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You can also use gdb to attach to the running process, 
set the Rails.logger to debug level and then detach. 
I have created the following 1 liner to do this for my puma process:

gdb attach $(pidof puma) -ex 'call(rb_eval_string("Rails.logger.level = Logger::DEBUG"))' -ex detach -ex quit
NOTE: pidof will return multiple pids, in descending order. 
So if you have multiple processes with the same name 
this will only run on the first one returned by pidof. 
The others will be discarded by the "gdb attach" command with the message: 
"Excess command line arguments ignored. (26762)". 
However you can safely ignore it if you only care about the first process returned by pidof.