How can I change a file with Chef?

Chef actually allows and uses this. You can find an example in the opscode’s

cookbooks/chef-server/recipes/default.rb:

ruby_block "ensure node can resolve API FQDN" do
  block do
    fe = Chef::Util::FileEdit.new("/etc/hosts")
    fe.insert_line_if_no_match(/#{node['chef-server']['api_fqdn']}/,
                               "127.0.0.1 #{node['chef-server']['api_fqdn']}")
    fe.write_file
  end
  not_if { Resolv.getaddress(node['chef-server']['api_fqdn']) rescue false }
end

Here’s the use-case. After the installation from source I had to uncomment lines in some created configuration file that hadn’t been the same in all versions of software, therefore use of templates hadn’t been appropriate. The methods I used were:

• (Object) search_file_replace(regex, replace)
• (Object) search_file_replace_line(regex, newline)

You may find the full documentation here:

• http://rubydoc.info/gems/chef/Chef/Util/FileEdit

TO STRESS: This method is to be used only when using templates and partials is inappropriate. As @StephenKing already said, templates are common way of doing this.