Quickly check JSON file syntax. Faster than `jq`. Good to use in git pre-commit hook.

jc.go

package main

import (
	"encoding/json"
	"fmt"
	"io/ioutil"
	"log"
	"os"
)

func must(err error) {
	if err != nil {
		log.Fatal(err)
	}
}

func main() {

	args := os.Args[1:]
	if len(args) <= 0 {
		fmt.Printf("usage: %s <file> \n", os.Args[0])
		os.Exit(1)
	}

	b, err := ioutil.ReadFile(args[0])
	must(err)

	var v interface{}
	err = json.Unmarshal(b, &v)

	// https: //adrianhesketh.com/2017/03/18/getting-line-and-character-positions-from-gos-json-unmarshal-errors/

	if jsonError, ok := err.(*json.SyntaxError); ok {
		line, character, lcErr := lineAndCharacter(string(b), int(jsonError.Offset))
		fmt.Fprintf(
			os.Stderr,
			"Cannot parse JSON schema due to a syntax error at line %d, character %d: %v\n",
			line, character, jsonError.Error())
		if lcErr != nil {
			fmt.Fprintf(
				os.Stderr,
				"Couldn't find the line and character position of the error due to error %v\n",
				lcErr)
		}
		os.Exit(1)
	}
	if jsonError, ok := err.(*json.UnmarshalTypeError); ok {
		line, character, lcErr := lineAndCharacter(string(b), int(jsonError.Offset))
		fmt.Fprintf(
			os.Stderr,
			"The JSON type '%v' cannot be converted into the Go '%v' type on struct "+
				"'%s', field '%v'. See input file line %d, character %d\n",
			jsonError.Value, jsonError.Type.Name(), jsonError.Struct,
			jsonError.Field, line, character)
		if lcErr != nil {
			fmt.Fprintf(
				os.Stderr,
				"Couldn't find the line and character position of the error due to error %v\n",
				lcErr)
		}
		os.Exit(1)
	}
	os.Exit(0)
}

func lineAndCharacter(input string, offset int) (line int, character int, err error) {
	lf := rune(0x0A)

	if offset > len(input) || offset < 0 {
		return 0, 0, fmt.Errorf("Couldn't find offset %d within the input.", offset)
	}

	// Humans tend to count from 1.
	line = 1

	for i, b := range input {
		if b == lf {
			line++
			character = 0
		}
		character++
		if i == offset {
			break
		}
	}

	return line, character, nil
}