10 Jun 2022 00:16:42 UTC
- Distribution: Log-ger-Manual
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- WHAT IS LOGGING LEVEL?
- WHICH LEVEL SHOULD I USE FOR WHICH PURPOSE?
- SETTING LEVEL
- CUSTOM LEVELS
- COPYRIGHT AND LICENSE
Log::ger::Manual::Tutorial::300_Level - Logging levels
When logging a message, you can choose from one of the several available levels:
log_info("This is just an informational message"); log_warn("This is a warning, you have been warned"); log_error("This is an error!"); log_fatal("This is a serious error!!! I can't continue"); exit;
Level signifies the importance, urgency, and/or severity of the message. One of the main ways the log messages are filtered is by level.
Log::ger comes with the following standard levels, sorted from the most important/severe to the least. The number in parentheses is the numeric representation for that level:
fatal (10) error (20) warn (30) info (40) debug (50) trace (60)
Aside from the above, there are also these category aliases:
off (0) warning (30)
Aliases don't get their own logging subroutines/methods (so there's no
log_warn), but they are recognized e.g. when you feed one to
There is no absolute set of rules on which level you should use for which purposes. The most important thing is to be consistent. Here are some links you can read:
Choosing a Log Level (Log::Any)
When to use the different log levels?
I personally use this set of rules:
warnand higher are to be shown to end users of application (non-developers) while
traceare meant only for developers. This means,
tracemessages tend to be more technical and precise.
infoonly for verbose output
infoshould only be shown when users specify increased verbosity, e.g. via command-line option
warnis for informing that there are some abnormality but not necessarily an error.
erroris for error condition (obviously) but the program can continue.
fatalis for a serious error that renders the program unable to continue.
traceis usually for dumping internal data structures or informing the flow of program execution (entering/leaving a subroutine), for everything else developer-related, use
At the start of program, the level is set to
warn. This means messages logged by
log_debug()by default won't be shown even after we set an output.
To change level, you can use:
use Log::ger::Util; Log::ger::Util::set_level("info"); # or ... Log::ger::Util::set_level(40);
set_level()will die if you feed it an unknown string level.
Normally you will only need to do this in an application, not in modules. One piece of advice is to allow user to change the level without her having to modify the source code, for example via environment variable and/or command-line option. An application framework like Perinci::CmdLine takes care of this for you, so you don't need to do
set_levelmanually at all.
Another module you can use for this purpose is Log::ger::Level::FromEnv. This module detects some environment variables (like
TRACE=1) then set the logging level according to it.
A more complex program might want to customize (increase the number of) levels. Log::ger allows you to do this. Basically all you have to do is set
%Log::ger::Levels, preferably before intializing logging for any package using
Some modules provide levels that mimic those in other frameworks, e.g. Log::ger::Level::Like::LogAny or other
This software is copyright (c) 2022, 2020, 2019, 2018, 2017 by perlancar <firstname.lastname@example.org>.
This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.
Module Install Instructions
To install Log::ger::Manual, copy and paste the appropriate command in to your terminal.
perl -MCPAN -e shell install Log::ger::Manual
For more information on module installation, please visit the detailed CPAN module installation guide.