# class 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 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) click to toggle 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 377
    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) click to toggle source

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

    # File lib/logger.rb, line 481
    def <<(msg)
      @logdev&.write(msg)
    end

Logger#add(severity, message = nil, progname = nil) { ... } click to toggle 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 455
    def add(severity, message = nil, progname = nil)
      severity ||= UNKNOWN
      if @logdev.nil? or severity < @level
        return true
      end
      if progname.nil?
        progname = @progname
      end
      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() click to toggle source

Close the logging device.

    # File lib/logger.rb, line 568
    def close
      @logdev&.close
    end

datetime_format() click to toggle source

Returns the date format being used. See `datetime_format=`

    # File lib/logger.rb, line 300
    def datetime_format
      @default_formatter.datetime_format
    end

datetime_format=(datetime_format) click to toggle source

`Set` date-time format.

`datetime_format`

  A string suitable for passing to `strftime`.

    # File lib/logger.rb, line 295
    def datetime_format=(datetime_format)
      @default_formatter.datetime_format = datetime_format
    end

debug(progname = nil, &block) click to toggle source

Log a `DEBUG` message.

See `info` for more information.

    # File lib/logger.rb, line 490
    def debug(progname = nil, &block)
      add(DEBUG, nil, progname, &block)
    end

debug?() click to toggle source

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

    # File lib/logger.rb, line 323
    def debug?; @level <= DEBUG; end

error(progname = nil, &block) click to toggle source

Log an `ERROR` message.

See `info` for more information.

    # File lib/logger.rb, line 542
    def error(progname = nil, &block)
      add(ERROR, nil, progname, &block)
    end

error?() click to toggle source

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

    # File lib/logger.rb, line 335
    def error?; @level <= ERROR; end

fatal(progname = nil, &block) click to toggle source

Log a `FATAL` message.

See `info` for more information.

    # File lib/logger.rb, line 551
    def fatal(progname = nil, &block)
      add(FATAL, nil, progname, &block)
    end

fatal?() click to toggle source

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

    # File lib/logger.rb, line 339
    def fatal?; @level <= FATAL; end

info(message) click to toggle 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 524
    def info(progname = nil, &block)
      add(INFO, nil, progname, &block)
    end

info?() click to toggle source

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

    # File lib/logger.rb, line 327
    def info?; @level <= INFO; end

level=(severity) click to toggle source

`Set` logging severity threshold.

`severity`

  The `Severity` of the log message.

    # File lib/logger.rb, line 266
    def level=(severity)
      if severity.is_a?(Integer)
        @level = severity
      else
        case severity.to_s.downcase
        when 'debug'
          @level = DEBUG
        when 'info'
          @level = INFO
        when 'warn'
          @level = WARN
        when 'error'
          @level = ERROR
        when 'fatal'
          @level = FATAL
        when 'unknown'
          @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 click to toggle 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 409
    def reopen(logdev = nil)
      @logdev.reopen(logdev)
      self
    end

sev_threshold=(severity)
Alias for: level=
unknown(progname = nil, &block) click to toggle 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 561
    def unknown(progname = nil, &block)
      add(UNKNOWN, nil, progname, &block)
    end

warn(progname = nil, &block) click to toggle source

Log a `WARN` message.

See `info` for more information.

    # File lib/logger.rb, line 533
    def warn(progname = nil, &block)
      add(WARN, nil, progname, &block)
    end

warn?() click to toggle source

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

    # File lib/logger.rb, line 331
    def warn?; @level <= WARN; end

### Private Instance Methods

format_message(severity, datetime, progname, msg) click to toggle source

    # File lib/logger.rb, line 581
    def format_message(severity, datetime, progname, msg)
      (@formatter || @default_formatter).call(severity, datetime, progname, msg)
    end

format_severity(severity) click to toggle source

    # File lib/logger.rb, line 577
    def format_severity(severity)
      SEV_LABEL[severity] || 'ANY'
    end