feat: Allow passing environment variables to stdio command

This commit adds a new `--env` (or `-e`) flag to the `stdio` command.
This allows the user to pass environment variables to the server process that is started by the command.

The implementation appends the provided environment variables to the existing environment of the `mcp-cli` process, ensuring that the child process inherits the parent's environment as well as the new variables.

The README.md file has been updated to document this new flag and its usage.

Author: google-labs-jules[bot]

README.md

@@ -47,13 +47,15 @@ The `mcp-cli` tool has three main commands, one for each transport protocol.
 Connect to an MCP server that communicates over standard input/output.
 
 ```sh
-mcp-cli stdio "<command-to-start-server>"
+mcp-cli stdio --env "VAR=value" --env "ANOTHER_VAR=another_value" "<command-to-start-server>"
 ```
 
+The `--env` (or `-e`) flag can be used multiple times to pass environment variables to the server process.
+
 **Example:**
 
 ```sh
-mcp-cli stdio "python /path/to/mcp/server.py"
+mcp-cli stdio -e "API_KEY=12345" "python /path/to/mcp/server.py"
 ```
 
 ### `sse`

main.go

@@ -37,6 +37,7 @@ func Execute() {
 
 func init() {
 	rootCmd.PersistentFlags().BoolVarP(&verbose, "verbose", "v", false, "verbose output")
+	stdioCmd.Flags().StringSliceP("env", "e", []string{}, "Environment variables to pass to the command")
 }
 
 var stdioCmd = &cobra.Command{
@@ -49,11 +50,15 @@ var stdioCmd = &cobra.Command{
 			log.Printf("Command: %s", command)
 		}
 
+		env, _ := cmd.Flags().GetStringSlice("env")
+
 		ctx := context.Background()
 		client := mcp.NewClient(&mcp.Implementation{Name: "mcp-cli", Version: "v0.1.0"}, nil)
 
 		cmdParts := strings.Fields(command)
-		transport := &mcp.CommandTransport{Command: exec.Command(cmdParts[0], cmdParts[1:]...)}
+		execCmd := exec.Command(cmdParts[0], cmdParts[1:]...)
+		execCmd.Env = append(os.Environ(), env...)
+		transport := &mcp.CommandTransport{Command: execCmd}
 		session, err := client.Connect(ctx, transport, nil)
 		if err != nil {
 			log.Fatalf("Failed to connect to stdio server: %v", err)