cmd/go: split go mod into multiple subcommands

The current "go mod" command does too many things.
The design is unclear.

It looks like "everything you might want to do with modules"
which causes people to think all module operations go through
"go mod", which is the opposite of the seamless integration we're
going for. In particular too many people think "go mod -require"
and "go get" are the same.

It does make sense to put the module-specific functionality
under "go mod", but not as flags. Instead, split "go mod" into
multiple subcommands:

	go mod edit   # old go mod -require ...
	go mod fix    # old go mod -fix
	go mod graph  # old go mod -graph
	go mod init   # old go mod -init
	go mod tidy   # old go mod -sync
	go mod vendor # old go mod -vendor
	go mod verify # old go mod -verify

Splitting out the individual commands makes both the docs
and the implementations dramatically easier to read.
It simplifies the command lines
(go mod -init -module m is now 'go mod init m')
and allows command-specific flags.

We've avoided subcommands in the go command to date, and we
should continue to avoid adding them unless it really makes
the experience significantly better. In this case, it does.

Creating subcommands required some changes in the core
command-parsing and help logic to generalize from one
level to multiple levels.

As part of having "go mod init" be a separate command,
this CL changes the failure behavior during module initialization
to be delayed until modules are actually needed.
Initialization can still happen early, but the base.Fatalf
is delayed until something needs to use modules.
This fixes a bunch of commands like 'go env' that were
unhelpfully failing with GO111MODULE=on when not in a
module directory.

Fixes #26432.
Fixes #26581.
Fixes #26596.
Fixes #26639.

Change-Id: I868db0babe8c288e8af684b29d4a5ae4825d6407
Reviewed-on: https://go-review.googlesource.com/126655
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/alldocs.go

