Implement :help command

Add the :help command to display available sqlcmd commands.
This improves compatibility with legacy ODBC sqlcmd.

Changes:
- Added HELP command to command registry
- Added helpCommand function with full command list
- Added tests for command parsing and functionality
- Updated README.md

Author: dlevy-msft-sql

README.md

@@ -154,6 +154,7 @@ switches are most important to you to have implemented next in the new sqlcmd.
 - `:Connect` now has an optional `-G` parameter to select one of the authentication methods for Azure SQL Database  - `SqlAuthentication`, `ActiveDirectoryDefault`, `ActiveDirectoryIntegrated`, `ActiveDirectoryServicePrincipal`, `ActiveDirectoryManagedIdentity`, `ActiveDirectoryPassword`. If `-G` is not provided, either Integrated security or SQL Authentication will be used, dependent on the presence of a `-U` username parameter.
 - The new `--driver-logging-level` command line parameter allows you to see traces from the `go-mssqldb` client driver. Use `64` to see all traces.
 - Sqlcmd can now print results using a vertical format. Use the new `--vertical` command line option to set it. It's also controlled by the `SQLCMDFORMAT` scripting variable.
+- `:help` displays a list of available sqlcmd commands.
 
 ```
 1> select session_id, client_interface_name, program_name from sys.dm_exec_sessions where session_id=@@spid

pkg/sqlcmd/commands.go

@@ -113,6 +113,11 @@ func newCommands() Commands {
 			action: xmlCommand,
 			name:   "XML",
 		},
+		"HELP": {
+			regex:  regexp.MustCompile(`(?im)^[ \t]*:HELP(?:[ \t]+(.*$)|$)`),
+			action: helpCommand,
+			name:   "HELP",
+		},
 	}
 }
 
@@ -596,6 +601,55 @@ func xmlCommand(s *Sqlcmd, args []string, line uint) error {
 	return nil
 }
 
+// helpCommand displays the list of available sqlcmd commands
+func helpCommand(s *Sqlcmd, args []string, line uint) error {
+	helpText := `:!! [<command>]
+  - Executes a command in the operating system shell.
+:connect server[\instance] [-l timeout] [-U user [-P password]]
+  - Connects to a SQL Server instance.
+:ed
+  - Edits the current or last executed statement cache.
+:error <dest>
+  - Redirects error output to a file, stderr, or stdout.
+:exit
+  - Quits sqlcmd immediately.
+:exit()
+  - Execute statement cache; quit with no return value.
+:exit(<query>)
+  - Execute the specified query; returns numeric result.
+go [<n>]
+  - Executes the statement cache (n times).
+:help
+  - Shows this list of commands.
+:list
+  - Prints the content of the statement cache.
+:listvar
+  - Lists the set sqlcmd scripting variables.
+:on error [exit|ignore]
+  - Action for batch or sqlcmd command errors.
+:out <filename>|stderr|stdout
+  - Redirects query output to a file, stderr, or stdout.
+:perftrace <filename>|stderr|stdout
+  - Redirects timing output to a file, stderr, or stdout.
+:quit
+  - Quits sqlcmd immediately.
+:r <filename>
+  - Append file contents to the statement cache.
+:reset
+  - Discards the statement cache.
+:serverlist
+  - Lists local and SQL Servers on the network.
+:setvar {variable}
+  - Removes a sqlcmd scripting variable.
+:setvar <variable> <value>
+  - Sets a sqlcmd scripting variable.
+:xml [on|off]
+  - Sets XML output mode.
+`
+	_, err := s.GetOutput().Write([]byte(helpText))
+	return err
+}
+
 func resolveArgumentVariables(s *Sqlcmd, arg []rune, failOnUnresolved bool) (string, error) {
 	var b *strings.Builder
 	end := len(arg)

pkg/sqlcmd/commands_test.go

@@ -54,6 +54,8 @@ func TestCommandParsing(t *testing.T) {
 		{`:XML ON `, "XML", []string{`ON `}},
 		{`:RESET`, "RESET", []string{""}},
 		{`RESET`, "RESET", []string{""}},
+		{`:HELP`, "HELP", []string{""}},
+		{`:help`, "HELP", []string{""}},
 	}
 
 	for _, test := range commands {
@@ -458,3 +460,24 @@ func TestExitCommandAppendsParameterToCurrentBatch(t *testing.T) {
 	}
 
 }
+
+func TestHelpCommand(t *testing.T) {
+	s, buf := setupSqlCmdWithMemoryOutput(t)
+	defer buf.Close()
+	s.SetOutput(buf)
+
+	err := helpCommand(s, []string{""}, 1)
+	assert.NoError(t, err, "helpCommand should not error")
+
+	output := buf.buf.String()
+	// Verify key commands are listed
+	assert.Contains(t, output, ":connect", "help should list :connect")
+	assert.Contains(t, output, ":exit", "help should list :exit")
+	assert.Contains(t, output, ":help", "help should list :help")
+	assert.Contains(t, output, ":setvar", "help should list :setvar")
+	assert.Contains(t, output, ":listvar", "help should list :listvar")
+	assert.Contains(t, output, ":out", "help should list :out")
+	assert.Contains(t, output, ":error", "help should list :error")
+	assert.Contains(t, output, ":r", "help should list :r")
+	assert.Contains(t, output, "go", "help should list go")
+}