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// Copyright 2018 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package modcmd
import (
var cmdDownload = &base.Command{
UsageLine: "go mod download [-dir] [-json] [modules]",
Short: "download modules to local cache",
Long: `
Download downloads the named modules, which can be module patterns selecting
dependencies of the main module or module queries of the form path@version.
With no arguments, download applies to all dependencies of the main module.
The go command will automatically download modules as needed during ordinary
execution. The "go mod download" command is useful mainly for pre-filling
the local cache or to compute the answers for a Go module proxy.
By default, download reports errors to standard error but is otherwise silent.
The -json flag causes download to print a sequence of JSON objects
to standard output, describing each downloaded module (or failure),
corresponding to this Go struct:
type Module struct {
Path string // module path
Version string // module version
Error string // error loading module
Info string // absolute path to cached .info file
GoMod string // absolute path to cached .mod file
Zip string // absolute path to cached .zip file
Dir string // absolute path to cached source root directory
Sum string // checksum for path, version (as in go.sum)
GoModSum string // checksum for go.mod (as in go.sum)
See 'go help modules' for more about module queries.
var downloadJSON = cmdDownload.Flag.Bool("json", false, "")
func init() {
cmdDownload.Run = runDownload // break init cycle
type moduleJSON struct {
Path string `json:",omitempty"`
Version string `json:",omitempty"`
Error string `json:",omitempty"`
Info string `json:",omitempty"`
GoMod string `json:",omitempty"`
Zip string `json:",omitempty"`
Dir string `json:",omitempty"`
Sum string `json:",omitempty"`
GoModSum string `json:",omitempty"`
func runDownload(cmd *base.Command, args []string) {
if len(args) == 0 {
args = []string{"all"}
var mods []*moduleJSON
var work par.Work
listU := false
listVersions := false
for _, info := range modload.ListModules(args, listU, listVersions) {
if info.Replace != nil {
info = info.Replace
if info.Version == "" {
m := &moduleJSON{
Path: info.Path,
Version: info.Version,
mods = append(mods, m)
work.Do(10, func(item interface{}) {
m := item.(*moduleJSON)
var err error
m.Info, err = modfetch.InfoFile(m.Path, m.Version)
if err != nil {
m.Error = err.Error()
m.GoMod, err = modfetch.GoModFile(m.Path, m.Version)
if err != nil {
m.Error = err.Error()
m.GoModSum, err = modfetch.GoModSum(m.Path, m.Version)
if err != nil {
m.Error = err.Error()
mod := module.Version{Path: m.Path, Version: m.Version}
m.Zip, err = modfetch.DownloadZip(mod)
if err != nil {
m.Error = err.Error()
m.Sum = modfetch.Sum(mod)
m.Dir, err = modfetch.Download(mod)
if err != nil {
m.Error = err.Error()
if *downloadJSON {
for _, m := range mods {
b, err := json.MarshalIndent(m, "", "\t")
if err != nil {
base.Fatalf("%v", err)
os.Stdout.Write(append(b, '\n'))