Class

Class Logger provides a simple but sophisticated logging utility that you can use to create one or more event logs for your program. Each such log contains a chronological sequence of entries that provides a record of the program’s activities.

About the Examples

All examples on this page assume that Logger has been required:

require 'logger'

Synopsis

Create a log with Logger.new:

# Single log file.
logger = Logger.new('t.log')
# Size-based rotated logging: 3 10-megabyte files.
logger = Logger.new('t.log', 3, 10485760)
# Period-based rotated logging: daily (also allowed: 'weekly', 'monthly').
logger = Logger.new('t.log', 'daily')
# Log to an IO stream.
logger = Logger.new($stdout)

Add entries (level, message) with Logger#add:

logger.add(Logger::DEBUG, 'Maximal debugging info')
logger.add(Logger::INFO, 'Non-error information')
logger.add(Logger::WARN, 'Non-error warning')
logger.add(Logger::ERROR, 'Non-fatal error')
logger.add(Logger::FATAL, 'Fatal error')
logger.add(Logger::UNKNOWN, 'Most severe')

Close the log with Logger#close:

logger.close

Entries

You can add entries with method Logger#add:

logger.add(Logger::DEBUG, 'Maximal debugging info')
logger.add(Logger::INFO, 'Non-error information')
logger.add(Logger::WARN, 'Non-error warning')
logger.add(Logger::ERROR, 'Non-fatal error')
logger.add(Logger::FATAL, 'Fatal error')
logger.add(Logger::UNKNOWN, 'Most severe')

These shorthand methods also add entries:

logger.debug('Maximal debugging info')
logger.info('Non-error information')
logger.warn('Non-error warning')
logger.error('Non-fatal error')
logger.fatal('Fatal error')
logger.unknown('Most severe')

When you call any of these methods, the entry may or may not be written to the log, depending on the entry’s severity and on the log level; see Log Level

An entry always has:

  • A severity (the required argument to add).

  • An automatically created timestamp.

And may also have:

  • A message.

  • A program name.

Example:

logger = Logger.new($stdout)
logger.add(Logger::INFO, 'My message.', 'mung')
# => I, [2022-05-07T17:21:46.536234 #20536]  INFO -- mung: My message.

The default format for an entry is:

"%s, [%s #%d] %5s -- %s: %s\n"

where the values to be formatted are:

  • Severity (one letter).

  • Timestamp.

  • Process id.

  • Severity (word).

  • Program name.

  • Message.

You can use a different entry format by:

  • Setting a custom format proc (affects following entries); see formatter=.

  • Calling any of the methods above with a block (affects only the one entry). Doing so can have two benefits:

    • Context: the block can evaluate the entire program context and create a context-dependent message.

    • Performance: the block is not evaluated unless the log level permits the entry actually to be written:

      logger.error { my_slow_message_generator }
      

      Contrast this with the string form, where the string is always evaluated, regardless of the log level:

      logger.error("#{my_slow_message_generator}")
      

Severity

The severity of a log entry has two effects:

  • Determines whether the entry is selected for inclusion in the log; see Log Level.

  • Indicates to any log reader (whether a person or a program) the relative importance of the entry.

Timestamp

The timestamp for a log entry is generated automatically when the entry is created.

The logged timestamp is formatted by method Time#strftime using this format string:

'%Y-%m-%dT%H:%M:%S.%6N'

Example:

logger = Logger.new($stdout)
logger.add(Logger::INFO)
# => I, [2022-05-07T17:04:32.318331 #20536]  INFO -- : nil

You can set a different format using method datetime_format=.

Message

The message is an optional argument to an entry method:

logger = Logger.new($stdout)
logger.add(Logger::INFO, 'My message')
# => I, [2022-05-07T18:15:37.647581 #20536]  INFO -- : My message

For the default entry formatter, Logger::Formatter, the message object may be:

  • A string: used as-is.

  • An Exception: message.message is used.

  • Anything else: message.inspect is used.

Note: Logger::Formatter does not escape or sanitize the message passed to it. Developers should be aware that malicious data (user input) may be in the message, and should explicitly escape untrusted data.

You can use a custom formatter to escape message data; see the example at formatter=.

Program Name

The program name is an optional argument to an entry method:

logger = Logger.new($stdout)
logger.add(Logger::INFO, 'My message', 'mung')
# => I, [2022-05-07T18:17:38.084716 #20536]  INFO -- mung: My message

The default program name for a new logger may be set in the call to Logger.new via optional keyword argument progname:

logger = Logger.new('t.log', progname: 'mung')

The default program name for an existing logger may be set by a call to method progname=:

logger.progname = 'mung'

The current program name may be retrieved with method progname:

logger.progname # => "mung"

Log Level

The log level setting determines whether an entry is actually written to the log, based on the entry’s severity.