@@ -9,7 +9,7 @@
 //
 // Usage:
 //
-// 	go command [arguments]
+// 	go <command> [arguments]
 //
 // The commands are:
 //
@@ -31,7 +31,7 @@
 // 	version     print Go version
 // 	vet         report likely mistakes in packages
 //
-// Use "go help [command]" for more information about a command.
+// Use "go help <command>" for more information about a command.
 //
 // Additional help topics:
 //
@@ -50,7 +50,7 @@
 // 	testflag    testing flags
 // 	testfunc    testing functions
 //
-// Use "go help [topic]" for more information about that topic.
+// Use "go help <topic>" for more information about that topic.
 //
 //
 // Start a bug report
@@ -852,35 +852,49 @@
 //
 // Module maintenance
 //
-// Usage:
-//
-// 	go mod [-v] [maintenance flags]
-//
-// Mod performs module maintenance operations as specified by the
-// following flags, which may be combined.
+// Go mod provides access to operations on modules.
 //
 // Note that support for modules is built into all the go commands,
 // not just 'go mod'. For example, day-to-day adding, removing, upgrading,
 // and downgrading of dependencies should be done using 'go get'.
 // See 'go help modules' for an overview of module functionality.
 //
-// The -v flag enables additional output about operations performed.
+// Usage:
 //
-// The first group of operations provide low-level editing operations
-// for manipulating go.mod from the command line or in scripts or
-// other tools. They read only go.mod itself; they do not look up any
-// information about the modules involved.
+// 	go mod <command> [arguments]
 //
-// The -init flag initializes and writes a new go.mod to the current directory,
-// in effect creating a new module rooted at the current directory.
-// The file go.mod must not already exist.
-// If possible, mod will guess the module path from import comments
-// (see 'go help importpath') or from version control configuration.
-// To override this guess, use the -module flag.
-// (Without -init, mod applies to the current module.)
+// The commands are:
+//
+// 	edit        edit go.mod from tools or scripts
+// 	fix         make go.mod semantically consistent
+// 	graph       print module requirement graph
+// 	init        initialize new module in current directory
+// 	tidy        add missing and remove unused modules
+// 	vendor      make vendored copy of dependencies
+// 	verify      verify dependencies have expected content
 //
-// The -module flag changes (or, with -init, sets) the module's path
-// (the go.mod file's module line).
+// Use "go help mod <command>" for more information about a command.
+//
+// Edit go.mod from tools or scripts
+//
+// Usage:
+//
+// 	go mod edit [editing flags] [go.mod]
+//
+// Edit provides a command-line interface for editing go.mod,
+// for use primarily by tools or scripts. It reads only go.mod;
+// it does not look up information about the modules involved.
+// By default, edit reads and writes the go.mod file of the main module,
+// but a different target file can be specified after the editing flags.
+//
+// The editing flags specify a sequence of editing operations.
+//
+// The -fmt flag reformats the go.mod file without making other changes.
+// This reformatting is also implied by any other modifications that use or
+// rewrite the go.mod file. The only time this flag is needed is if no other
+// flags are specified, as in 'go mod edit -fmt'.
+//
+// The -module flag changes the module's path (the go.mod file's module line).
 //
 // The -go flag changes the minimum required version of Go listed in go.mod.
 //
@@ -903,21 +917,15 @@
 // new path should be a directory on the local system, not a module path.
 // Note that -replace overrides any existing replacements for old@v.
 //
-// These editing flags (-require, -droprequire, -exclude, -dropexclude,
-// -replace, and -dropreplace) may be repeated.
-//
-// The -fmt flag reformats the go.mod file without making other changes.
-// This reformatting is also implied by any other modifications that use or
-// rewrite the go.mod file. The only time this flag is needed is if no other
-// flags are specified, as in 'go mod -fmt'.
+// The -require, -droprequire, -exclude, -dropexclude, -replace,
+// and -dropreplace editing flags may be repeated, and the changes
+// are applied in the order given.
 //
-// The -graph flag prints the module requirement graph (with replacements applied)
-// in text form. Each line in the output has two space-separated fields: a module
-// and one of its requirements. Each module is identified as a string of the form
-// path@version, except for the main module, which has no @version suffix.
+// The -print flag prints the final go.mod in its text format instead of
+// writing it back to go.mod.
 //
-// The -json flag prints the go.mod file in JSON format corresponding to these
-// Go types:
+// The -json flag prints the final go.mod file in JSON format instead of
+// writing it back to go.mod. The JSON output corresponds to these Go types:
 //
 // 	type Module struct {
 // 		Path string
@@ -946,14 +954,18 @@
 // referred to indirectly. For the full set of modules available to a build,
 // use 'go list -m -json all'.
 //
-// The next group of operations provide higher-level editing and maintenance
-// of a module, beyond the go.mod file.
+// For example, a tool can obtain the go.mod as a data structure by
+// parsing the output of 'go mod edit -json' and can then make changes
+// by invoking 'go mod edit' with -require, -exclude, and so on.
+//
+//
+// Make go.mod semantically consistent
 //
-// The -packages flag prints a list of packages in the module.
-// It only identifies directories containing Go source code;
-// it does not check that those directories contain code that builds.
+// Usage:
+//
+// 	go mod fix
 //
-// The -fix flag updates go.mod to use canonical version identifiers and
+// Fix updates go.mod to use canonical version identifiers and
 // to be semantically consistent. For example, consider this go.mod file:
 //
 // 	module M
@@ -968,42 +980,95 @@
 //
 // 	exclude D v1.2.3
 //
-// First, -fix rewrites non-canonical version identifiers to semver form, so
+// First, fix rewrites non-canonical version identifiers to semver form, so
 // A's v1 becomes v1.0.0 and E's dev becomes the pseudo-version for the latest
 // commit on the dev branch, perhaps v0.0.0-20180523231146-b3f5c0f6e5f1.
 //
-// Next, -fix updates requirements to respect exclusions, so the requirement
+// Next, fix updates requirements to respect exclusions, so the requirement
 // on the excluded D v1.2.3 is updated to use the next available version of D,
 // perhaps D v1.2.4 or D v1.3.0.
 //
-// Finally, -fix removes redundant or misleading requirements.
+// Finally, fix removes redundant or misleading requirements.
 // For example, if A v1.0.0 itself requires B v1.2.0 and C v1.0.0,
 // then go.mod's requirement of B v1.0.0 is misleading (superseded
 // by B's need for v1.2.0), and its requirement of C v1.0.0 is redundant
 // (implied by B's need for the same version), so both will be removed.
 //
-// Although -fix runs the fix-up operation in isolation, the fix-up also
+// Although fix runs the fix-up operation in isolation, the fix-up also
 // runs automatically any time a go command uses the module graph,
-// to update go.mod to reflect reality. For example, the -sync, -vendor,
-// and -verify flags all effectively imply -fix. And because the module
-// graph defines the meaning of import statements, any commands
-// that load packages—'go build', 'go test', 'go list', and so on—also
-// effectively imply 'go mod -fix'.
+// to update go.mod to reflect reality. Because the module graph defines
+// the meaning of import statements, any commands that load packages
+// also use and therefore fix the module graph. For example,
+// go build, go get, go install, go list, go test, go mod graph, go mod tidy,
+// and other commands all effectively imply go mod fix.
+//
+//
+// Print module requirement graph
 //
-// The -sync flag synchronizes go.mod with the source code in the module.
+// Usage:
+//
+// 	go mod graph
+//
+// Graph prints the module requirement graph (with replacements applied)
+// in text form. Each line in the output has two space-separated fields: a module
+// and one of its requirements. Each module is identified as a string of the form
+// path@version, except for the main module, which has no @version suffix.
+//
+//
+// Initialize new module in current directory
+//
+// Usage:
+//
+// 	go mod init [module]
+//
+// Init initializes and writes a new go.mod to the current directory,
+// in effect creating a new module rooted at the current directory.
+// The file go.mod must not already exist.
+// If possible, init will guess the module path from import comments
+// (see 'go help importpath') or from version control configuration.
+// To override this guess, supply the module path as an argument.
+//
+//
+// Add missing and remove unused modules
+//
+// Usage:
+//
+// 	go mod tidy [-v]
+//
+// Tidy makes sure go.mod matches the source code in the module.
 // It adds any missing modules necessary to build the current module's
 // packages and dependencies, and it removes unused modules that
 // don't provide any relevant packages. It also adds any missing entries
 // to go.sum and removes any unnecessary ones.
 //
-// The -vendor flag resets the module's vendor directory to include all
-// packages needed to build and test all the module's packages.
-// It does not include any test code for the vendored packages.
+// The -v flag causes tidy to print information about removed modules
+// to standard error.
+//
+//
+// Make vendored copy of dependencies
+//
+// Usage:
+//
+// 	go mod vendor [-v]
+//
+// Vendor resets the main module's vendor directory to include all packages
+// needed to build and test all the main module's packages.
+// It does not include test code for vendored packages.
+//
+// The -v flag causes vendor to print the names of vendored
+// modules and packages to standard error.
+//
+//
+// Verify dependencies have expected content
+//
+// Usage:
+//
+// 	go mod verify
 //
-// The -verify flag checks that the dependencies of the current module,
+// Verify checks that the dependencies of the current module,
 // which are stored in a local downloaded source cache, have not been
 // modified since being downloaded. If all the modules are unmodified,
-// -verify prints "all modules verified." Otherwise it reports which
+// verify prints "all modules verified." Otherwise it reports which
 // modules have been changed and causes 'go mod' to exit with a
 // non-zero status.
 //
@@ -1949,12 +2014,12 @@
 //
 // To start a new module, simply create a go.mod file in the root of the
 // module's directory tree, containing only a module statement.
-// The 'go mod' command can be used to do this:
+// The 'go mod init' command can be used to do this:
 //
-// 	go mod -init -module example.com/m
+// 	go mod init example.com/m
 //
 // In a project already using an existing dependency management tool like
-// godep, glide, or dep, 'go mod -init' will also add require statements
+// godep, glide, or dep, 'go mod init' will also add require statements
 // matching the existing configuration.
 //
 // Once the go.mod file exists, no additional steps are required:
@@ -2011,7 +2076,7 @@
 // is no longer necessary and can be deleted requires a full view of
 // all packages in the module, across all possible build configurations
 // (architectures, operating systems, build tags, and so on).
-// The 'go mod -sync' command builds that view and then
+// The 'go mod tidy' command builds that view and then
 // adds any missing module requirements and removes unnecessary ones.
 //
 // As part of maintaining the require statements in go.mod, the go command
@@ -2201,7 +2266,7 @@
 // and records the cryptographic checksum of each package at download time.
 // In normal operation, the go command checks these pre-computed checksums
 // against the main module's go.sum file, instead of recomputing them on
-// each command invocation. The 'go mod -verify' command checks that
+// each command invocation. The 'go mod verify' command checks that
 // the cached copies of module downloads still match both their recorded
 // checksums and the entries in go.sum.
 //
@@ -2220,7 +2285,7 @@
 // from their sources and using those downloaded copies (after verification,
 // as described in the previous section). To allow interoperation with older
 // versions of Go, or to ensure that all files used for a build are stored
-// together in a single file tree, 'go mod -vendor' creates a directory named
+// together in a single file tree, 'go mod vendor' creates a directory named
 // vendor in the root directory of the main module and stores there all the
 // packages from dependency modules that are needed to support builds and
 // tests of packages in the main module.
@@ -2239,7 +2304,7 @@
 // This help text, accessible as 'go help module-get' even in legacy GOPATH mode,
 // describes 'go get' as it operates in module-aware mode.
 //
-// Usage: get [-d] [-m] [-u] [-v] [-insecure] [build flags] [packages]
+// Usage: go get [-d] [-m] [-u] [-v] [-insecure] [build flags] [packages]
 //
 // Get resolves and adds dependencies to the current development module
 // and then builds and installs them.

src/cmd/go/internal/base/base.go

@@ -45,25 +45,43 @@ type Command struct {
 	// CustomFlags indicates that the command will do its own
 	// flag parsing.
 	CustomFlags bool
+
+	// Commands lists the available commands and help topics.
+	// The order here is the order in which they are printed by 'go help'.
+	// Note that subcommands are in general best avoided.
+	Commands []*Command
 }
 
