Chef: Read variable from file and use it in one converge

The correct solution is to use a lazy property:

execute 'download_joiner' do
  command "aws s3 cp s3://bucket/foo /root/foo"
  creates '/root/foo'
  sensitive true
end


execute 'join_domain' do
  command lazy {
    password = IO.read('/root/foo').strip
    "net ads join -U joiner%#{password}"
  }
  sensitive true
end

That delays the file read until after it is written. Also I included the sensitive property so the password is not displayed.

You can download the file at compile time by using run_action and wrap the second part in the conditional block which will be executed at run time.

execute 'download_joiner' do
  command "aws s3 cp s3://bucket/foo /root/foo"
  not_if { ::File.exist?('/root/foo') }
end.run_action(:run)

if File.exist?('/root/foo')
  password = ::File.read('/root/foo').chomp

  execute 'join_domain' do
    command "net ads join -U joiner%#{password}"
  end
end