semantic_logger_trap.rb

#!/usr/bin/env ruby

# Illustrates cycling through the SemanticLogger log levels with signals.
# For reference by https://github.com/reidmorrison/semantic_logger/issues/231.

require 'semantic_logger'

# Redefine add_signal_handler to output to STDERR (omit TTIN behavior for brevity):
module SemanticLogger
  def self.add_signal_handler(log_level_signal = "USR2", thread_dump_signal = "TTIN", gc_log_microseconds = 100_000)
    if log_level_signal
      Signal.trap(log_level_signal) do

        # This line added:
        STDERR.print "Original level: #{default_level.to_s.ljust(5)}. "

        index     = default_level == :trace ? LEVELS.find_index(:error) : LEVELS.find_index(default_level)
        new_level = LEVELS[index - 1]

        # This line disabled:
        #self["SemanticLogger"].warn "Changed global default log level to #{new_level.inspect}"
        
        self.default_level = new_level
        
        # This line added:
        STDERR.puts "New level is #{default_level}. "
      end
    end
  end
end

SemanticLogger.add_signal_handler('USR1')

7.times do
  print 'Sending signal...'
  Process.kill('USR1', Process.pid)
  sleep 0.01
end

=begin
Output is:

Sending signal...Original level: info . New level is debug. 
Sending signal...Original level: debug. New level is trace. 
Sending signal...Original level: trace. New level is warn. 
Sending signal...Original level: warn . New level is info. 
Sending signal...Original level: info . New level is debug. 
Sending signal...Original level: debug. New level is trace. 
Sending signal...Original level: trace. New level is warn. 

=end