test script for https://github.com/spf13/viper/issues/233

viper_subflags.go

package main

import (
	"fmt"

	"github.com/spf13/cobra"
	"github.com/spf13/viper"
)

func main() {
	var flagValue string

	var cmdPing = &cobra.Command{
		Use:   "ping",
		Short: "ping command",
		Long:  `ping command`,
		Run: func(cmd *cobra.Command, args []string) {
			fmt.Println("ping called")
			fmt.Printf("flag: %s\n", viper.GetString("flag"))
		},
	}

	var cmdPong = &cobra.Command{
		Use:   "pong",
		Short: "pong command",
		Long:  `pong command`,
		Run: func(cmd *cobra.Command, args []string) {
			fmt.Println("pong called")
			fmt.Printf("flag: %s\n", viper.GetString("flag"))
		},
	}

	cmdPing.Flags().StringP("flag", "f", "", "testing flag")
	cmdPong.Flags().StringP("flag", "f", "", "testing flag")

	viper.BindPFlag("flag", cmdPing.Flags().Lookup("flag"))
	viper.BindPFlag("flag", cmdPong.Flags().Lookup("flag"))

	viper.SetDefault("flag", "")

	var rootCmd = &cobra.Command{Use: "app"}
	rootCmd.AddCommand(cmdPing, cmdPong)
	rootCmd.Execute()
}

The problem:

[21/06/23 4:08:58] ~/GitHub/demo (master) $ ./app  ping --flag hello
ping called
flag: hello
[21/06/23 4:08:59] ~/GitHub/demo (master) $ ./app  pong --flag hello
pong called
flag: 

The fix workaround:

package main

import (
	"fmt"

	"github.com/spf13/cobra"
	"github.com/spf13/viper"
)

func main() {
	var cmdPing = &cobra.Command{
		Use:   "ping",
		Short: "ping command",
		Long:  `ping command`,
		Run: func(cmd *cobra.Command, args []string) {
			fmt.Println("ping called")
			fmt.Printf("flag: %s\n", viper.GetString("flag"))
		},
		PreRun: func(cmd *cobra.Command, args []string) {
			viper.BindPFlag("flag", cmd.Flags().Lookup("flag"))
		},
	}

	var cmdPong = &cobra.Command{
		Use:   "pong",
		Short: "pong command",
		Long:  `pong command`,
		Run: func(cmd *cobra.Command, args []string) {
			fmt.Println("pong called")
			fmt.Printf("flag: %s\n", viper.GetString("flag"))
		},
		PreRun: func(cmd *cobra.Command, args []string) {
			viper.BindPFlag("flag", cmd.Flags().Lookup("flag"))
		},
	}

	cmdPing.Flags().StringP("flag", "f", "", "testing flag")
	cmdPong.Flags().StringP("flag", "f", "", "testing flag")

	viper.SetDefault("flag", "")

	var rootCmd = &cobra.Command{Use: "app"}
	rootCmd.AddCommand(cmdPing, cmdPong)
	rootCmd.Execute()
}

This is a well known workaround, not a fix. It doesn't fix the fact that you cannot configure the command with flags as intended. There is unintended behavior when you use the BindPFlag function as intended.