These are the defined severities (least severe to most severe):

logger = Logger.new($stdout)
logger.add(Logger::DEBUG, 'Maximal debugging info')
# => D, [2022-05-07T17:57:41.776220 #20536] DEBUG -- : Maximal debugging info
logger.add(Logger::INFO, 'Non-error information')
# => I, [2022-05-07T17:59:14.349167 #20536]  INFO -- : Non-error information
logger.add(Logger::WARN, 'Non-error warning')
# => W, [2022-05-07T18:00:45.337538 #20536]  WARN -- : Non-error warning
logger.add(Logger::ERROR, 'Non-fatal error')
# => E, [2022-05-07T18:02:41.592912 #20536] ERROR -- : Non-fatal error
logger.add(Logger::FATAL, 'Fatal error')
# => F, [2022-05-07T18:05:24.703931 #20536] FATAL -- : Fatal error
logger.add(Logger::UNKNOWN, 'Most severe')
# => A, [2022-05-07T18:07:54.657491 #20536]   ANY -- : Most severe

The default initial level setting is Logger::DEBUG, the lowest level, which means that all entries are to be written, regardless of severity:

logger = Logger.new($stdout)
logger.level # => 0
logger.add(0, "My message")
# => D, [2022-05-11T15:10:59.773668 #20536] DEBUG -- : My message

You can specify a different setting in a new logger using keyword argument level with an appropriate value:

logger = Logger.new($stdout, level: Logger::ERROR)
logger = Logger.new($stdout, level: 'error')
logger = Logger.new($stdout, level: :error)
logger.level # => 3

With this level, entries with severity Logger::ERROR and higher are written, while those with lower severities are not written:

logger = Logger.new($stdout, level: Logger::ERROR)
logger.add(3)
# => E, [2022-05-11T15:17:20.933362 #20536] ERROR -- : nil
logger.add(2) # Silent.

You can set the log level for an existing logger with method level=:

logger.level = Logger::ERROR

These shorthand methods also set the level:

logger.debug! # => 0
logger.info!  # => 1
logger.warn!  # => 2
logger.error! # => 3
logger.fatal! # => 4

You can retrieve the log level with method level:

logger.level = Logger::ERROR
logger.level # => 3

These methods return whether a given level is to be written:

logger.level = Logger::ERROR
logger.debug? # => false
logger.info?  # => false
logger.warn?  # => false
logger.error? # => true
logger.fatal? # => true

Log File Rotation

By default, a log file is a single file that grows indefinitely (until explicitly closed); there is no file rotation.

To keep log files to a manageable size, you can use log file rotation, which uses multiple log files:

  • Each log file has entries for a non-overlapping time interval.

  • Only the most recent log file is open and active; the others are closed and inactive.

Size-Based Rotation

For size-based log file rotation, call Logger.new with:

  • Argument logdev as a file path.

  • Argument shift_age with a positive integer: the number of log files to be in the rotation.

  • Argument shift_size as a positive integer: the maximum size (in bytes) of each log file; defaults to 1048576 (1 megabyte).

Examples:

logger = Logger.new('t.log', 3)           # Three 1-megabyte files.
logger = Logger.new('t.log', 5, 10485760) # Five 10-megabyte files.

For these examples, suppose:

logger = Logger.new('t.log', 3)

Logging begins in the new log file, t.log; the log file is “full” and ready for rotation when a new entry would cause its size to exceed shift_size.

The first time t.log is full:

  • t.log is closed and renamed to t.log.0.

  • A new file t.log is opened.

The second time t.log is full:

  • +t.log.0 is renamed as t.log.1.

  • t.log is closed and renamed to t.log.0.

  • A new file t.log is opened.

Each subsequent time that t.log is full, the log files are rotated:

  • t.log.1 is removed.

  • +t.log.0 is renamed as t.log.1.

  • t.log is closed and renamed to t.log.0.

  • A new file t.log is opened.

Periodic Rotation

For periodic rotation, call Logger.new with:

  • Argument logdev as a file path.

  • Argument shift_age as a string period indicator.

Examples:

logger = Logger.new('t.log', 'daily')   # Rotate log files daily.
logger = Logger.new('t.log', 'weekly')  # Rotate log files weekly.
logger = Logger.new('t.log', 'monthly') # Rotate log files monthly.

Example:

logger = Logger.new('t.log', 'daily')

When the given period expires:

  • The base log file, t.log is closed and renamed with a date-based suffix such as t.log.20220509.

  • A new log file t.log is opened.

  • Nothing is removed.

The default format for the suffix is '%Y%m%d', which produces a suffix similar to the one above. You can set a different format using create-time option shift_period_suffix; see details and suggestions at Time#strftime.

Constants
No documentation available

Severity label for logging (max 5 chars).

No documentation available
Attributes

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

Read & Write

Program name to include in log messages.

Read & Write

Sets or retrieves the logger entry formatter proc.

When formatter is nil, the logger uses Logger::Formatter.

When formatter is a proc, a new entry is formatted by the proc, which is called with four arguments:

  • severity: The severity of the entry.

  • time: A Time object representing the entry’s timestamp.

  • progname: The program name for the entry.

  • msg: The message for the entry (string or string-convertible object).

The proc should return a string containing the formatted entry.

