BUG: https://github.com/urfave/cli/issues/1373

go.mod

module bla

go 1.18

require github.com/urfave/cli/v2 v2.4.8

require (
	github.com/BurntSushi/toml v1.1.0 // indirect
	github.com/cpuguy83/go-md2man/v2 v2.0.1 // indirect
	github.com/russross/blackfriday/v2 v2.1.0 // indirect
	gopkg.in/yaml.v2 v2.4.0 // indirect
)

go.sum

github.com/BurntSushi/toml v1.1.0 h1:ksErzDEI1khOiGPgpwuI7x2ebx/uXQNw7xJpn9Eq1+I=
github.com/BurntSushi/toml v1.1.0/go.mod h1:CxXYINrC8qIiEnFrOxCa7Jy5BFHlXnUU2pbicEuybxQ=
github.com/cpuguy83/go-md2man/v2 v2.0.1 h1:r/myEWzV9lfsM1tFLgDyu0atFtJ1fXn261LKYj/3DxU=
github.com/cpuguy83/go-md2man/v2 v2.0.1/go.mod h1:tgQtvFlXSQOSOSIRvRPT7W67SCa46tRHOmNcaadrF8o=
github.com/russross/blackfriday/v2 v2.1.0 h1:JIOH55/0cWyOuilr9/qlrm0BSXldqnqwMsf35Ld67mk=
github.com/russross/blackfriday/v2 v2.1.0/go.mod h1:+Rmxgy9KzJVeS9/2gXHxylqXiyQDYRxCVz55jmeOWTM=
github.com/urfave/cli/v2 v2.4.8 h1:9HuvvddU3oEJr1tJlwUVVsk3snVWMuKSpyAO+SzTNuI=
github.com/urfave/cli/v2 v2.4.8/go.mod h1:oDzoM7pVwz6wHn5ogWgFUU1s4VJayeQS+aEZDqXIEJs=
gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
gopkg.in/yaml.v2 v2.4.0 h1:D8xgwECY7CYvx+Y2n4sBz93Jn9JRvxdiyyo8CTfuKaY=
gopkg.in/yaml.v2 v2.4.0/go.mod h1:RDklbk79AGWmwhnvt/jBztapEOGDOx6ZbXqjP6csGnQ=

main.go

package main

import (
	"fmt"
	"github.com/urfave/cli/v2"
	"github.com/urfave/cli/v2/altsrc"
	"log"
	"os"
	"time"
)

func main() {
	c := &cli.App{
		Name:                   "ntfy",
		Usage:                  "Simple pub-sub notification service",
		UsageText:              "ntfy [OPTION..]",
		HideVersion:            true,
		UseShortOptionHandling: true,
		Commands: []*cli.Command{
			cmdServe,
		},
	}
	log.Fatalln(c.Run(os.Args))
}

var flagsServe = []cli.Flag{
	&cli.StringFlag{Name: "config", Aliases: []string{"c"}, EnvVars: []string{"NTFY_CONFIG_FILE"}, Value: "/etc/ntfy/server.yml", DefaultText: "/etc/ntfy/server.yml", Usage: "config file"},
	altsrc.NewDurationFlag(&cli.DurationFlag{Name: "keepalive-interval", Aliases: []string{"k"}, EnvVars: []string{"NTFY_KEEPALIVE_INTERVAL"}, Value: 45 * time.Second, Usage: "interval of keepalive messages"}),
}

var cmdServe = &cli.Command{
	Name:      "serve",
	Usage:     "Run the ntfy server",
	UsageText: "ntfy serve [OPTIONS..]",
	Action:    execServe,
	Flags:     flagsServe,
	Before:    initConfigFileInputSource("config", flagsServe),
}

func execServe(c *cli.Context) error {
	keepaliveInterval := c.Duration("keepalive-interval")
	log.Printf("keepalive %#v", keepaliveInterval)
	return nil
}

// initConfigFileInputSource is like altsrc.InitInputSourceWithContext and altsrc.NewYamlSourceFromFlagFunc, but checks
// if the config flag is exists and only loads it if it does. If the flag is set and the file exists, it fails.
func initConfigFileInputSource(configFlag string, flags []cli.Flag) cli.BeforeFunc {
	return func(context *cli.Context) error {
		configFile := context.String(configFlag)
		if context.IsSet(configFlag) && !FileExists(configFile) {
			return fmt.Errorf("config file %s does not exist", configFile)
		} else if !context.IsSet(configFlag) && !FileExists(configFile) {
			return nil
		}
		inputSource, err := altsrc.NewYamlSourceFromFile(configFile)
		if err != nil {
			return err
		}
		return altsrc.ApplyInputSourceValues(context, inputSource, flags)
	}
}

// FileExists checks if a file exists, and returns true if it does
func FileExists(filename string) bool {
	stat, _ := os.Stat(filename)
	return stat != nil
}

run.sh

```
$ go run main.go serve --config=server.yml 
2022/04/24 21:38:54 keepalive 0
2022/04/24 21:38:54 <nil>
exit status 1
```

server.yml

# empty file