Attach environment variables to standard library flags

envVarsWithFlags.go

package main

import (
  "flag"
  "fmt"
  "os"
  "strings"
)

package main() {
  flags := flag.NewFlagSet("run", flag.ContinueOnError)
  
  flags.String("port", "7777", "port on which app will listen and serve")
  
  flags.VisitAll(func (f *flag.Flag) {
    envVar := strings.Replace(strings.ToUpper(fmt.Sprintf("prefix_%s", f.Name)), "-", "_", -1)
    if val, ok := os.LookupEnv(envVar); ok {
      flags.Set(f.Name, val) 
    }
  })
}

Precedence

1. flag
2. env var
3. default value