"github.com/urfave/cli"

App 结构体定义了命令行的应用结构，如下很庞大的样子

// App is the main structure of a cli application. It is recommended that
// an app be created with the cli.NewApp() function
type App struct {
       // The name of the program. Defaults to path.Base(os.Args[0])
       Name string
       // Full name of command for help,defaults to Name
       HelpName string
       // Description of the program.
       Usage string
       // Text to override the USAGE section of help
       UsageText string
       // Description of the program argument format.
       ArgsUsage string
       // Version of the program
       Version string
       // List of commands to execute
       Commands []Command
       // List of flags to parse
       Flags []Flag
       // Boolean to enable bash completion commands
       EnableBashCompletion bool
       // Boolean to hide built-in help command
       HideHelp bool
       // Boolean to hide built-in version flag and the VERSION section of help
       HideVersion bool
       // Populate on app startup,only gettable through method Categories()
       categories CommandCategories
       // An action to execute when the bash-completion flag is set
       BashComplete BashCompleteFunc
       // An action to execute before any subcommands are run,but after the context is ready
       // If a non-nil error is returned,no subcommands are run
       Before BeforeFunc
       // An action to execute after any subcommands are run,but after the subcommand has finished
       // It is run even if Action() panics
       After AfterFunc

       // The action to execute when no subcommands are specified
       // Expects a `cli.ActionFunc` but will accept the *deprecated* signature of `func(*cli.Context) {}`
       // *Note*: support for the deprecated `Action` signature will be removed in a future version
       Action interface{}

       // Execute this function if the proper command cannot be found
       CommandNotFound CommandNotFoundFunc
       // Execute this function if an usage error occurs
       OnUsageError OnUsageErrorFunc
       // Compilation date
       Compiled time.Time
       // List of all authors who contributed
       Authors []Author
       // Copyright of the binary if any
       Copyright string
       // Name of Author (Note: Use App.Authors,this is deprecated)
       Author string
       // Email of Author (Note: Use App.Authors,this is deprecated)
       Email string
       // Writer writer to write output to
       Writer io.Writer
       // ErrWriter writes error output
       ErrWriter io.Writer
       // Other custom info
       Metadata map[string]interface{}

       didSetup bool
}

// Context is a type that is passed through to
// each Handler action in a cli application. Context
// can be used to retrieve context-specific Args and
// parsed command-line options.
type Context struct {
       App           *App
       Command       Command
       flagSet       *flag.FlagSet
       setFlags      map[string]bool
       parentContext *Context
}

func main() {
       app := cli.NewApp()
       app.Name = "name"
       app.Usage = usage 
       app.Version = "xxxxx"
       app.Flags = []cli.Flag{
              cli.BoolFlag{
                     Name:  "debug",                     Usage: "enable debug output for logging"              }              cli.StringFlag{
                     Name:  "log"                     Value: "/dev/null""set the log file path where internal debug information is written"
       }
       app.Commands = []cli.Command{               createCommand,deleteCommand,       }
       app.Before = func(context *cli.Context) error {
              if context.GlobalBool("debug") {
                     logrus.SetLevel(logrus.DebugLevel)
              }
              if path := context.GlobalString("log"); path != "" {
                     ferr := os.OpenFile(pathos.O_CREATE|os.O_WRONLY|os.O_APPEND|os.O_SYNC0666)
                     if err != nil {
                            return err
                     }
                     logrus.SetOutput(f)
              }
              return nil
       }
 
       if err := app.Run(os.Args); err != nil {
              fatal(err)
       }
}

// loadSpec loads the specification from the provided path.
func loadSpec(cPath string) (spec *specs.Spec,err error) {
       cf,err := os.Open(cPath)
       if err != nil {
              if os.IsNotExist(err) {
                     return nil,fmt.Errorf("JSON specification file %s not found",cPath)
              }
              return nil,err
       }
       defer cf.Close()

       if err = json.NewDecoder(cf).Decode(&spec); err != nil {
              return nil,err
       }
       return spec,validateProcessSpec(spec.Process)
}