after.txt

[user@work cmdtest]$ ./cmdtest --header aa --header bb --url foo
invoked via main app
  ctx.IsSet(HttpHeaderFlag.Name): true
    0: aa
    1: bb
  ctx.IsSet(UrlFlag.Name): true
    foo
[user@work cmdtest]$ ./cmdtest connect --header aa --header bb --url foo
invoked via subcommand
  ctx.IsSet(HttpHeaderFlag.Name): true
    0: aa
    1: bb
  ctx.IsSet(UrlFlag.Name): true
    foo
[user@work cmdtest]$ ./cmdtest  --header aa --header bb --url foo connect
invoked via subcommand
  ctx.IsSet(HttpHeaderFlag.Name): true
    0: aa
    1: bb
  ctx.IsSet(UrlFlag.Name): true
    foo

before.txt

[user@work cmdtest]$ ./cmdtest --header aa --header bb --url foo
invoked via main app
  ctx.IsSet(HttpHeaderFlag.Name): true
    0: aa
    1: bb
  ctx.IsSet(UrlFlag.Name): true
    foo
[user@work cmdtest]$ ./cmdtest connect --header aa --header bb --url foo
invoked via subcommand
  ctx.IsSet(HttpHeaderFlag.Name): true
    0: aa
    1: bb
  ctx.IsSet(UrlFlag.Name): true
    foo
[user@work cmdtest]$ ./cmdtest  --header aa --header bb --url foo connect
invoked via subcommand
  ctx.IsSet(HttpHeaderFlag.Name): true
    0: [aa bb]
  ctx.IsSet(UrlFlag.Name): true
    foo

main.go

package main

import (
	"fmt"
	"os"

	"github.com/ethereum/go-ethereum/internal/flags"
	"github.com/urfave/cli/v2"
)

// NewApp creates an app with sane defaults.
func NewApp() *cli.App {
	app := cli.NewApp()
	app.Before = func(ctx *cli.Context) error {
		flags.MigrateGlobalFlags(ctx)
		return nil
	}
	return app
}

var (
	// The app that holds all commands and flags.
	app            = NewApp()
	HttpHeaderFlag = &cli.StringSliceFlag{
		Name:    "header",
		Aliases: []string{"H"},
		Usage:   "Pass custom header(s) to server",
	}
	UrlFlag = &cli.StringFlag{
		Name:    "url",
		Aliases: []string{"U"},
		Usage:   "URL to connect",
	}
	subCmd = &cli.Command{
		Action: subCommand,
		Name:   "connect",
		Usage:  "",
		Flags: []cli.Flag{
			HttpHeaderFlag,
			UrlFlag,
		},
		Description: "Shows metadata about the chain status.",
	}
)

func init() {
	// Initialize the CLI app and start Geth
	app.Action = mainApp
	app.Name = "Main app"
	app.Commands = []*cli.Command{
		subCmd,
	}
	app.Flags = []cli.Flag{
		HttpHeaderFlag,
		UrlFlag,
	}
}
func mainApp(ctx *cli.Context) error {
	fmt.Printf("invoked via main app\n")
	printCtx(ctx)
	return nil
}

func subCommand(ctx *cli.Context) error {
	fmt.Printf("invoked via subcommand\n")
	printCtx(ctx)
	return nil
}

func printCtx(ctx *cli.Context) {
	fmt.Printf("  ctx.IsSet(HttpHeaderFlag.Name): %v\n", ctx.IsSet(HttpHeaderFlag.Name))
	if ctx.IsSet(HttpHeaderFlag.Name) {
		for i, v := range ctx.StringSlice(HttpHeaderFlag.Name) {
			fmt.Printf("    %d: %v\n", i, v)
		}
	}
	fmt.Printf("  ctx.IsSet(UrlFlag.Name): %v\n", ctx.IsSet(UrlFlag.Name))
	if ctx.IsSet(UrlFlag.Name) {
		fmt.Printf("    %v\n", ctx.String(UrlFlag.Name))
	}
}

func main() {
	if err := app.Run(os.Args); err != nil {
		fmt.Fprintln(os.Stderr, err)
		os.Exit(1)
	}
}