is it possible to watch tracked env vars?

[MushiTheMoshi]

Hi. greatly appreciated the awesome work done here.

I am wondering whether is possible to watch for changes over environment variables?

Regards,
Julio

[vynazevedo]

Viper doesn't have an environment-variable change callback. WatchConfig watches the config file through fsnotify.

It does reread the current process's environment on each getter call, though. Checked with Viper 1.21.0:

v := viper.New()
if err := v.BindEnv("mode", "APP_MODE"); err != nil {
    panic(err)
}
if err := os.Setenv("APP_MODE", "before"); err != nil {
    panic(err)
}
fmt.Println(v.GetString("mode")) // before
if err := os.Setenv("APP_MODE", "after"); err != nil {
    panic(err)
}
fmt.Println(v.GetString("mode")) // after

Changing an export in a shell after starting the application won't change the running application's environment. Neither will editing a .env file unless something explicitly reloads it. Also, a value you've already copied into a variable or unmarshaled into a struct doesn't update itself.

If the goal is live configuration from outside the process, a config file plus WatchConfig/OnConfigChange fits that use case. For changes made with os.Setenv inside the process, you can read the getter again, but Viper won't notify you automatically.

Reference: working with environment variables.