Get the size of a container image (the linux/amd64 platform-specific image)

main.go

package main

import (
	"fmt"
	"log"
	"os"

	"github.com/google/go-containerregistry/pkg/authn"
	"github.com/google/go-containerregistry/pkg/name"
	v1 "github.com/google/go-containerregistry/pkg/v1"
	"github.com/google/go-containerregistry/pkg/v1/remote"
)

// Given a string reference to an remote image, fetch the manifest
func fetchImageManifest(img string) (*v1.Manifest, error) {
	ref, err := name.ParseReference(img)
	if err != nil {
		return nil, err
	}
	oimg, err := remote.Image(ref, // TODO: allow alternate platform?
		remote.WithPlatform(v1.Platform{OS: "linux", Architecture: "amd64"}),
		remote.WithAuthFromKeychain(authn.DefaultKeychain),
	)
	if err != nil {
		return nil, err
	}
	return oimg.Manifest()
}

// Given a manifest, get size (size of manifest plus size of each layer)
func calculateSize(manifest *v1.Manifest) (int64, error) {
	size := manifest.Config.Size
	for _, layer := range manifest.Layers {
		size += layer.Size
	}
	return size, nil
}

// Convert a size to human readable format
// Adapted from https://yourbasic.org/golang/formatting-byte-size-to-human-readable-format/
func humanReadable(b int64) string {
	if b < 1024 {
		return fmt.Sprintf("%d B", b)
	}
	div, exp := 1024, 0
	for n := b / 1024; n >= 1024; n /= 1024 {
		div *= 1024
		exp++
	}
	return fmt.Sprintf("%.1f %ciB", float64(b)/float64(div), "KMGTPE"[exp])
}

func main() {
	args := os.Args
	if len(args) != 2 {
		log.Fatal("Usage: go run . <img>")
	}
	img := args[1]
	manifest, err := fetchImageManifest(img)
	if err != nil {
		log.Fatalf("fetching image manifest: %v", err)
	}
	size, err := calculateSize(manifest)
	if err != nil {
		log.Fatalf("calculating size: %v", err)
	}
	fmt.Println(humanReadable(size))
}