orchard ssh vm: introduce exec mode (#72)

* orchard ssh vm: introduce exec mode

* Document "tart vm ssh" exec mode

README.md

@@ -32,6 +32,8 @@ orchard list vms
 
 ### Accessing Virtual Machines
 
+#### SSH
+
 To SSH into a VM use the `orchard ssh` command:
 
 ```shell
@@ -40,6 +42,20 @@ orchard ssh vm ventura-base
 
 You can specify the `--username` and `--password` flags to specify the username/password pair to SSH. By default, `admin`/`admin` is used.
 
+You can also execute remote commands instead of spawning a login shell, similarly to the OpenSSH's `ssh` command:
+
+```shell
+orchard ssh vm ventura-base "uname -a"
+```
+
+You can execute scripts remotely this way, by telling the remote command-line interpreter to read from the standard input and using the redirection operator as follows:
+
+```shell
+orchard ssh vm ventura-base "bash -s" < script.sh
+```
+
+#### VNC
+
 Similar to `ssh` command, you can use `vnc` command to open Screen Sharing into a remote VM:
 
 ```shell

internal/command/ssh/vm.go

@@ -22,9 +22,9 @@ var wait uint16
 
 func newSSHVMCommand() *cobra.Command {
 	command := &cobra.Command{
-		Use:   "vm VM_NAME",
+		Use:   "vm VM_NAME [COMMAND]",
 		Short: "SSH into the VM",
-		Args:  cobra.ExactArgs(1),
+		Args:  cobra.RangeArgs(1, 2),
 		RunE:  runSSHVM,
 	}
 
@@ -39,8 +39,16 @@ func newSSHVMCommand() *cobra.Command {
 }
 
 func runSSHVM(cmd *cobra.Command, args []string) error {
+	// Required NAME argument
 	name := args[0]
 
+	// Optional [COMMAND] argument
+	var command string
+
+	if len(args) > 1 {
+		command = args[1]
+	}
+
 	client, err := client.New()
 	if err != nil {
 		return err
@@ -70,13 +78,27 @@ func runSSHVM(cmd *cobra.Command, args []string) error {
 	if err != nil {
 		return fmt.Errorf("%w: failed to establish an SSH connection: %v", ErrFailed, err)
 	}
+	defer func() {
+		_ = sshConn.Close()
+	}()
 
 	sshClient := ssh.NewClient(sshConn, chans, reqs)
 
 	sshSess, err := sshClient.NewSession()
 	if err != nil {
 		return fmt.Errorf("%w: failed to open an SSH session: %v", ErrFailed, err)
 	}
+	defer func() {
+		_ = sshSess.Close()
+	}()
+
+	if command != "" {
+		sshSess.Stdout = os.Stdout
+		sshSess.Stderr = os.Stderr
+		sshSess.Stdin = os.Stdin
+
+		return sshSess.Run(command)
+	}
 
 	// Switch controlling terminal into raw mode,
 	// otherwise ANSI escape sequences that allow
@@ -109,7 +131,6 @@ func runSSHVM(cmd *cobra.Command, args []string) error {
 	go func() {
 		_, _ = io.Copy(sshSessStdinPipe, os.Stdin)
 		_ = sshSessStdinPipe.Close()
-		_ = sshSess.Close()
 	}()
 
 	// Periodically adjust remote terminal size
@@ -176,7 +197,7 @@ func ChooseUsernameAndPassword(
 	}
 
 	// Fall back
-	fmt.Println("no credentials specified or found, trying default admin:admin credentials...")
+	_, _ = fmt.Fprintf(os.Stderr, "no credentials specified or found, trying default admin:admin credentials...")
 
 	return "admin", "admin"
 }