Read from stdin as JSON or not JSON

main.go

package main

import (
	"bufio"
	"encoding/json"
	"fmt"
	"os"
)

func main() {
	reader := bufio.NewReader(os.Stdin)
	b, _ := reader.Peek(1)
	if string(b) == "{" {
		var v interface{}
		dec := json.NewDecoder(reader)
		dec.Decode(&v)
		fmt.Println("json", v)
	} else {
		line, _, _ := reader.ReadLine()
		fmt.Println("line", string(line))
	}
}

result.txt

$ echo foo | go run main.go
line foo
$ echo '{"foo":"bar"}' | go run main.go
json map[foo:bar]