How can I change a file with Chef?

As of 2020, the solutions provided are discouraged - see here and here.

The modern way of performing file manual editing is to use the official, community-maintained, cookbook line.

Example:

replace_or_add "why hello" do
  path "/some/file"
  pattern "Why hello there.*"
  line "Why hello there, you beautiful person, you."
end

It's crucial, before using such strategy, to be 100% sure that manual editing is really required, but this is strictly dependent on the use case.

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')