chef server with knife backup showing error

Question

This is on Ubuntu machine. I installed knife backup gem on chef server for backup the server.

sudo gem install knife-backup

Now I create a knife.rb file the following configuration.

log_level                 :info 
log_location              STDOUT 
validation_client_name    'chef-validator' 
validation_key            '/etc/chef-server/chef-validator.pem' 
chef_server_url           'https://ip'

When I run the following command,

sudo knife backup export -D /etc/chef/backups -c knife.rb

it is showing me error:-

/opt/vagrant_ruby/lib/ruby/gems/1.8/gems/ohai-7.0.4/lib/ohai/loader.rb:188: warning: character class has [' without escape /opt/vagrant_ruby/lib/ruby/gems/1.8/gems/ohai-7.0.4/lib/ohai/loader.rb:188: warning: regexp has]' without escape
Backing up clients
ERROR: Your private key could not be loaded from /etc/chef/client.pem
Check your configuration file and ensure that your private key is readable

Answer 1

I have solved this using the below command. This is admin task and we need to describe user and admin key for the same.

sudo knife backup export -D /etc/chef/backups --user admin --key /etc/chef-server/admin.pem -c knife.rb

Please give me some more solutions for the same and better description