-// Commands lists the available commands and help topics.
-// The order here is the order in which they are printed by 'go help'.
-var Commands []*Command
+var Go = &Command{
+	UsageLine: "go",
+	Long:      `Go is a tool for managing Go source code.`,
+	// Commands initialized in package main
+}
 
-// Name returns the command's name: the first word in the usage line.
-func (c *Command) Name() string {
+// LongName returns the command's long name: all the words in the usage line between "go" and a flag or argument,
+func (c *Command) LongName() string {
 	name := c.UsageLine
-	i := strings.Index(name, " ")
-	if i >= 0 {
+	if i := strings.Index(name, " ["); i >= 0 {
 		name = name[:i]
 	}
+	if name == "go" {
+		return ""
+	}
+	return strings.TrimPrefix(name, "go ")
+}
+
+// Name returns the command's short name: the last word in the usage line before a flag or argument.
+func (c *Command) Name() string {
+	name := c.LongName()
+	if i := strings.LastIndex(name, " "); i >= 0 {
+		name = name[i+1:]
+	}
 	return name
 }
 
 func (c *Command) Usage() {
 	fmt.Fprintf(os.Stderr, "usage: %s\n", c.UsageLine)
-	fmt.Fprintf(os.Stderr, "Run 'go help %s' for details.\n", c.Name())
+	fmt.Fprintf(os.Stderr, "Run 'go help %s' for details.\n", c.LongName())
 	os.Exit(2)
 }
 

src/cmd/go/internal/bug/bug.go

@@ -25,7 +25,7 @@ import (
 
 var CmdBug = &base.Command{
 	Run:       runBug,
-	UsageLine: "bug",
+	UsageLine: "go bug",
 	Short:     "start a bug report",
 	Long: `
 Bug opens the default browser and starts a new bug report.
@@ -38,6 +38,9 @@ func init() {
 }
 
 func runBug(cmd *base.Command, args []string) {
+	if len(args) > 0 {
+		base.Fatalf("go bug: bug takes no arguments")
+	}
 	var buf bytes.Buffer
 	buf.WriteString(bugHeader)
 	inspectGoVersion(&buf)

src/cmd/go/internal/clean/clean.go

@@ -18,11 +18,12 @@ import (
 	"cmd/go/internal/cfg"
 	"cmd/go/internal/load"
 	"cmd/go/internal/modfetch"
+	"cmd/go/internal/modload"
 	"cmd/go/internal/work"
 )
 
 var CmdClean = &base.Command{
-	UsageLine: "clean [clean flags] [build flags] [packages]",
+	UsageLine: "go clean [clean flags] [build flags] [packages]",
 	Short:     "remove object files and cached files",
 	Long: `
 Clean removes object files from package source directories.
@@ -102,9 +103,14 @@ func init() {
 }
 
 func runClean(cmd *base.Command, args []string) {
+	if len(args) == 0 && modload.Failed() {
+		// Don't try to clean current directory,
+		// which will cause modload to base.Fatalf.
+	} else {
 		for _, pkg := range load.PackagesAndErrors(args) {
 			clean(pkg)
 		}
+	}
 
 	if cleanCache {
 		var b work.Builder

src/cmd/go/internal/doc/doc.go

@@ -12,7 +12,7 @@ import (
 
 var CmdDoc = &base.Command{
 	Run:         runDoc,
-	UsageLine:   "doc [-u] [-c] [package|[package.]symbol[.methodOrField]]",
+	UsageLine:   "go doc [-u] [-c] [package|[package.]symbol[.methodOrField]]",
 	CustomFlags: true,
 	Short:       "show documentation for package or symbol",
 	Long: `

src/cmd/go/internal/envcmd/env.go

@@ -22,7 +22,7 @@ import (
 )
 
 var CmdEnv = &base.Command{
-	UsageLine: "env [-json] [var ...]",
+	UsageLine: "go env [-json] [var ...]",
 	Short:     "print Go environment information",
 	Long: `
 Env prints Go environment information.

src/cmd/go/internal/fix/fix.go

@@ -17,7 +17,7 @@ import (
 
 var CmdFix = &base.Command{
 	Run:       runFix,
-	UsageLine: "fix [packages]",
+	UsageLine: "go fix [packages]",
 	Short:     "update packages to use new APIs",
 	Long: `
 Fix runs the Go fix command on the packages named by the import paths.

src/cmd/go/internal/fmtcmd/fmt.go

@@ -26,7 +26,7 @@ func init() {
 
 var CmdFmt = &base.Command{
 	Run:       runFmt,
-	UsageLine: "fmt [-n] [-x] [packages]",
+	UsageLine: "go fmt [-n] [-x] [packages]",
 	Short:     "gofmt (reformat) package sources",
 	Long: `
 Fmt runs the command 'gofmt -l -w' on the packages named

src/cmd/go/internal/generate/generate.go

@@ -27,7 +27,7 @@ import (
 
 var CmdGenerate = &base.Command{
 	Run:       runGenerate,
-	UsageLine: "generate [-run regexp] [-n] [-v] [-x] [build flags] [file.go... | packages]",
+	UsageLine: "go generate [-run regexp] [-n] [-v] [-x] [build flags] [file.go... | packages]",
 	Short:     "generate Go files by processing source",
 	Long: `
 Generate runs commands described by directives within existing

src/cmd/go/internal/get/get.go

@@ -23,7 +23,7 @@ import (
 )
 
 var CmdGet = &base.Command{
-	UsageLine: "get [-d] [-f] [-t] [-u] [-v] [-fix] [-insecure] [build flags] [packages]",
+	UsageLine: "go get [-d] [-f] [-t] [-u] [-v] [-fix] [-insecure] [build flags] [packages]",
 	Short:     "download and install packages and dependencies",
 	Long: `
 Get downloads the packages named by the import paths, along with their

src/cmd/go/internal/help/help.go

@@ -21,20 +21,8 @@ import (
 
 // Help implements the 'help' command.
 func Help(args []string) {
-	if len(args) == 0 {
-		PrintUsage(os.Stdout)
-		// not exit 2: succeeded at 'go help'.
-		return
-	}
-	if len(args) != 1 {
-		fmt.Fprintf(os.Stderr, "usage: go help command\n\nToo many arguments given.\n")
-		os.Exit(2) // failed at 'go help'
-	}
-
-	arg := args[0]
-
 	// 'go help documentation' generates doc.go.
-	if arg == "documentation" {
+	if len(args) == 1 && args[0] == "documentation" {
 		fmt.Println("// Copyright 2011 The Go Authors. All rights reserved.")
 		fmt.Println("// Use of this source code is governed by a BSD-style")
 		fmt.Println("// license that can be found in the LICENSE file.")
@@ -43,68 +31,85 @@ func Help(args []string) {
 		fmt.Println("// Edit the documentation in other files and rerun mkalldocs.sh to generate this one.")
 		fmt.Println()
 		buf := new(bytes.Buffer)
-		PrintUsage(buf)
+		PrintUsage(buf, base.Go)
 		usage := &base.Command{Long: buf.String()}
 		cmds := []*base.Command{usage}
-		for _, cmd := range base.Commands {
+		for _, cmd := range base.Go.Commands {
 			if cmd.UsageLine == "gopath-get" {
 				// Avoid duplication of the "get" documentation.
 				continue
 			}
 			cmds = append(cmds, cmd)
+			cmds = append(cmds, cmd.Commands...)
 		}
 		tmpl(&commentWriter{W: os.Stdout}, documentationTemplate, cmds)
 		fmt.Println("package main")
 		return
 	}
 
