diff --git a/Library/Homebrew/completions.rb b/Library/Homebrew/completions.rb
index 67c27d965c..87d5b23278 100644
--- a/Library/Homebrew/completions.rb
+++ b/Library/Homebrew/completions.rb
@@ -97,7 +97,7 @@ module Homebrew
 
     sig { params(command: String).returns(T::Boolean) }
     def command_gets_completions?(command)
-      return false if command.start_with? "cask " # TODO: remove when `brew cask` commands are removed
+      return false if command.start_with? "cask " # TODO: (2.8) remove when `brew cask` commands are removed
 
       command_options(command).any?
     end
diff --git a/Library/Homebrew/test/completions_spec.rb b/Library/Homebrew/test/completions_spec.rb
index efdcee115e..d6d59a1547 100644
--- a/Library/Homebrew/test/completions_spec.rb
+++ b/Library/Homebrew/test/completions_spec.rb
@@ -4,6 +4,7 @@
 require "completions"
 
 describe Homebrew::Completions do
+  let(:completions_dir) { HOMEBREW_REPOSITORY/"completions" }
   let(:internal_path) { HOMEBREW_REPOSITORY/"Library/Taps/homebrew/homebrew-bar" }
   let(:external_path) { HOMEBREW_REPOSITORY/"Library/Taps/foo/homebrew-bar" }
 
@@ -11,130 +12,233 @@ describe Homebrew::Completions do
     HOMEBREW_REPOSITORY.cd do
       system "git", "init"
     end
+    described_class::SHELLS.each do |shell|
+      (completions_dir/shell).mkpath
+    end
     internal_path.mkpath
     external_path.mkpath
   end
 
-  def setup_completions(external:)
-    (internal_path/"completions/bash/foo_internal").write "#foo completions"
-    if external
-      (external_path/"completions/bash/foo_external").write "#foo completions"
-    elsif (external_path/"completions/bash/foo_external").exist?
-      (external_path/"completions/bash/foo_external").delete
-    end
-  end
-
-  def setup_completions_setting(state, setting: "linkcompletions")
-    HOMEBREW_REPOSITORY.cd do
-      system "git", "config", "--replace-all", "homebrew.#{setting}", state.to_s
-    end
-  end
-
-  def read_completions_setting(setting: "linkcompletions")
-    HOMEBREW_REPOSITORY.cd do
-      Utils.popen_read("git", "config", "--get", "homebrew.#{setting}").chomp.presence
-    end
-  end
-
-  def delete_completions_setting(setting: "linkcompletions")
-    HOMEBREW_REPOSITORY.cd do
-      system "git", "config", "--unset-all", "homebrew.#{setting}"
-    end
-  end
-
   after do
+    FileUtils.rm_rf completions_dir
     FileUtils.rm_rf internal_path
     FileUtils.rm_rf external_path.dirname
   end
 
-  describe ".link!" do
-    it "sets homebrew.linkcompletions to true" do
-      setup_completions_setting false
-      expect { described_class.link! }.not_to raise_error
-      expect(read_completions_setting).to eq "true"
+  context "when linking or unlinking completions" do
+    def setup_completions(external:)
+      (internal_path/"completions/bash/foo_internal").write "#foo completions"
+      if external
+        (external_path/"completions/bash/foo_external").write "#foo completions"
+      elsif (external_path/"completions/bash/foo_external").exist?
+        (external_path/"completions/bash/foo_external").delete
+      end
     end
 
-    it "sets homebrew.linkcompletions to true if unset" do
-      delete_completions_setting
-      expect { described_class.link! }.not_to raise_error
-      expect(read_completions_setting).to eq "true"
+    def setup_completions_setting(state, setting: "linkcompletions")
+      HOMEBREW_REPOSITORY.cd do
+        system "git", "config", "--replace-all", "homebrew.#{setting}", state.to_s
+      end
     end
 
