update driver lookup to allow for cli drivers

pkg/driver/command_driver_nix.go

@@ -0,0 +1,20 @@
+// +build !windows
+
+package driver
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+)
+
+// CheckDriverExists checks to see if the named driver exists
+func (d *CommandDriver) CheckDriverExists() bool {
+	cmd := exec.Command("/bin/sh", "-c", fmt.Sprintf("command -v %s", d.cliName()))
+	cmd.Env = os.Environ()
+	if err := cmd.Run(); err != nil {
+		return false
+	}
+
+	return true
+}

pkg/driver/command_driver_test.go

@@ -1,5 +1,46 @@
 package driver
 
-import "github.com/deislabs/cnab-go/driver"
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
+	"strings"
+	"testing"
+
+	"github.com/deislabs/cnab-go/driver"
+)
 
 var _ driver.Driver = &CommandDriver{}
+
+func TestCheckDriverExists(t *testing.T) {
+	name := "missing-driver"
+	cmddriver := &CommandDriver{Name: name}
+	if cmddriver.CheckDriverExists() {
+		t.Errorf("Expected driver %s not to exist", name)
+	}
+
+	name = "existing-driver"
+	cmddriver = &CommandDriver{Name: name}
+	dirname, err := ioutil.TempDir("", "duffle")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	defer os.RemoveAll(dirname)
+	filename := fmt.Sprintf("%s/duffle-%s", dirname, name)
+	newfile, err := os.Create(filename)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	newfile.Chmod(0755)
+	path := os.Getenv("PATH")
+	pathlist := []string{dirname, path}
+	newpath := strings.Join(pathlist, string(os.PathListSeparator))
+	defer os.Setenv("PATH", path)
+	os.Setenv("PATH", newpath)
+	if !cmddriver.CheckDriverExists() {
+		t.Fatalf("Expected driver %s to exist", name)
+	}
+
+}

pkg/driver/command_driver_windows.go

@@ -0,0 +1,19 @@
+// +build windows
+
+package driver
+
+import (
+	"os"
+	"os/exec"
+)
+
+// CheckDriverExists checks to see if the named driver exists
+func (d *CommandDriver) CheckDriverExists() bool {
+	cmd := exec.Command("where", d.cliName())
+	cmd.Env = os.Environ()
+	if err := cmd.Run(); err != nil {
+		return false
+	}
+
+	return true
+}

pkg/driver/lookup.go

@@ -15,9 +15,12 @@ func Lookup(name string) (driver.Driver, error) {
 		return &KubernetesDriver{}, nil
 	case "debug":
 		return &driver.DebugDriver{}, nil
-	case "command":
-		return &CommandDriver{Name: name}, nil
 	default:
-		return nil, fmt.Errorf("unsupported driver: %s", name)
+		cmddriver := &CommandDriver{Name: name}
+		if cmddriver.CheckDriverExists() {
+			return cmddriver, nil
+		}
+
+		return nil, fmt.Errorf("unsupported driver or driver not found in PATH: %s", name)
 	}
 }