Logger

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"