-    it "keeps homebrew.linkcompletions set to true" do
-      setup_completions_setting true
-      expect { described_class.link! }.not_to raise_error
-      expect(read_completions_setting).to eq "true"
+    def read_completions_setting(setting: "linkcompletions")
+      HOMEBREW_REPOSITORY.cd do
+        Utils.popen_read("git", "config", "--get", "homebrew.#{setting}").chomp.presence
+      end
+    end
+
+    def delete_completions_setting(setting: "linkcompletions")
+      HOMEBREW_REPOSITORY.cd do
+        system "git", "config", "--unset-all", "homebrew.#{setting}"
+      end
+    end
+
+    describe ".link!" do
+      it "sets homebrew.linkcompletions to true" do
+        setup_completions_setting false
+        expect { described_class.link! }.not_to raise_error
+        expect(read_completions_setting).to eq "true"
+      end
+
+      it "sets homebrew.linkcompletions to true if unset" do
+        delete_completions_setting
+        expect { described_class.link! }.not_to raise_error
+        expect(read_completions_setting).to eq "true"
+      end
+
+      it "keeps homebrew.linkcompletions set to true" do
+        setup_completions_setting true
+        expect { described_class.link! }.not_to raise_error
+        expect(read_completions_setting).to eq "true"
+      end
+    end
+
+    describe ".unlink!" do
+      it "sets homebrew.linkcompletions to false" do
+        setup_completions_setting true
+        expect { described_class.unlink! }.not_to raise_error
+        expect(read_completions_setting).to eq "false"
+      end
+
+      it "sets homebrew.linkcompletions to false if unset" do
+        delete_completions_setting
+        expect { described_class.unlink! }.not_to raise_error
+        expect(read_completions_setting).to eq "false"
+      end
+
+      it "keeps homebrew.linkcompletions set to false" do
+        setup_completions_setting false
+        expect { described_class.unlink! }.not_to raise_error
+        expect(read_completions_setting).to eq "false"
+      end
+    end
+
+    describe ".link_completions?" do
+      it "returns true if homebrew.linkcompletions is true" do
+        setup_completions_setting true
+        expect(described_class.link_completions?).to be true
+      end
+
+      it "returns false if homebrew.linkcompletions is false" do
+        setup_completions_setting false
+        expect(described_class.link_completions?).to be false
+      end
+
+      it "returns false if homebrew.linkcompletions is not set" do
+        expect(described_class.link_completions?).to be false
+      end
+    end
+
+    describe ".completions_to_link?" do
+      it "returns false if only internal taps have completions" do
+        setup_completions external: false
+        expect(described_class.completions_to_link?).to be false
+      end
+
+      it "returns true if external taps have completions" do
+        setup_completions external: true
+        expect(described_class.completions_to_link?).to be true
+      end
+    end
+
+    describe ".show_completions_message_if_needed" do
+      it "doesn't show the message if there are no completions to link" do
+        setup_completions external: false
+        delete_completions_setting setting: :completionsmessageshown
+        expect { described_class.show_completions_message_if_needed }.not_to output.to_stdout
+      end
+
+      it "doesn't show the message if there are completions to link but the message has already been shown" do
+        setup_completions external: true
+        setup_completions_setting true, setting: :completionsmessageshown
+        expect { described_class.show_completions_message_if_needed }.not_to output.to_stdout
+      end
+
+      it "shows the message if there are completions to link and the message hasn't already been shown" do
+        setup_completions external: true
+        delete_completions_setting setting: :completionsmessageshown
+
+        message = /Homebrew completions for external commands are unlinked by default!/
+        expect { described_class.show_completions_message_if_needed }
+          .to output(message).to_stdout
+      end
     end
   end
 
-  describe ".unlink!" do
-    it "sets homebrew.linkcompletions to false" do
-      setup_completions_setting true
-      expect { described_class.unlink! }.not_to raise_error
-      expect(read_completions_setting).to eq "false"
+  context "when generating completions" do
+    describe ".update_shell_completions!" do
+      it "generates shell completions" do
+        described_class.update_shell_completions!
+        expect(completions_dir/"bash/brew").to be_a_file
+      end
     end
 
-    it "sets homebrew.linkcompletions to false if unset" do
-      delete_completions_setting
-      expect { described_class.unlink! }.not_to raise_error
-      expect(read_completions_setting).to eq "false"
+    describe ".command_options" do
+      it "returns an array of options for a ruby command" do
+        expected_options = %w[--debug --help --hide --quiet --verbose]
+        expect(described_class.command_options("missing")).to eq expected_options
+      end
+
+      it "returns an array of options for a shell command" do
+        expected_options = %w[--debug --force --help --merge --preinstall --verbose]
+        expect(described_class.command_options("update")).to eq expected_options
+      end
+
+      it "handles --[no]- options correctly" do
+        options = described_class.command_options("audit")
+        expect(options.include?("--appcast")).to eq true
+        expect(options.include?("--no-appcast")).to eq true
+      end
+
+      it "return an empty array if command is not found" do
+        expect(described_class.command_options("foobar")).to eq []
+      end
+
+      it "return an empty array for a command with no options" do
+        expect(described_class.command_options("help")).to eq []
+      end
+
+      it "will list global options only once if overriden" do
+        count = 0
+        described_class.command_options("upgrade").each do |opt|
+          count += 1 if opt == "--verbose"
+        end
+        expect(count).to eq 1
+      end
     end
 