-	for _, cmd := range base.Commands {
-		if cmd.Name() == arg {
-			tmpl(os.Stdout, helpTemplate, cmd)
-			// not exit 2: succeeded at 'go help cmd'.
-			return
+	cmd := base.Go
+Args:
+	for i, arg := range args {
+		for _, sub := range cmd.Commands {
+			if sub.Name() == arg {
+				cmd = sub
+				continue Args
 			}
 		}
 
-	fmt.Fprintf(os.Stderr, "Unknown help topic %#q. Run 'go help'.\n", arg)
+		// helpSuccess is the help command using as many args as possible that would succeed.
+		helpSuccess := "go help"
+		if i > 0 {
+			helpSuccess = " " + strings.Join(args[:i], " ")
+		}
+		fmt.Fprintf(os.Stderr, "go help %s: unknown help topic. Run '%s'.\n", strings.Join(args, " "), helpSuccess)
 		os.Exit(2) // failed at 'go help cmd'
+	}
+
+	if len(cmd.Commands) > 0 {
+		PrintUsage(os.Stdout, cmd)
+	} else {
+		tmpl(os.Stdout, helpTemplate, cmd)
+	}
+	// not exit 2: succeeded at 'go help cmd'.
+	return
 }
 
-var usageTemplate = `Go is a tool for managing Go source code.
+var usageTemplate = `{{.Long | trim}}
 
 Usage:
 
-	go command [arguments]
+	{{.UsageLine}} <command> [arguments]
 
 The commands are:
-{{range .}}{{if .Runnable}}
+{{range .Commands}}{{if or (.Runnable) .Commands}}
 	{{.Name | printf "%-11s"}} {{.Short}}{{end}}{{end}}
 
-Use "go help [command]" for more information about a command.
-
+Use "go help{{with .LongName}} {{.}}{{end}} <command>" for more information about a command.
+{{if eq (.UsageLine) "go"}}
 Additional help topics:
-{{range .}}{{if not .Runnable}}
+{{range .Commands}}{{if and (not .Runnable) (not .Commands)}}
 	{{.Name | printf "%-11s"}} {{.Short}}{{end}}{{end}}
 
-Use "go help [topic]" for more information about that topic.
-
+Use "go help{{with .LongName}} {{.}}{{end}} <topic>" for more information about that topic.
+{{end}}
 `
 
-var helpTemplate = `{{if .Runnable}}usage: go {{.UsageLine}}
+var helpTemplate = `{{if .Runnable}}usage: {{.UsageLine}}
 
 {{end}}{{.Long | trim}}
 `
 
 var documentationTemplate = `{{range .}}{{if .Short}}{{.Short | capitalize}}
 
-{{end}}{{if .Runnable}}Usage:
+{{end}}{{if .Commands}}` + usageTemplate + `{{else}}{{if .Runnable}}Usage:
 
-	go {{.UsageLine}}
+	{{.UsageLine}}
 
 {{end}}{{.Long | trim}}
 
 
-{{end}}`
+{{end}}{{end}}`
 
 // commentWriter writes a Go comment to the underlying io.Writer,
 // using line comment form (//).
@@ -179,8 +184,8 @@ func capitalize(s string) string {
 	return string(unicode.ToTitle(r)) + s[n:]
 }
 
-func PrintUsage(w io.Writer) {
+func PrintUsage(w io.Writer, cmd *base.Command) {
 	bw := bufio.NewWriter(w)
-	tmpl(bw, usageTemplate, base.Commands)
+	tmpl(bw, usageTemplate, cmd)
 	bw.Flush()
 }

src/cmd/go/internal/list/list.go

@@ -26,7 +26,7 @@ import (
 var CmdList = &base.Command{
 	// Note: -f -json -m are listed explicitly because they are the most common list flags.
 	// Do not send CLs removing them because they're covered by [list flags].
-	UsageLine: "list [-f format] [-json] [-m] [list flags] [build flags] [packages]",
+	UsageLine: "go list [-f format] [-json] [-m] [list flags] [build flags] [packages]",
 	Short:     "list packages or modules",
 	Long: `
 List lists the named packages, one per line.

src/cmd/go/internal/load/pkg.go

@@ -28,6 +28,9 @@ import (
 )
 
 var (
+	// module initialization hook; never nil, no-op if module use is disabled
+	ModInit func()
+
 	// module hooks; nil if module use is disabled
 	ModBinDir            func() string                                                   // return effective bin directory
 	ModLookup            func(parentPath, path string) (dir, realPath string, err error) // lookup effective meaning of import
@@ -1817,7 +1820,7 @@ func ImportPaths(args []string) []string {
 	if cmdlineMatchers == nil {
 		SetCmdlinePatterns(search.CleanImportPaths(args))
 	}
-	if cfg.ModulesEnabled {
+	if ModInit(); cfg.ModulesEnabled {
 		return ModImportPaths(args)
 	}
 	return search.ImportPaths(args)
@@ -1877,6 +1880,8 @@ func PackagesForBuild(args []string) []*Package {
 // (typically named on the command line). The target is named p.a for
 // package p or named after the first Go file for package main.
 func GoFilesPackage(gofiles []string) *Package {
+	ModInit()
+
 	// TODO: Remove this restriction.
 	for _, f := range gofiles {
 		if !strings.HasSuffix(f, ".go") {

