Debugger CLI: Improve "help" command

Given a command name it prints the command's usage.

src/apps/debugger/user_interface/cli/CommandLineUserInterface.cpp

@@ -56,16 +56,23 @@ private:
 struct CommandLineUserInterface::HelpCommand : CliCommand {
 	HelpCommand(CommandLineUserInterface* userInterface)
 		:
-		CliCommand("print a list of all commands",
-			"%s\n"
-			"Prints a list of all commands."),
+		CliCommand("print help for a command or a list of all commands",
+			"%s [ <command> ]\n"
+			"Prints help for command <command>, if given, or a list of all "
+				"commands\n"
+			"otherwise."),
 		fUserInterface(userInterface)
 	{
 	}
 
 	virtual void Execute(int argc, const char* const* argv, CliContext& context)
 	{
-		fUserInterface->_PrintHelp();
+		if (argc > 2) {
+			PrintUsage(argv[0]);
+			return;
+		}
+
+		fUserInterface->_PrintHelp(argc == 2 ? argv[1] : NULL);
 	}
 
 private:
@@ -282,33 +289,63 @@ CommandLineUserInterface::_RegisterCommand(const BString& name,
 void
 CommandLineUserInterface::_ExecuteCommand(int argc, const char* const* argv)
 {
-	const char* commandName = argv[0];
+	CommandEntry* commandEntry = _FindCommand(argv[0]);
+	if (commandEntry != NULL)
+		commandEntry->Command()->Execute(argc, argv, fContext);
+}
+
+
+CommandLineUserInterface::CommandEntry*
+CommandLineUserInterface::_FindCommand(const char* commandName)
+{
 	size_t commandNameLength = strlen(commandName);
 
-	CommandEntry* firstEntry = NULL;
+	// try to find an exact match first
+	CommandEntry* commandEntry = NULL;
 	for (int32 i = 0; CommandEntry* entry = fCommands.ItemAt(i); i++) {
-		if (entry->Name().Compare(commandName, commandNameLength) == 0) {
-			if (firstEntry != NULL) {
-				printf("Ambiguous command \"%s\".\n", commandName);
-				return;
-			}
-
-			firstEntry = entry;
+		if (entry->Name() == commandName) {
+			commandEntry = entry;
+			break;
 		}
 	}
 
-	if (firstEntry == NULL) {
-		printf("Unknown command \"%s\".\n", commandName);
-		return;
+	// If nothing found yet, try partial matches, but only, if they are
+	// unambiguous.
+	if (commandEntry == NULL) {
+		for (int32 i = 0; CommandEntry* entry = fCommands.ItemAt(i); i++) {
+			if (entry->Name().Compare(commandName, commandNameLength) == 0) {
+				if (commandEntry != NULL) {
+					printf("Error: Ambiguous command \"%s\".\n", commandName);
+					return NULL;
+				}
+
+				commandEntry = entry;
+			}
+		}
 	}
 
-	firstEntry->Command()->Execute(argc, argv, fContext);
+	if (commandEntry == NULL) {
+		printf("Error: Unknown command \"%s\".\n", commandName);
+		return NULL;
+	}
+
+	return commandEntry;
 }
 
 
 void
-CommandLineUserInterface::_PrintHelp()
+CommandLineUserInterface::_PrintHelp(const char* commandName)
 {
+	// If a command name is given, print the usage for that one.
+	if (commandName != NULL) {
+		CommandEntry* commandEntry = _FindCommand(commandName);
+		if (commandEntry != NULL)
+			commandEntry->Command()->PrintUsage(commandEntry->Name().String());
+		return;
+	}
+
+	// No command name given -- print a list of all commands.
+
 	// determine longest command name
 	int32 longestCommandName = 0;
 	for (int32 i = 0; CommandEntry* entry = fCommands.ItemAt(i); i++) {