SystemCommand with both STDOUT and STDERR output from upstream with print_stdout its result stderr is expected to eq "2\n4\n6\n" Failure/Error: its(:stderr) { is_expected.to eq([2, 4, 6, nil].join("\n")) } expected: "2\n4\n6\n" got: "" (compared using ==) Diff: @@ -1,4 +1 @@ -2 -4 -6 Shared Example Group: "it returns '1 2 3 4 5 6'" called from ./test/system_command_spec.rb:180