src/cmd/go/internal/modcmd/edit.go

+// 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.
+
+// go mod edit
+
+package modcmd
+
+import (
+	"encoding/json"
+	"fmt"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/modfile"
+	"cmd/go/internal/modload"
+	"cmd/go/internal/module"
+)
+
+var cmdEdit = &base.Command{
+	UsageLine: "go mod edit [editing flags] [go.mod]",
+	Short:     "edit go.mod from tools or scripts",
+	Long: `
+Edit provides a command-line interface for editing go.mod,
+for use primarily by tools or scripts. It reads only go.mod;
+it does not look up information about the modules involved.
+By default, edit reads and writes the go.mod file of the main module,
+but a different target file can be specified after the editing flags.
+
+The editing flags specify a sequence of editing operations.
+
+The -fmt flag reformats the go.mod file without making other changes.
+This reformatting is also implied by any other modifications that use or
+rewrite the go.mod file. The only time this flag is needed is if no other
+flags are specified, as in 'go mod edit -fmt'.
+
+The -module flag changes the module's path (the go.mod file's module line).
+
+The -require=path@version and -droprequire=path flags
+add and drop a requirement on the given module path and version.
+Note that -require overrides any existing requirements on path.
+These flags are mainly for tools that understand the module graph.
+Users should prefer 'go get path@version' or 'go get path@none',
+which make other go.mod adjustments as needed to satisfy
+constraints imposed by other modules.
+
+The -exclude=path@version and -dropexclude=path@version flags
+add and drop an exclusion for the given module path and version.
+Note that -exclude=path@version is a no-op if that exclusion already exists.
+
+The -replace=old[@v]=new[@v] and -dropreplace=old[@v] flags
+add and drop a replacement of the given module path and version pair.
+If the @v in old@v is omitted, the replacement applies to all versions
+with the old module path. If the @v in new@v is omitted, the new path
+should be a local module root directory, not a module path.
+Note that -replace overrides any existing replacements for old[@v].
+
+The -require, -droprequire, -exclude, -dropexclude, -replace,
+and -dropreplace editing flags may be repeated, and the changes
+are applied in the order given.
+
+The -print flag prints the final go.mod in its text format instead of
+writing it back to go.mod.
+
+The -json flag prints the final go.mod file in JSON format instead of
+writing it back to go.mod. The JSON output corresponds to these Go types:
+
+	type Module struct {
+		Path string
+		Version string
+	}
+
+	type GoMod struct {
+		Module Module
+		Require []Require
+		Exclude []Module
+		Replace []Replace
+	}
+
+	type Require struct {
+		Path string
+		Version string
+		Indirect bool
+	}
+
+	type Replace struct {
+		Old Module
+		New Module
+	}
+
+Note that this only describes the go.mod file itself, not other modules
+referred to indirectly. For the full set of modules available to a build,
+use 'go list -m -json all'.
+
+For example, a tool can obtain the go.mod as a data structure by
+parsing the output of 'go mod edit -json' and can then make changes
+by invoking 'go mod edit' with -require, -exclude, and so on.
+	`,
+}
+
+var (
+	editFmt = cmdEdit.Flag.Bool("fmt", false, "")
+	// editGo     = cmdEdit.Flag.String("go", "", "")
+	editJSON   = cmdEdit.Flag.Bool("json", false, "")
+	editPrint  = cmdEdit.Flag.Bool("print", false, "")
+	editModule = cmdEdit.Flag.String("module", "", "")
+	edits      []func(*modfile.File) // edits specified in flags
+)
+
+type flagFunc func(string)
+
+func (f flagFunc) String() string     { return "" }
+func (f flagFunc) Set(s string) error { f(s); return nil }
+
+func init() {
+	cmdEdit.Run = runEdit // break init cycle
+
+	cmdEdit.Flag.Var(flagFunc(flagRequire), "require", "")
+	cmdEdit.Flag.Var(flagFunc(flagDropRequire), "droprequire", "")
+	cmdEdit.Flag.Var(flagFunc(flagExclude), "exclude", "")
+	cmdEdit.Flag.Var(flagFunc(flagDropReplace), "dropreplace", "")
+	cmdEdit.Flag.Var(flagFunc(flagReplace), "replace", "")
+	cmdEdit.Flag.Var(flagFunc(flagDropExclude), "dropexclude", "")
+
+	base.AddBuildFlagsNX(&cmdEdit.Flag)
+}
+
+func runEdit(cmd *base.Command, args []string) {
+	anyFlags :=
+		*editModule != "" ||
+			*editJSON ||
+			*editPrint ||
+			*editFmt ||
+			len(edits) > 0
+
+	if !anyFlags {
+		base.Fatalf("go mod edit: no flags specified (see 'go help mod edit').")
+	}
+
+	if *editJSON && *editPrint {
+		base.Fatalf("go mod edit: cannot use both -json and -print")
+	}
+
+	if len(args) > 1 {
+		base.Fatalf("go mod edit: too many arguments")
+	}
+	var gomod string
+	if len(args) == 1 {
+		gomod = args[0]
+	} else {
+		modload.MustInit()
+		gomod = filepath.Join(modload.ModRoot, "go.mod")
+	}
+
+	if *editModule != "" {
+		if err := module.CheckPath(*editModule); err != nil {
+			base.Fatalf("go mod: invalid -module: %v", err)
+		}
+	}
+
+	// TODO(rsc): Implement -go= once we start advertising it.
+
+	data, err := ioutil.ReadFile(gomod)
+	if err != nil {
+		base.Fatalf("go: %v", err)
+	}
+
+	modFile, err := modfile.Parse(gomod, data, nil)
+	if err != nil {
+		base.Fatalf("go: errors parsing %s:\n%s", base.ShortPath(gomod), err)
+	}
+
+	if *editModule != "" {
+		modFile.AddModuleStmt(modload.CmdModModule)
+	}
+
+	if len(edits) > 0 {
+		for _, edit := range edits {
+			edit(modFile)
+		}
+	}
+	modFile.SortBlocks()
+	modFile.Cleanup() // clean file after edits
+
+	if *editJSON {
+		editPrintJSON(modFile)
+		return
+	}
+
+	data, err = modFile.Format()
+	if err != nil {
+		base.Fatalf("go: %v", err)
+	}
+
+	if *editPrint {
+		os.Stdout.Write(data)
+		return
+	}
+
+	if err := ioutil.WriteFile(gomod, data, 0666); err != nil {
+		base.Fatalf("go: %v", err)
+	}
+}
+
+// parsePathVersion parses -flag=arg expecting arg to be path@version.
+func parsePathVersion(flag, arg string) (path, version string) {
+	i := strings.Index(arg, "@")
+	if i < 0 {
+		base.Fatalf("go mod: -%s=%s: need path@version", flag, arg)
+	}
+	path, version = strings.TrimSpace(arg[:i]), strings.TrimSpace(arg[i+1:])
+	if err := module.CheckPath(path); err != nil {
+		base.Fatalf("go mod: -%s=%s: invalid path: %v", flag, arg, err)
+	}
+
+	// We don't call modfile.CheckPathVersion, because that insists
+	// on versions being in semver form, but here we want to allow
+	// versions like "master" or "1234abcdef", which the go command will resolve
+	// the next time it runs (or during -fix).
+	// Even so, we need to make sure the version is a valid token.
+	if modfile.MustQuote(version) {
+		base.Fatalf("go mod: -%s=%s: invalid version %q", flag, arg, version)
+	}
+
+	return path, version
+}
+
+// parsePath parses -flag=arg expecting arg to be path (not path@version).
+func parsePath(flag, arg string) (path string) {
+	if strings.Contains(arg, "@") {
+		base.Fatalf("go mod: -%s=%s: need just path, not path@version", flag, arg)
+	}
+	path = arg
+	if err := module.CheckPath(path); err != nil {
+		base.Fatalf("go mod: -%s=%s: invalid path: %v", flag, arg, err)
+	}
+	return path
+}
+
+// parsePathVersionOptional parses path[@version], using adj to
+// describe any errors.
+func parsePathVersionOptional(adj, arg string, allowDirPath bool) (path, version string, err error) {
+	if i := strings.Index(arg, "@"); i < 0 {
+		path = arg
+	} else {
+		path, version = strings.TrimSpace(arg[:i]), strings.TrimSpace(arg[i+1:])
+	}
+	if err := module.CheckPath(path); err != nil {
+		if !allowDirPath || !modfile.IsDirectoryPath(path) {
+			return path, version, fmt.Errorf("invalid %s path: %v", adj, err)
+		}
+	}
+	if path != arg && modfile.MustQuote(version) {
+		return path, version, fmt.Errorf("invalid %s version: %q", adj, version)
+	}
+	return path, version, nil
+}
+
+// flagRequire implements the -require flag.
+func flagRequire(arg string) {
+	path, version := parsePathVersion("require", arg)
+	edits = append(edits, func(f *modfile.File) {
+		if err := f.AddRequire(path, version); err != nil {
+			base.Fatalf("go mod: -require=%s: %v", arg, err)
+		}
+	})
+}
+
+// flagDropRequire implements the -droprequire flag.
+func flagDropRequire(arg string) {
+	path := parsePath("droprequire", arg)
+	edits = append(edits, func(f *modfile.File) {
+		if err := f.DropRequire(path); err != nil {
+			base.Fatalf("go mod: -droprequire=%s: %v", arg, err)
+		}
+	})
+}
+
+// flagExclude implements the -exclude flag.
+func flagExclude(arg string) {
+	path, version := parsePathVersion("exclude", arg)
+	edits = append(edits, func(f *modfile.File) {
+		if err := f.AddExclude(path, version); err != nil {
+			base.Fatalf("go mod: -exclude=%s: %v", arg, err)
+		}
+	})
+}
+
+// flagDropExclude implements the -dropexclude flag.
+func flagDropExclude(arg string) {
+	path, version := parsePathVersion("dropexclude", arg)
+	edits = append(edits, func(f *modfile.File) {
+		if err := f.DropExclude(path, version); err != nil {
+			base.Fatalf("go mod: -dropexclude=%s: %v", arg, err)
+		}
+	})
+}
+
+// flagReplace implements the -replace flag.
+func flagReplace(arg string) {
+	var i int
+	if i = strings.Index(arg, "="); i < 0 {
+		base.Fatalf("go mod: -replace=%s: need old[@v]=new[@w] (missing =)", arg)
+	}
+	old, new := strings.TrimSpace(arg[:i]), strings.TrimSpace(arg[i+1:])
+	if strings.HasPrefix(new, ">") {
+		base.Fatalf("go mod: -replace=%s: separator between old and new is =, not =>", arg)
+	}
+	oldPath, oldVersion, err := parsePathVersionOptional("old", old, false)
+	if err != nil {
+		base.Fatalf("go mod: -replace=%s: %v", arg, err)
+	}
+	newPath, newVersion, err := parsePathVersionOptional("new", new, true)
+	if err != nil {
+		base.Fatalf("go mod: -replace=%s: %v", arg, err)
+	}
+	if newPath == new && !modfile.IsDirectoryPath(new) {
+		base.Fatalf("go mod: -replace=%s: unversioned new path must be local directory", arg)
+	}
+
+	edits = append(edits, func(f *modfile.File) {
+		if err := f.AddReplace(oldPath, oldVersion, newPath, newVersion); err != nil {
+			base.Fatalf("go mod: -replace=%s: %v", arg, err)
+		}
+	})
+}
+
+// flagDropReplace implements the -dropreplace flag.
+func flagDropReplace(arg string) {
+	path, version, err := parsePathVersionOptional("old", arg, true)
+	if err != nil {
+		base.Fatalf("go mod: -dropreplace=%s: %v", arg, err)
+	}
+	edits = append(edits, func(f *modfile.File) {
+		if err := f.DropReplace(path, version); err != nil {
+			base.Fatalf("go mod: -dropreplace=%s: %v", arg, err)
+		}
+	})
+}
+
+// fileJSON is the -json output data structure.
+type fileJSON struct {
+	Module  module.Version
+	Require []requireJSON
+	Exclude []module.Version
+	Replace []replaceJSON
+}
+
+type requireJSON struct {
+	Path     string
+	Version  string `json:",omitempty"`
+	Indirect bool   `json:",omitempty"`
+}
+
+type replaceJSON struct {
+	Old module.Version
+	New module.Version
+}
+
+// editPrintJSON prints the -json output.
+func editPrintJSON(modFile *modfile.File) {
+	var f fileJSON
+	f.Module = modFile.Module.Mod
+	for _, r := range modFile.Require {
+		f.Require = append(f.Require, requireJSON{Path: r.Mod.Path, Version: r.Mod.Version, Indirect: r.Indirect})
+	}
+	for _, x := range modFile.Exclude {
+		f.Exclude = append(f.Exclude, x.Mod)
+	}
+	for _, r := range modFile.Replace {
+		f.Replace = append(f.Replace, replaceJSON{r.Old, r.New})
+	}
+	data, err := json.MarshalIndent(&f, "", "\t")
+	if err != nil {
+		base.Fatalf("go: internal error: %v", err)
+	}
+	data = append(data, '\n')
+	os.Stdout.Write(data)
+}

