前往：从文件中读取命令行标志

Question

When creating a command line program in Go, is there a way (either provided by the core libraries, or a widely accepted practice) to implement a command line flag that reads its contents from a file?

If that's not clear, I'm thinking of something like the @ symbol in the command line curl program. Many of curl's arguments allow you to do something like this to read a flag's value in from a file

# setting the value 
curl --data-binary '{...}' http://example.com

# setting the value by reading from a file
curl --data-binary @path/to/data.txt http://example.com  

Does go have any code for automatically implementing these sorts of flags? I've read through the official docs and didn't see anything obvious, but I'm still getting the hang of navigating those docs.

If there's nothing official, is there a de-facto standard "better flags" library provided by someone in the go community that includes this functionality?

Or is it down to an individual programmer to create a string flag, scan it for the @, and handle reading the file's contents in myself?

Answer 1

flag.Value allows you to implement arbitrary flag behaviour:

package main

import (
        "flag"
        "fmt"
        "io/ioutil"
        "strings"
)

type T string

// String implements flag.Value
func (t *T) String() string {
        return string(*t)
}

// Set implements flag.Value
func (t *T) Set(maybeFilename string) error {
        if !strings.HasPrefix(maybeFilename, "@") {
                *t = T(maybeFilename)
                return nil
        }

        filename := maybeFilename[1:]
        b, err := ioutil.ReadFile(filename)
        if err != nil {
                return err
        }

        *t = T(string(b))
        return nil
}

func main() {
        var t T
        flag.Var(&t, "data-binary", "(description)")
        flag.Parse()
        fmt.Println(t)
}