This custom formatter uses String#dump to escape the message string:

logger = Logger.new($stdout, progname: 'mung')
original_formatter = logger.formatter || Logger::Formatter.new
logger.formatter = proc { |severity, time, progname, msg|
  original_formatter.call(severity, time, progname, msg.dump)
}
logger.add(Logger::INFO, "hello \n ''")
logger.add(Logger::INFO, "\f\x00\xff\\\"")

Output:

I, [2022-05-13T13:16:29.637488 #8492]  INFO -- mung: "hello \n ''"
I, [2022-05-13T13:16:29.637610 #8492]  INFO -- mung: "\f\x00\xFF\\\""

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

Class Methods

With the single argument logdev, returns a new logger with all default options:

Logger.new('t.log') # => #<Logger:0x000001e685dc6ac8>

Argument logdev must be one of:

  • A string filepath: entries are to be written to the file at that path; if the file at that path exists, new entries are appended.

  • An IO stream (typically +$stdout+, +$stderr+. or an open file): entries are to be written to the given stream.

  • nil or File::NULL: no entries are to be written.

Examples:

Logger.new('t.log')
Logger.new($stdout)

The keyword options are:

  • level: sets the log level; default value is Logger::DEBUG. See Log Level:

    Logger.new('t.log', level: Logger::ERROR)
    
  • progname: sets the default program name; default is nil. See Program Name:

    Logger.new('t.log', progname: 'mung')
    
  • formatter: sets the entry formatter; default is nil. See formatter=.

  • datetime_format: sets the format for entry timestamp; default is nil. See datetime_format=.

  • binmode: sets whether the logger writes in binary mode; default is false.

  • shift_period_suffix: sets the format for the filename suffix for periodic log file rotation; default is '%Y%m%d'. See Periodic Rotation.

Instance Methods

Writes the given msg to the log with no formatting; returns the number of characters written, or nil if no log device exists:

logger = Logger.new($stdout)
logger << 'My message.' # => 10

Output:

My message.

Creates a log entry, which may or may not be written to the log, depending on the entry’s severity and on the log level. See Log Level and Entries for details.

Examples:

logger = Logger.new($stdout, progname: 'mung')
logger.add(Logger::INFO)
logger.add(Logger::ERROR, 'No good')
logger.add(Logger::ERROR, 'No good', 'gnum')

Output:

I, [2022-05-12T16:25:31.469726 #36328]  INFO -- mung: mung
E, [2022-05-12T16:25:55.349414 #36328] ERROR -- mung: No good
E, [2022-05-12T16:26:35.841134 #36328] ERROR -- gnum: No good

These convenience methods have implicit severity:

Closes the logger; returns nil:

logger = Logger.new('t.log')
logger.close       # => nil
logger.info('foo') # Prints "log writing failed. closed stream"

Related: Logger#reopen.

Returns the date-time format; see datetime_format=.

Sets the date-time format.

Argument datetime_format should be either of these:

  • A string suitable for use as a format for method Time#strftime.

  • nil: the logger uses '%Y-%m-%dT%H:%M:%S.%6N'.

Equivalent to calling add with severity Logger::DEBUG.

Sets the log level to Logger::DEBUG. See Log Level.

Returns true if the log level allows entries with severity Logger::DEBUG to be written, false otherwise. See Log Level.

Equivalent to calling add with severity Logger::ERROR.

Sets the log level to Logger::ERROR. See Log Level.

Returns true if the log level allows entries with severity Logger::ERROR to be written, false otherwise. See Log Level.

Equivalent to calling add with severity Logger::FATAL.

Sets the log level to Logger::FATAL. See Log Level.

Returns true if the log level allows entries with severity Logger::FATAL to be written, false otherwise. See Log Level.

No documentation available
No documentation available

Equivalent to calling add with severity Logger::INFO.

Sets the log level to Logger::INFO. See Log Level.

Returns true if the log level allows entries with severity Logger::INFO to be written, false otherwise. See Log Level.

Sets the log level; returns severity. See Log Level.

Argument severity may be an integer, a string, or a symbol:

logger.level = Logger::ERROR # => 3
logger.level = 3             # => 3
logger.level = 'error'       # => "error"
logger.level = :error        # => :error

Logger#sev_threshold= is an alias for Logger#level=.

Sets the logger’s output stream:

  • If logdev is nil, reopens the current output stream.

  • If logdev is a filepath, opens the indicated file for append.

  • If logdev is an IO stream (usually $stdout, $stderr, or an open File object), opens the stream for append.

Example:

logger = Logger.new('t.log')
logger.add(Logger::ERROR, 'one')
logger.close
logger.add(Logger::ERROR, 'two') # Prints 'log writing failed. closed stream'
logger.reopen
logger.add(Logger::ERROR, 'three')
logger.close
File.readlines('t.log')
# =>
# ["# Logfile created on 2022-05-12 14:21:19 -0500 by logger.rb/v1.5.0\n",
#  "E, [2022-05-12T14:21:27.596726 #22428] ERROR -- : one\n",
#  "E, [2022-05-12T14:23:05.847241 #22428] ERROR -- : three\n"]

Equivalent to calling add with severity Logger::UNKNOWN.

Equivalent to calling add with severity Logger::WARN.

Sets the log level to Logger::WARN. See Log Level.

Returns true if the log level allows entries with severity Logger::WARN to be written, false otherwise. See Log Level.