src/cmd/go/internal/modcmd/fix.go

+// 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.
+
+// go mod fix
+
+package modcmd
+
+import (
+	"cmd/go/internal/base"
+	"cmd/go/internal/modload"
+)
+
+var cmdFix = &base.Command{
+	UsageLine: "go mod fix",
+	Short:     "make go.mod semantically consistent",
+	Long: `
+Fix updates go.mod to use canonical version identifiers and
+to be semantically consistent. For example, consider this go.mod file:
+
+	module M
+
+	require (
+		A v1
+		B v1.0.0
+		C v1.0.0
+		D v1.2.3
+		E dev
+	)
+
+	exclude D v1.2.3
+
+First, fix rewrites non-canonical version identifiers to semver form, so
+A's v1 becomes v1.0.0 and E's dev becomes the pseudo-version for the latest
+commit on the dev branch, perhaps v0.0.0-20180523231146-b3f5c0f6e5f1.
+
+Next, fix updates requirements to respect exclusions, so the requirement
+on the excluded D v1.2.3 is updated to use the next available version of D,
+perhaps D v1.2.4 or D v1.3.0.
+
+Finally, fix removes redundant or misleading requirements.
+For example, if A v1.0.0 itself requires B v1.2.0 and C v1.0.0, then go.mod's
+requirement of B v1.0.0 is misleading (superseded by A's need for v1.2.0),
+and its requirement of C v1.0.0 is redundant (implied by A's need for the
+same version), so both will be removed. If module M contains packages
+that directly import packages from B or C, then the requirements will be
+kept but updated to the actual versions being used.
+
+Although fix runs the fix-up operation in isolation, the fix-up also
+runs automatically any time a go command uses the module graph,
+to update go.mod to reflect reality. Because the module graph defines
+the meaning of import statements, any commands that load packages
+also use and therefore fix the module graph. For example,
+go build, go get, go install, go list, go test, go mod graph, go mod tidy,
+and other commands all effectively imply go mod fix.
+	`,
+	Run: runFix,
+}
+
+func runFix(cmd *base.Command, args []string) {
+	if len(args) != 0 {
+		base.Fatalf("go mod fix: fix takes no arguments")
+	}
+	modload.LoadBuildList() // writes go.mod
+}

