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.
-
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')
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.
-
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.
How to close a logger¶ ↑
logger.close
Setting severity threshold¶ ↑
-
Original interface.
logger.sev_threshold = Logger::WARN
-
Log4r (somewhat) compatible interface.
logger.level = Logger::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
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.
Logging severity threshold (e.g. Logger::INFO
).
Program name to include in log messages.
Logging severity threshold (e.g. Logger::INFO
).
Logging severity threshold (e.g. Logger::INFO
).
Public Class Methods
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
ormonthly
). shift_size
-
Maximum logfile size (only applies when
shift_age
is a number).
Description¶ ↑
Create an instance.
# File lib/logger.rb, line 311 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
Dump given message to the log device without any formatting. If no log
device exists, return nil
.
# File lib/logger.rb, line 388 def <<(msg) unless @logdev.nil? @logdev.write(msg) end end
Args¶ ↑
severity
-
Severity. Constants are defined in Logger namespace:
DEBUG
,INFO
,WARN
,ERROR
,FATAL
, orUNKNOWN
. message
-
The log message. A String or Exception.
progname
-
Program name string. Can be omitted. Treated as a message if no
message
andblock
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 364 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 lib/logger.rb, line 477 def close @logdev.close if @logdev end
Returns the date format being used. See datetime_format=
# File lib/logger.rb, line 250 def datetime_format @default_formatter.datetime_format end
Set date-time format.
datetime_format
-
A string suitable for passing to
strftime
.
# File lib/logger.rb, line 245 def datetime_format=(datetime_format) @default_formatter.datetime_format = datetime_format end
Log a DEBUG
message.
See info for more information.
# File lib/logger.rb, line 399 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 lib/logger.rb, line 273 def debug?; @level <= DEBUG; end
Log an ERROR
message.
See info for more information.
# File lib/logger.rb, line 451 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 lib/logger.rb, line 285 def error?; @level <= ERROR; end
Log a FATAL
message.
See info for more information.
# File lib/logger.rb, line 460 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 lib/logger.rb, line 289 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.
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 433 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 lib/logger.rb, line 277 def info?; @level <= INFO; end
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 470 def unknown(progname = nil, &block) add(UNKNOWN, nil, progname, &block) end
Log a WARN
message.
See info for more information.
# File lib/logger.rb, line 442 def warn(progname = nil, &block) add(WARN, nil, progname, &block) end
Returns true
iff the current severity level allows for the
printing of WARN
messages.
# File lib/logger.rb, line 281 def warn?; @level <= WARN; end
Private Instance Methods
# File lib/logger.rb, line 490 def format_message(severity, datetime, progname, msg) (@formatter || @default_formatter).call(severity, datetime, progname, msg) end
# File lib/logger.rb, line 486 def format_severity(severity) SEV_LABEL[severity] || 'ANY' end