decode and print claims in JWT

main.go

package main

import (
	"encoding/base64"
	"fmt"
	"io"
	"os"
	"strings"

	"golang.org/x/term"
)

func main() {
	var jwtString string
	if term.IsTerminal(int(os.Stdin.Fd())) {
		if len(os.Args) < 2 {
			println("usage:\n claims <base64 encoded JWT>")
		}
		jwtString = os.Args[1]
	} else {
		// read from stdin
		b, err := io.ReadAll(os.Stdin)
		if err != nil {
			fmt.Printf("failed to read from stdin: %s", err.Error())
		}
		jwtString = string(b)
	}

	if err := decode(jwtString); err != nil {
		println(err.Error())
	}
}

func decode(jwtString string) error {
	components := strings.Split(jwtString, ".")
	if len(components) < 3 {
		return fmt.Errorf("string has only %d components. %s", len(components), jwtString)
	}

	b, err := base64.RawURLEncoding.DecodeString(components[1])
	if err != nil {
		return fmt.Errorf("base64 decode: %w %s", err, components[1])
	}
	println(string(b))
	return nil
}