In my program, I utilize a JSON file to set up the arguments and also use the flag package to configure the same arguments.
Whenever an argument is parsed by both the JSON file and the flag at the same time, I prefer to use the argument parsed through the flag.
The issue arises when the JSON file path is also parsed through the flag. The JSON path can be retrieved after calling `flag.parse()`, but since the arguments are already parsed, unmarshaling the JSON will overwrite the arguments from flag parsing.
Example JSON:
{
"opt1": 1,
"opt2": "hello"
}
Example code:
var Config = struct {
Opt1 int `json:"opt1"`
Opt2 string `json:"opt2"`
}{
Opt1: 0,
Opt2: "none",
}
func main() {
// Parse config file path
var configFile string
flag.StringVar(&configFile, "config", "", "config file path")
// Parse options
flag.IntVar(&Config.Opt1, "opt1", Config.Opt1, "")
flag.StringVar(&Config.Opt2, "opt2", Config.Opt2, "")
// Parse flags
flag.Parse()
// Load config options from config.json file
if configFile != "" {
if data, err := ioutil.ReadFile(configFile); err != nil {
fmt.Printf("read config file error: %v\n", err)
} else if err = json.Unmarshal(data, &Config); err != nil {
fmt.Printf("parse config file error: %v\n", err)
}
}
fmt.Printf("%+v", Config)
}
Program example output:
./foo.exe -opt2 world
out: {Opt1:0 Opt2:world}
./foo.exe -config config.json
out: {Opt1:1 Opt2:hello}
./foo.exe -config config.json -opt2 worldreal out:
{Opt1:1 Opt2:hello}desired out:
{Opt1:1 Opt2:world}