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Log filename, line and function name in ruby automatically

published on 11 May 2013, 2 minute read

Suppose you have a app with a logger:

require 'logger'

module MyApp
  Log = Logger.new(STDOUT)

  class Foo
    def self.hello
      Log.info "test message"
    end
  end
end

MyApp::Foo.hello

When you run it, everything is as usual:

I, [2013-05-11T21:26:40.701590 #4326]  INFO -- : test message

Now imagine you have a ton of such messages flooding the output. Wouldn’t be nice if the logger would also include the filename, the line and the function name where the Log output was issued? Well, you can do that in ruby:

require 'logger'

module MyApp
  Log = Logger.new(STDOUT)
  Log.formatter = proc { |severity, datetime, progname, msg|
    "#{severity} #{caller[4]} #{msg}\n"
  }

  class Foo
    def self.hello
      Log.info "test message"
    end
  end
end

MyApp::Foo.hello

If we will run this, we’ll see the following output:

INFO log_test.rb:14:in `hello': test message

How it is done

Ruby has a magic array - caller. It contains the ordered list of the files, lines, and functions being executed.

class Foo
  def self.baz
    hello
  end

  def self.hello
    p caller
  end
end

Foo.baz #=> ["/home/andrei/Temp/log_test.rb:21:in `baz'", "/home/andrei/Temp/log_test.rb:29:in `<main>'"]

We used that magic array for our logger formatter, and we used its 4th element because the code which calls the logger is 4 levels above in call stack relative to formatter proc.

Note: If you use a different version of logger, you might need to use a different number, not 4. In any case, inspect the caller array and see at which position in that array your code info is located.

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