL, nil, progname, &block)
end
fatal?()

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

# File lib/logger.rb, line 324
def fatal?; @level <= FATAL; end
info(message)
info(progname, &block)

Log an INFO message.

message

The message to log; does not need to be a String.

progname

In the block form, this is the progname to use in the log message. The default can be set with progname=.

block

Evaluates to the message to log. This is not evaluated unless the logger's level is sufficient to log the message. This allows you to create potentially expensive logging messages that are only called when the logger is configured to show them.

Examples

logger.info("MainApp") { "Received connection from #{ip}" }
# ...
logger.info "Waiting for input from user"
# ...
logger.info { "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 progname= as well).

Return

See add.

# File lib/logger.rb, line 488
def info(progname = nil, &block)
  add(INFO, nil, progname, &block)
end
info?()

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

# File lib/logger.rb, line 312
def info?; @level <= INFO; end
level=(severity)

Set logging severity threshold.

severity

The Severity of the log message.

Also aliased as: sev_threshold=
# File lib/logger.rb, line 250
def level=(severity)
  if severity.is_a?(Integer)
    @level = severity
  else
    _severity = severity.to_s.downcase
    case _severity
    when 'debug'.freeze
      @level = DEBUG
    when 'info'.freeze
      @level = INFO
    when 'warn'.freeze
      @level = WARN
    when 'error'.freeze
      @level = ERROR
    when 'fatal'.freeze
      @level = FATAL
    when 'unknown'.freeze
      @level = UNKNOWN
    else
      raise ArgumentError, "invalid log level: #{severity}"
    end
  end
end
log(severity, message = nil, progname = nil)
Alias for: add
Logger#reopen
Logger#reopen(logdev)

Args

logdev

The log device. This is a filename (String) or IO object (typically STDOUT, STDERR, or an open file).

Description

Reopen a log device.

# File lib/logger.rb, line 373
def reopen(logdev = nil)
  @logdev.reopen(logdev)
  self
end
sev_threshold=(severity)
Alias for: level=
unknown(progname = nil, &block)

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

See info for more information.

# File lib/logger.rb, line 525
def unknown(progname = nil, &block)
  add(UNKNOWN, nil, progname, &block)
end
warn(progname = nil, &block)

Log a WARN message.

See info for more information.

# File lib/logger.rb, line 497
def warn(progname = nil, &block)
  add(WARN, nil, progname, &block)
end
warn?()

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

# File lib/logger.rb, line 316
def warn?; @level <= WARN; end