-    it "keeps homebrew.linkcompletions set to false" do
-      setup_completions_setting false
-      expect { described_class.unlink! }.not_to raise_error
-      expect(read_completions_setting).to eq "false"
-    end
-  end
+    describe ".command_gets_completions?" do
+      it "returns true for a non-cask command with options" do
+        expect(described_class.command_gets_completions?("install")).to eq true
+      end
 
-  describe ".link_completions?" do
-    it "returns true if homebrew.linkcompletions is true" do
-      setup_completions_setting true
-      expect(described_class.link_completions?).to be true
+      it "returns false for a non-cask command with no options" do
+        expect(described_class.command_gets_completions?("help")).to eq false
+      end
+
+      it "returns false for a cask command" do
+        expect(described_class.command_gets_completions?("cask install")).to eq false
+      end
     end
 
-    it "returns false if homebrew.linkcompletions is false" do
-      setup_completions_setting false
-      expect(described_class.link_completions?).to be false
+    describe ".generate_bash_subcommand_completion" do
+      it "returns nil if completions aren't needed" do
+        expect(described_class.generate_bash_subcommand_completion("help")).to be_nil
+      end
+
+      it "returns appropriate completion for a ruby command" do
+        completion = described_class.generate_bash_subcommand_completion("missing")
+        expect(completion).to match(/^_brew_missing\(\) {/)
+        expect(completion).to match(/__brewcomp "\n +--debug\n +--help\n +--hide\n +--quiet\n +--verbose/s)
+        expect(completion).to match(/__brew_complete_formulae\n}$/)
+      end
+
+      it "returns appropriate completion for a shell command" do
+        completion = described_class.generate_bash_subcommand_completion("update")
+        options_regex = /__brewcomp "\n +--debug\n +--force\n +--help\n +--merge\n +--preinstall\n +--verbose/
+        expect(completion).to match(/^_brew_update\(\) {/)
+        expect(completion).to match(options_regex)
+      end
+
+      it "returns appropriate completion for a command with multiple named arg types" do
+        completion = described_class.generate_bash_subcommand_completion("upgrade")
+        expect(completion).to match(/__brew_complete_outdated_formulae\n  __brew_complete_outdated_casks\n}$/)
+      end
     end
 
-    it "returns false if homebrew.linkcompletions is not set" do
-      expect(described_class.link_completions?).to be false
-    end
-  end
-
-  describe ".completions_to_link?" do
-    it "returns false if only internal taps have completions" do
-      setup_completions external: false
-      expect(described_class.completions_to_link?).to be false
-    end
-
-    it "returns true if external taps have completions" do
-      setup_completions external: true
-      expect(described_class.completions_to_link?).to be true
-    end
-  end
-
-  describe ".show_completions_message_if_needed" do
-    it "doesn't show the message if there are no completions to link" do
-      setup_completions external: false
-      delete_completions_setting setting: :completionsmessageshown
-      expect { described_class.show_completions_message_if_needed }.not_to output.to_stdout
-    end
-
-    it "doesn't show the message if there are completions to link but the message has already been shown" do
-      setup_completions external: true
-      setup_completions_setting true, setting: :completionsmessageshown
-      expect { described_class.show_completions_message_if_needed }.not_to output.to_stdout
-    end
-
-    it "shows the message if there are completions to link and the message hasn't already been shown" do
-      setup_completions external: true
-      delete_completions_setting setting: :completionsmessageshown
-
-      message = /Homebrew completions for external commands are unlinked by default!/
-      expect { described_class.show_completions_message_if_needed }
-        .to output(message).to_stdout
+    describe ".generate_bash_completion_file" do
+      it "returns the correct completion file" do
+        file = described_class.generate_bash_completion_file(%w[install missing update])
+        expect(file).to match(/^__brewcomp\(\) {$/)
+        expect(file).to match(/^_brew_install\(\) {$/)
+        expect(file).to match(/^_brew_missing\(\) {$/)
+        expect(file).to match(/^_brew_update\(\) {$/)
+        expect(file).to match(/^_brew\(\) {$/)
+        expect(file).to match(/^ {4}install\) _brew_install ;;/)
+        expect(file).to match(/^ {4}missing\) _brew_missing ;;/)
+        expect(file).to match(/^ {4}update\) _brew_update ;;/)
+        expect(file).to match(/^complete -o bashdefault -o default -F _brew brew$/)
+      end
     end
   end
 end