Class: Logger (Ruby 2.3.7)
Description: Class : Logger - Ruby 2.3.7

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Support for the Ruby 2.3 series has ended. See here for reference.

### Home Classes Methods

### In Files

- logger.rb

### Parent

Object

### Namespace

- MODULE Logger::Period
- MODULE Logger::Severity
- CLASS Logger::Formatter
- CLASS Logger::LogDevice

### Methods

- ::new
- #<<
- #add
- #close
- #datetime_format
- #datetime_format=
- #debug
- #debug?
- #error
- #error?
- #fatal
- #fatal?
- #info
- #info?
- #level=
- #log
- #reopen
- #sev_threshold=
- #unknown
- #warn
- #warn?

### Included Modules

- Logger::Severity

### Class/Module Index

Quicksearch

- Logger
- Logger::Formatter
- Logger::LogDevice
- Logger::Period
- Logger::Severity

No matching classes.

# 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"

### 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 = 7, shift_size = 1048576) click to toggle source
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 logger.rb, line 345
    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

### 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 logger.rb, line 442
    def <<(msg)
      unless @logdev.nil?
        @logdev.write(msg)
      end
    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 logger.rb, line 418
    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() click to toggle source

Close the logging device.

                   # File logger.rb, line 531
    def close
      @logdev.close if @logdev
    end

datetime_format() click to toggle source

Returns the date format being used. See datetime_format=

                   # File logger.rb, line 284
    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 logger.rb, line 279
    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 logger.rb, line 453
    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 logger.rb, line 307
    def debug?; @level <= DEBUG; end

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

Log an `ERROR` message.

See info for more information.

                   # File logger.rb, line 505
    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 logger.rb, line 319
    def error?; @level <= ERROR; end

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

Log a `FATAL` message.

See info for more information.

                   # File logger.rb, line 514
    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 logger.rb, line 323
    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 logger.rb, line 487
    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 logger.rb, line 311
    def info?; @level <= INFO; end

level=(severity) click to toggle source

Set logging severity threshold.

`severity`

  The Severity of the log message.

                   # File logger.rb, line 249
    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

Also aliased as: sev_threshold=
log(severity, message = nil, progname = nil) click to toggle source
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).

### Description¶ ↑

Reopen a log device.

                   # File logger.rb, line 372
    def reopen(logdev = nil)
      @logdev.reopen(logdev)
      self
    end

sev_threshold=(severity) click to toggle source
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 logger.rb, line 524
    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 logger.rb, line 496
    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 logger.rb, line 315
    def warn?; @level <= WARN; end