jq.go

package main

import (
	"encoding/json"
	"flag"
	"fmt"
	"io"
	"io/ioutil"
	"os"
	"path/filepath"
	"text/template"
)

var format = flag.String("f", "{{json .}}", "Format JSON with the given text/template.")
var input = flag.String("i", "-", "Input file; reads from stdin by default.")

func die(v interface{}) {
	fmt.Fprintln(os.Stderr, v)
	os.Exit(1)
}

func isStdin() bool {
	stat, err := os.Stdin.Stat()
	if err != nil {
		return false
	}
	return stat.Mode()&os.ModeCharDevice == 0
}

var funcs = template.FuncMap{
	"json": func(v interface{}) (string, error) {
		p, err := json.MarshalIndent(v, "", "\t")
		if err != nil {
			return "", err
		}
		return string(p), nil
	},
	"base": func(s string) string {
		return filepath.Base(s)
	},
	"dir": func(s string) string {
		return filepath.Dir(s)
	},
}

func parse() (*template.Template, io.ReadCloser) {
	flag.Parse()
	if *format == "" {
		die("empty value passed for -f")
	}
	tmpl, err := template.New("gojq").Funcs(funcs).Parse(*format)
	if err != nil {
		die(err)
	}
	switch *input {
	case "":
		die("empty value passed for -i")
	case "-":
		if !isStdin() {
			die("no data on stdin")
		}
		return tmpl, ioutil.NopCloser(os.Stdin)
	}
	f, err := os.Open(*input)
	if err != nil {
		die(err)
	}
	return tmpl, f
}

func main() {
	var m interface{}
	tmpl, rc := parse()
	defer rc.Close()
	if err := json.NewDecoder(rc).Decode(&m); err != nil {
		die(err)
	}
	if err := tmpl.Execute(os.Stdout, m); err != nil {
		die(err)
	}
	fmt.Println()
}