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.

Here is an example of how you can use Chef to uncomment a line in a configuration file. The ruby_block is protected with a ::File::grep. The test for Debian is just for fun.

pam_config = "/etc/pam.d/su"
commented_limits = /^#\s+(session\s+\w+\s+pam_limits\.so)\b/m

ruby_block "add pam_limits to su" do
  block do
    sed = Chef::Util::FileEdit.new(pam_config)
    sed.search_file_replace(commented_limits, '\1')
    sed.write_file
  end
  only_if { ::File.readlines(pam_config).grep(commented_limits).any? }
end if platform_family?('debian')

How can I change a file with Chef?

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