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/Ruby 2.4

class Logger

Parent:
Object
Included modules:
Logger::Severity

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
    
  4. Constructor

    Logger.new(logdev, level: Logger::INFO)
    Logger.new(logdev, level: :info)
    Logger.new(logdev, level: 'INFO')
    

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 via the constructor.

Logger.new(logdev, datetime_format: '%Y-%m-%d %H:%M:%S')

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"

or via the constructor.

Logger.new(logdev, formatter: proc {|severity, datetime, progname, msg|
  "#{datetime}: #{msg}\n"
})

Constants

ProgName
SEV_LABEL

Severity label for logging (max 5 chars).

VERSION

Attributes

formatter[RW]

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.

level[R]

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

progname[RW]

Program name to include in log messages.

sev_threshold[R]

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

Public Class Methods

new(logdev, shift_age = 0, shift_size = 1048576) Show source
new(logdev, shift_age = 'weekly')
new(logdev, level: :info)
new(logdev, progname: 'progname')
new(logdev, formatter: formatter)
new(logdev, datetime_format: '%Y-%m-%d %H:%M:%S')

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). Default value is 0.

shift_size

Maximum logfile size in bytes (only applies when shift_age is a number). Defaults to 1048576 (1MB).

level

Logging severity threshold. Default values is Logger::DEBUG.

progname

Program name to include in log messages. Default value is nil.

formatter

Logging formatter. Default values is an instance of Logger::Formatter.

datetime_format

Date and time format. Default value is '%Y-%m-%d %H:%M:%S'.

shift_period_suffix

The log file suffix format for daily, weekly or monthly rotation. Default is '%Y%m%d'.

Description

Create an instance.

# File lib/logger.rb, line 376
def initialize(logdev, shift_age = 0, shift_size = 1048576, level: DEBUG,
               progname: nil, formatter: nil, datetime_format: nil,
               shift_period_suffix: '%Y%m%d')
  self.level = level
  self.progname = progname
  @default_formatter = Formatter.new
  self.datetime_format = datetime_format
  self.formatter = formatter
  @logdev = nil
  if logdev
    @logdev = LogDevice.new(logdev, :shift_age => shift_age,
      :shift_size => shift_size,
      :shift_period_suffix => shift_period_suffix)
  end
end

Public Instance Methods

<<(msg) Show source

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

# File lib/logger.rb, line 478
def <<(msg)
  unless @logdev.nil?
    @logdev.write(msg)
  end
end
Logger#add(severity, message = nil, progname = nil) { ... } Show source

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.

# File lib/logger.rb, line 454
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
Also aliased as: log
close() Show source

Close the logging device.

# File lib/logger.rb, line 567
def close
  @logdev.close if @logdev
end
datetime_format() Show source

Returns the date format being used. See datetime_format=

# File lib/logger.rb, line 299
def datetime_format
  @default_formatter.datetime_format
end
datetime_format=(datetime_format) Show source

Set date-time format.

datetime_format

A string suitable for passing to strftime.

# File lib/logger.rb, line 294
def datetime_format=(datetime_format)
  @default_formatter.datetime_format = datetime_format
end
debug(progname = nil, &block) Show source

Log a DEBUG message.

See info for more information.

# File lib/logger.rb, line 489
def debug(progname = nil, &block)
  add(DEBUG, nil, progname, &block)
end
debug?() Show source

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

# File lib/logger.rb, line 322
def debug?; @level <= DEBUG; end
error(progname = nil, &block) Show source

Log an ERROR message.

See info for more information.

# File lib/logger.rb, line 541
def error(progname = nil, &block)
  add(ERROR, nil, progname, &block)
end
error?() Show source

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

# File lib/logger.rb, line 334
def error?; @level <= ERROR; end
fatal(progname = nil, &block) Show source

Log a FATAL message.

See info for more information.

# File lib/logger.rb, line 550
def fatal(progname = nil, &block)
  add(FATAL, nil, progname, &block)
end
fatal?() Show source

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

# File lib/logger.rb, line 338
def fatal?; @level <= FATAL; end
info(message) Show source
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 523
def info(progname = nil, &block)
  add(INFO, nil, progname, &block)
end
info?() Show source

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

# File lib/logger.rb, line 326
def info?; @level <= INFO; end
level=(severity) Show source

Set logging severity threshold.

severity

The Severity of the log message.

# File lib/logger.rb, line 265
def level=(severity)
  if severity.is_a?(Integer)
    @level = severity
  else
    case severity.to_s.downcase
    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
Also aliased as: sev_threshold=
log(severity, message = nil, progname = nil)
Alias for: add
Logger#reopen Show source
Logger#reopen(logdev)

Args

logdev

The log device. This is a filename (String) or IO object (typically STDOUT, STDERR, or an open file). reopen the same filename if it is nil, do nothing for IO. Default is nil.

Description

Reopen a log device.

# File lib/logger.rb, line 408
def reopen(logdev = nil)
  @logdev.reopen(logdev)
  self
end
sev_threshold=(severity)
Alias for: level=
unknown(progname = nil, &block) Show source

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 560
def unknown(progname = nil, &block)
  add(UNKNOWN, nil, progname, &block)
end
warn(progname = nil, &block) Show source

Log a WARN message.

See info for more information.

# File lib/logger.rb, line 532
def warn(progname = nil, &block)
  add(WARN, nil, progname, &block)
end
warn?() Show source

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

# File lib/logger.rb, line 330
def warn?; @level <= WARN; end

Private Instance Methods

format_message(severity, datetime, progname, msg) Show source
# File lib/logger.rb, line 580
def format_message(severity, datetime, progname, msg)
  (@formatter || @default_formatter).call(severity, datetime, progname, msg)
end
format_severity(severity) Show source
# File lib/logger.rb, line 576
def format_severity(severity)
  SEV_LABEL[severity] || 'ANY'
end

Ruby Core © 1993–2017 Yukihiro Matsumoto
Licensed under the Ruby License.
Ruby Standard Library © contributors
Licensed under their own licenses.