diff --git a/Library/Homebrew/brew.rb b/Library/Homebrew/brew.rb
index ca712613e4..cab8b2a852 100644
--- a/Library/Homebrew/brew.rb
+++ b/Library/Homebrew/brew.rb
@@ -131,6 +131,8 @@ begin
     end
     exec "brew-#{cmd}", *ARGV
   else
+    raise "command made by bash not ruby: #{cmd}" if Commands.only_bash_command_list.include?(cmd)
+
     possible_tap = OFFICIAL_CMD_TAPS.find { |_, cmds| cmds.include?(cmd) }
     possible_tap = Tap.fetch(possible_tap.first) if possible_tap
 
diff --git a/Library/Homebrew/commands.rb b/Library/Homebrew/commands.rb
index 0fd56c5838..d725a0bafd 100644
--- a/Library/Homebrew/commands.rb
+++ b/Library/Homebrew/commands.rb
@@ -197,4 +197,8 @@ module Commands
     file = HOMEBREW_CACHE/"all_commands_list.txt"
     file.atomic_write("#{commands(aliases: true).sort.join("\n")}\n")
   end
+
+  def only_bash_command_list
+    internal_commands.reject { |cmd| valid_internal_cmd?(cmd) }
+  end
 end