src/cmd/go/internal/modcmd/graph.go

+// 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.
+
+// go mod graph
+
+package modcmd
+
+import (
+	"bufio"
+	"os"
+	"sort"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/modload"
+	"cmd/go/internal/module"
+	"cmd/go/internal/par"
+)
+
+var cmdGraph = &base.Command{
+	UsageLine: "go mod graph",
+	Short:     "print module requirement graph",
+	Long: `
+Graph prints the module requirement graph (with replacements applied)
+in text form. Each line in the output has two space-separated fields: a module
+and one of its requirements. Each module is identified as a string of the form
+path@version, except for the main module, which has no @version suffix.
+	`,
+	Run: runGraph,
+}
+
+func runGraph(cmd *base.Command, args []string) {
+	if len(args) > 0 {
+		base.Fatalf("go mod graph: graph takes no arguments")
+	}
+	modload.LoadBuildList()
+
+	reqs := modload.MinReqs()
+	format := func(m module.Version) string {
+		if m.Version == "" {
+			return m.Path
+		}
+		return m.Path + "@" + m.Version
+	}
+
+	// Note: using par.Work only to manage work queue.
+	// No parallelism here, so no locking.
+	var out []string
+	var deps int // index in out where deps start
+	var work par.Work
+	work.Add(modload.Target)
+	work.Do(1, func(item interface{}) {
+		m := item.(module.Version)
+		list, _ := reqs.Required(m)
+		for _, r := range list {
+			work.Add(r)
+			out = append(out, format(m)+" "+format(r)+"\n")
+		}
+		if m == modload.Target {
+			deps = len(out)
+		}
+	})
+
+	sort.Slice(out[deps:], func(i, j int) bool {
+		return out[deps+i][0] < out[deps+j][0]
+	})
+
+	w := bufio.NewWriter(os.Stdout)
+	for _, line := range out {
+		w.WriteString(line)
+	}
+	w.Flush()
+}

src/cmd/go/internal/modcmd/init.go

+// 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.
+
+// go mod init
+
+package modcmd
+
+import (
+	"cmd/go/internal/base"
+	"cmd/go/internal/modload"
+	"os"
+)
+
+var cmdInit = &base.Command{
+	UsageLine: "go mod init [module]",
+	Short:     "initialize new module in current directory",
+	Long: `
+Init initializes and writes a new go.mod to the current directory,
+in effect creating a new module rooted at the current directory.
+The file go.mod must not already exist.
+If possible, init will guess the module path from import comments
+(see 'go help importpath') or from version control configuration.
+To override this guess, supply the module path as an argument.
+	`,
+	Run: runInit,
+}
+
+func runInit(cmd *base.Command, args []string) {
+	modload.CmdModInit = true
+	if len(args) > 1 {
+		base.Fatalf("go mod init: too many arguments")
+	}
+	if len(args) == 1 {
+		modload.CmdModModule = args[0]
+	}
+	if _, err := os.Stat("go.mod"); err == nil {
+		base.Fatalf("go mod init: go.mod already exists")
+	}
+	modload.InitMod() // does all the hard work
+}

src/cmd/go/internal/modcmd/mod.go

@@ -5,566 +5,26 @@
 // Package modcmd implements the ``go mod'' command.
 package modcmd
 
-import (
-	"bufio"
-	"encoding/json"
-	"fmt"
-	"os"
-	"sort"
-	"strings"
-
-	"cmd/go/internal/base"
-	"cmd/go/internal/modfetch"
-	"cmd/go/internal/modfile"
-	"cmd/go/internal/modload"
-	"cmd/go/internal/module"
-	"cmd/go/internal/par"
-)
+import "cmd/go/internal/base"
 
 var CmdMod = &base.Command{
-	UsageLine: "mod [-v] [maintenance flags]",
+	UsageLine: "go mod",
 	Short:     "module maintenance",
-	Long: `
-Mod performs module maintenance operations as specified by the
-following flags, which may be combined.
+	Long: `Go mod provides access to operations on modules.
 
