diff --git a/pkgs/ruby/spec/lib/kernaux/cmdline_spec.rb b/pkgs/ruby/spec/lib/kernaux/cmdline_spec.rb
index 72b2e16..44eddb7 100644
--- a/pkgs/ruby/spec/lib/kernaux/cmdline_spec.rb
+++ b/pkgs/ruby/spec/lib/kernaux/cmdline_spec.rb
@@ -12,4 +12,70 @@ RSpec.describe KernAux, '.cmdline' do
   it { is_expected.to all be_instance_of String }
   it { is_expected.to all be_frozen }
   it { is_expected.to eq ['foo', 'bar baz', 'car cdr'] }
+
+  context 'when str is empty' do
+    let(:str) { '' }
+
+    it { is_expected.to eq [] }
+  end
+
+  context 'when str has invalid type' do
+    let(:str) { 123 }
+
+    specify { expect { cmdline }.to raise_error TypeError }
+  end
+
+  context 'when str has EOL after backslash' do
+    let(:str) { '\\' }
+
+    specify do
+      expect { cmdline }.to \
+        raise_error described_class::CmdlineError, 'EOL after backslash'
+    end
+  end
+
+  context 'when str has EOL after backslash inside quote' do
+    let(:str) { '"\\' }
+
+    specify do
+      expect { cmdline }.to raise_error \
+        described_class::CmdlineError, 'EOL after backslash inside quote'
+    end
+  end
+
+  context 'when str has unescaped quotation mark' do
+    let(:str) { 'foo"' }
+
+    specify do
+      expect { cmdline }.to \
+        raise_error described_class::CmdlineError, 'unescaped quotation mark'
+    end
+  end
+
+  context 'when str has EOL inside quote' do
+    let(:str) { '"' }
+
+    specify do
+      expect { cmdline }.to \
+        raise_error described_class::CmdlineError, 'EOL inside quote'
+    end
+  end
+
+  xcontext 'when there are too many args' do
+    let(:str) { 'a ' * 257 }
+
+    specify do
+      expect { cmdline }.to \
+        raise_error described_class::CmdlineError, 'too many args'
+    end
+  end
+
+  xcontext 'when args cause buffer overflow' do
+    let(:str) { 'a' * 4096 }
+
+    specify do
+      expect { cmdline }.to \
+        raise_error described_class::CmdlineError, 'buffer overflow'
+    end
+  end
 end