Support for the Ruby 2.4 series has ended. See here for reference.
Object
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)
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.
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.
The options below give you various choices, in more or less increasing complexity.
Create a logger which logs messages to STDERR/STDOUT.
logger = Logger.new(STDERR) logger = Logger.new(STDOUT)
Create a logger for the file which has the specified name.
logger = Logger.new('logfile.log')
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)
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)
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')
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.
Message in a block.
logger.fatal { "Argument 'foo' not given." }
Message as a string.
logger.error "Argument #{@foo} mismatch."
With progname.
logger.info('initialize') { "Initializing..." }
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.
logger.close
Original interface.
logger.sev_threshold = Logger::WARN
Log4r (somewhat) compatible interface.
logger.level = Logger::INFO # DEBUG < INFO < WARN < ERROR < FATAL < UNKNOWN
Symbol or String (case insensitive)
logger.level = :info logger.level = 'INFO' # :debug < :info < :warn < :error < :fatal < :unknown
Constructor
Logger.new(logdev, level: Logger::INFO) Logger.new(logdev, level: :info) Logger.new(logdev, level: 'INFO')
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" })
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.
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'.
Create an instance.
# File 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
Dump given message to the log device without any formatting. If no log device exists, return nil
.
# File logger.rb, line 479 def <<(msg) unless @logdev.nil? @logdev.write(msg) end end
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.
When the given severity is not high enough (for this particular logger), log no message, and return true
.
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.
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 455 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
Close the logging device.
# File logger.rb, line 568 def close @logdev.close if @logdev end
Returns the date format being used. See datetime_format=
# File logger.rb, line 300 def datetime_format @default_formatter.datetime_format end
Set date-time format.
datetime_format
A string suitable for passing to strftime
.
# File logger.rb, line 295 def datetime_format=(datetime_format) @default_formatter.datetime_format = datetime_format end
Log a DEBUG
message.
See info
for more information.
# File logger.rb, line 490 def debug(progname = nil, &block) add(DEBUG, nil, progname, &block) end
Returns true
iff the current severity level allows for the printing of DEBUG
messages.
# File logger.rb, line 323 def debug?; @level <= DEBUG; end
Log an ERROR
message.
See info
for more information.
# File logger.rb, line 542 def error(progname = nil, &block) add(ERROR, nil, progname, &block) end
Returns true
iff the current severity level allows for the printing of ERROR
messages.
# File logger.rb, line 335 def error?; @level <= ERROR; end
Log a FATAL
message.
See info
for more information.
# File logger.rb, line 551 def fatal(progname = nil, &block) add(FATAL, nil, progname, &block) end
Returns true
iff the current severity level allows for the printing of FATAL
messages.
# File logger.rb, line 339 def fatal?; @level <= FATAL; end
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.
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).
See add
.
# File logger.rb, line 524 def info(progname = nil, &block) add(INFO, nil, progname, &block) end
Returns true
iff the current severity level allows for the printing of INFO
messages.
# File logger.rb, line 327 def info?; @level <= INFO; end
Set logging severity threshold.
severity
The Severity
of the log message.
# File logger.rb, line 266 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
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
.
Reopen a log device.
# File logger.rb, line 409 def reopen(logdev = nil) @logdev.reopen(logdev) self end
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 561 def unknown(progname = nil, &block) add(UNKNOWN, nil, progname, &block) end
Log a WARN
message.
See info
for more information.
# File logger.rb, line 533 def warn(progname = nil, &block) add(WARN, nil, progname, &block) end