// getCommands
func getCommands() []cli.Command {
    command := cli.Command{
        Name:   "web",
        Usage:  "run web server",
        Action: runWeb,
        Flags: []cli.Flag{
            cli.StringFlag{
                Name:  "host",
                Value: "0.0.0.0",
                Usage: "bind host",
            },
            cli.IntFlag{
                Name:  "port,p",
                Value: DefaultPort,
                Usage: "bind port",
            },
            cli.StringFlag{
                Name:  "env,e",
                Value: "prod",
                Usage: "runtime environment, dev|test|prod",
            },
        },
    }

    return []cli.Command{command}
}

The above code is not easy to understand. We need to break it up
When we instantiate a class directly, if the braces are arranged vertically, a comma should be added after the member assignment.

    b := Taoshihan{
        Name: "taoshihan",
    }
    fmt.Println(b.Name)

Define an interface with a member method

type Flag interface {
    GetName() string
}

If you define another type that just has this method, you can think that this type implements the interface

type StringFlag struct {
}
func (t StringFlag) GetName() string {
    return "taoshihan"
}

At this time, if you define variables of Flag type, StringFlag can also be assigned to the past

var a Flag
a = StringFlag{}
a.GetName()

Back to the logic in the original code, it's easy to understand if you use the following method

var myflag []Flag
myflag = append(myflag, StringFlag{}, StringFlag{})

command := Command{
    Flags: myflag,
}

Full source:

package main

import "fmt"

type Flag interface {
    GetName() string
}

type Command struct {
    Flags []Flag
}
type StringFlag struct {
}

func (t StringFlag) GetName() string {
    return "taoshihan"
}

type Taoshihan struct {
    Name string
}

func main() {
    // var a Flag
    // a = StringFlag{}
    // a.GetName()
    // b := Taoshihan{
    //     Name: "taoshihan",
    // }
    // fmt.Println(b.Name)

    var myflag []Flag
    myflag = append(myflag, StringFlag{}, StringFlag{})

    command := Command{
        Flags: myflag,
    }
    for _, p := range command.Flags {
        fmt.Println(p.GetName())
    }
}