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// Package envflag is an extremely small bit of code to make configuration
// via environment variables a little bit less of a hassle when using
// the flag package in the standard library.
//
// Example:
//
// package main
//
// import (
// "flag"
// "fmt"
// "os"
//
// "riedstra.dev/go/envflag"
// )
//
// func main() {
// fl := flag.NewFlagSet("envflag", flag.ExitOnError)
//
// var aFlag string = "some default"
// fmt.Println("aFlag:", aFlag)
//
// envflag.String(fl, &aFlag, "a", "FLAG_A", "Set's the value of -a")
//
// // By default, command line flags override environment vars.
// fl.Parse(os.Args[1:])
//
// fmt.Println("aFlag:", aFlag)
// }
//
// Known caveat, variables set in the environment will override what the flag
// package sees as the "default" value.
package envflag
import (
"flag"
"fmt"
"os"
"strconv"
)
// String is a convent way to set value from environment variable,
// and allow override when a command line flag is set. It's assumed `p` is
// not nil.
func String(fl *flag.FlagSet, p *string, name, envvar, usage string) {
if v := os.Getenv(envvar); v != "" {
*p = v
}
fl.StringVar(p, name, *p, fmt.Sprintf("%s (Environ: '%s')", usage, envvar))
}
// Bool is a convent way to set value from environment variable,
// and allow override when a command line flag is set. It's assumed `p` is
// not nil.
func Bool(fl *flag.FlagSet, p *bool, name, envvar, usage string) error {
if v := os.Getenv(envvar); v != "" {
res, err := strconv.ParseBool(v)
if err != nil {
return fmt.Errorf("Bool: cannot parse '%s=%s', %w",
envvar, v, err)
}
*p = res
}
fl.BoolVar(p, name, *p, fmt.Sprintf("%s (Environ: '%s')", usage, envvar))
return nil
}
// Int is a convent way to set value from environment variable,
// and allow override when a command line flag is set. It's assumed `p` is
// not nil.
func Int(fl *flag.FlagSet, p *int, name, envvar, usage string) error {
if v := os.Getenv(envvar); v != "" {
res, err := strconv.ParseInt(v, 10, 32)
if err != nil {
return fmt.Errorf("Int: cannot parse '%s=%s', %w",
envvar, v, err)
}
*p = int(res)
}
fl.IntVar(p, name, *p, fmt.Sprintf("%s (Environ: '%s')", usage, envvar))
return nil
}
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