jq but for MessagePack: go build mq.go -o mq

mq.go

package main

import (
	"bytes"
	"fmt"
	"io"
	"io/ioutil"
	"log"
	"os"
	"os/exec"
	"strings"

	"github.com/tinylib/msgp/msgp"
)

// usage invokes jq but replaced jq with mq
func usage() {
	out, err := exec.Command("jq", "--help").CombinedOutput()
	if err != nil {
		log.Fatalf("failed to execute jq: %v", err)
	}
	out = bytes.ReplaceAll(out, []byte("jq"), []byte("mq"))
	out = bytes.ReplaceAll(out, []byte("JSON"), []byte("MessagePack"))
	fmt.Print(string(out))
	os.Exit(0)
}

// convert takes a input converts to json and returns the path
func convert(inputPath string) (string, error) {
	output, err := ioutil.TempFile("", "mq*")
	if err != nil {
		return output.Name(), err
	}
	defer output.Close()
	input, err := os.Open(inputPath)
	if err != nil {
		return output.Name(), err
	}
	defer input.Close()
	if _, err := msgp.CopyToJSON(output, input); err != nil {
		return output.Name(), err
	}
	return output.Name(), nil
}

// Entry point
func main() {
	// Extract the jq args from the filenames
	var files []string
	var args []string
	flags := true
	for _, arg := range os.Args[1:] {
		switch {
		case arg == "--":
			flags = false
		case arg == "--help" || arg == "-h":
			usage()
		case flags && strings.HasPrefix(arg, "-"):
			args = append(args, arg)
		default:
			files = append(files, arg)
		}
	}
	cmd := exec.Command("jq", files[0])
	files = files[1:]
	// If 1 or less files, use stdin to pipe
	if len(files) <= 1 {
		src := os.Stdin
		if len(files) == 1 {
			f, err := os.Open(files[0])
			if err != nil {
				log.Fatalf("failed to open file: %v", err)
			}
			defer f.Close()
			src = f
		}
		// Use a pipe for minimal i/o
		var dst io.WriteCloser
		cmd.Stdin, dst = io.Pipe()
		go func() {
			defer dst.Close()
			if _, err := msgp.CopyToJSON(dst, src); err != nil {
				log.Fatalf("failed to decode msgpack: %v", err)
			}
		}()
	} else {
		// Convert the files on disk with temporary files blocking until done
		for _, file := range files {
			tmp, err := convert(file)
			defer os.Remove(tmp)
			if err != nil {
				log.Fatalf("failed to convert file: %v", err)
			}
			cmd.Args = append(cmd.Args, tmp)
		}
	}
	cmd.Args = append(cmd.Args, args...)
	cmd.Stdout = os.Stdout
	cmd.Stderr = os.Stderr
	if err := cmd.Run(); err != nil {
		if exitError, ok := err.(*exec.ExitError); ok {
			os.Exit(exitError.ExitCode())
		} else {
			log.Fatalf("failed to execute jq: %v", err)
		}
	}
}