 Note that support for modules is built into all the go commands,
 not just 'go mod'. For example, day-to-day adding, removing, upgrading,
 and downgrading of dependencies should be done using 'go get'.
 See 'go help modules' for an overview of module functionality.
-
-The -v flag enables additional output about operations performed.
-
-The first group of operations provide low-level editing operations
-for manipulating go.mod from the command line or in scripts or
-other tools. They read only go.mod itself; they do not look up any
-information about the modules involved.
-
-The -init flag initializes and writes a new go.mod to the current directory,
-in effect creating a new module rooted at the current directory.
-The file go.mod must not already exist.
-If possible, mod will guess the module path from import comments
-(see 'go help importpath') or from version control configuration.
-To override this guess, use the -module flag.
-(Without -init, mod applies to the current module.)
-
-The -module flag changes (or, with -init, sets) the module's path
-(the go.mod file's module line).
-
-The -go flag changes the minimum required version of Go listed in go.mod.
-
-The -require=path@version and -droprequire=path flags
-add and drop a requirement on the given module path and version.
-Note that -require overrides any existing requirements on path.
-These flags are mainly for tools that understand the module graph.
-Users should prefer 'go get path@version' or 'go get path@none',
-which make other go.mod adjustments as needed to satisfy
-constraints imposed by other modules.
-
-The -exclude=path@version and -dropexclude=path@version flags
-add and drop an exclusion for the given module path and version.
-Note that -exclude=path@version is a no-op if that exclusion already exists.
-
-The -replace=old[@v]=new[@w] and -dropreplace=old[@v] flags
-add and drop a replacement of the given module path and version pair.
-If the @v in old@v is omitted, the replacement applies to all versions
-with the old module path. If the @w in new@w is omitted, the
-new path should be a directory on the local system containing
-source for a module, not a module path.
-Note that -replace overrides any existing replacements for old[@v].
-
-These editing flags (-require, -droprequire, -exclude, -dropexclude,
--replace, and -dropreplace) may be repeated.
-
-The -fmt flag reformats the go.mod file without making other changes.
-This reformatting is also implied by any other modifications that use or
-rewrite the go.mod file. The only time this flag is needed is if no other
-flags are specified, as in 'go mod -fmt'.
-
-The -graph flag prints the module requirement graph (with replacements applied)
-in text form. Each line in the output has two space-separated fields: a module
-and one of its requirements. Each module is identified as a string of the form
-path@version, except for the main module, which has no @version suffix.
-
-The -json flag prints the go.mod file in JSON format corresponding to these
-Go types:
-
-	type Module struct {
-		Path string
-		Version string
-	}
-
-	type GoMod struct {
-		Module Module
-		Require []Require
-		Exclude []Module
-		Replace []Replace
-	}
-	
-	type Require struct {
-		Path string
-		Version string
-		Indirect bool
-	}
-	
-	type Replace struct {
-		Old Module
-		New Module
-	}
-
-Note that this only describes the go.mod file itself, not other modules
-referred to indirectly. For the full set of modules available to a build,
-use 'go list -m -json all'.
-
-The next group of operations provide higher-level editing and maintenance
-of a module, beyond the go.mod file.
-
-The -packages flag prints a list of packages in the module.
-It only identifies directories containing Go source code;
-it does not check that those directories contain code that builds.
-
-The -fix flag updates go.mod to use canonical version identifiers and
-to be semantically consistent. For example, consider this go.mod file:
-
-	module M
-
-	require (
-		A v1
-		B v1.0.0
-		C v1.0.0
-		D v1.2.3
-		E dev
-	)
-
-	exclude D v1.2.3
-
-First, -fix rewrites non-canonical version identifiers to semver form, so
-A's v1 becomes v1.0.0 and E's dev becomes the pseudo-version for the latest
-commit on the dev branch, perhaps v0.0.0-20180523231146-b3f5c0f6e5f1.
-
-Next, -fix updates requirements to respect exclusions, so the requirement
-on the excluded D v1.2.3 is updated to use the next available version of D,
-perhaps D v1.2.4 or D v1.3.0.
-
-Finally, -fix removes redundant or misleading requirements.
-For example, if A v1.0.0 itself requires B v1.2.0 and C v1.0.0,
-then go.mod's requirement of B v1.0.0 is misleading (superseded
-by B's need for v1.2.0), and its requirement of C v1.0.0 is redundant
-(implied by B's need for the same version), so both will be removed.
-
-Although -fix runs the fix-up operation in isolation, the fix-up also
-runs automatically any time a go command uses the module graph,
-to update go.mod to reflect reality. For example, the -sync, -vendor,
-and -verify flags all effectively imply -fix. And because the module
-graph defines the meaning of import statements, any commands
-that load packages—'go build', 'go test', 'go list', and so on—also
-effectively imply 'go mod -fix'.
-
-The -sync flag synchronizes go.mod with the source code in the module.
-It adds any missing modules necessary to build the current module's
-packages and dependencies, and it removes unused modules that
-don't provide any relevant packages. It also adds any missing entries
-to go.sum and removes any unnecessary ones.
-
-The -vendor flag resets the module's vendor directory to include all
-packages needed to build and test all the module's packages.
-It does not include any test code for the vendored packages.
-
-The -verify flag checks that the dependencies of the current module,
-which are stored in a local downloaded source cache, have not been
-modified since being downloaded. If all the modules are unmodified,
--verify prints "all modules verified." Otherwise it reports which
-modules have been changed and causes 'go mod' to exit with a
-non-zero status.
 	`,
-}
-
-var (
-	modV = CmdMod.Flag.Bool("v", false, "")
-
-	modFmt      = CmdMod.Flag.Bool("fmt", false, "")
-	modFix      = CmdMod.Flag.Bool("fix", false, "")
-	modGraph    = CmdMod.Flag.Bool("graph", false, "")
-	modJSON     = CmdMod.Flag.Bool("json", false, "")
-	modPackages = CmdMod.Flag.Bool("packages", false, "")
-	modSync     = CmdMod.Flag.Bool("sync", false, "")
-	modVendor   = CmdMod.Flag.Bool("vendor", false, "")
-	modVerify   = CmdMod.Flag.Bool("verify", false, "")
-
-	modEdits []func(*modfile.File) // edits specified in flags
-)
-
-type flagFunc func(string)
-
-func (f flagFunc) String() string     { return "" }
-func (f flagFunc) Set(s string) error { f(s); return nil }
-
-func init() {
-	CmdMod.Run = runMod // break init cycle
-
-	CmdMod.Flag.BoolVar(&modload.CmdModInit, "init", modload.CmdModInit, "")
-	CmdMod.Flag.StringVar(&modload.CmdModModule, "module", modload.CmdModModule, "")
-
-	CmdMod.Flag.Var(flagFunc(flagRequire), "require", "")
-	CmdMod.Flag.Var(flagFunc(flagDropRequire), "droprequire", "")
-	CmdMod.Flag.Var(flagFunc(flagExclude), "exclude", "")
-	CmdMod.Flag.Var(flagFunc(flagDropReplace), "dropreplace", "")
-	CmdMod.Flag.Var(flagFunc(flagReplace), "replace", "")
-	CmdMod.Flag.Var(flagFunc(flagDropExclude), "dropexclude", "")
-
-	base.AddBuildFlagsNX(&CmdMod.Flag)
-}
-
-func runMod(cmd *base.Command, args []string) {
-	if modload.Init(); !modload.Enabled() {
-		base.Fatalf("go mod: cannot use outside module; see 'go help modules'")
-	}
-	if len(args) != 0 {
-		base.Fatalf("go mod: mod takes no arguments")
-	}
-
-	anyFlags :=
-		modload.CmdModInit ||
-			modload.CmdModModule != "" ||
-			*modVendor ||
-			*modVerify ||
-			*modJSON ||
-			*modFmt ||
-			*modFix ||
-			*modGraph ||
-			*modPackages ||
-			*modSync ||
-			len(modEdits) > 0
-
-	if !anyFlags {
-		base.Fatalf("go mod: no flags specified (see 'go help mod').")
-	}
-
-	if modload.CmdModModule != "" {
-		if err := module.CheckPath(modload.CmdModModule); err != nil {
-			base.Fatalf("go mod: invalid -module: %v", err)
-		}
-	}
-
-	if modload.CmdModInit {
-		if _, err := os.Stat("go.mod"); err == nil {
-			base.Fatalf("go mod -init: go.mod already exists")
-		}
-	}
-	modload.InitMod()
-
-	// Syntactic edits.
-
-	modFile := modload.ModFile()
-	if modload.CmdModModule != "" {
-		modFile.AddModuleStmt(modload.CmdModModule)
-	}
-
-	if len(modEdits) > 0 {
-		for _, edit := range modEdits {
-			edit(modFile)
-		}
-	}
-	modFile.SortBlocks()
-	modload.WriteGoMod() // write back syntactic changes
-
-	// Semantic edits.
-
-	needBuildList := *modFix || *modGraph
-
-	if *modSync || *modVendor || needBuildList {
-		var pkgs []string
-		if *modSync || *modVendor {
-			pkgs = modload.LoadALL()
-		} else {
-			modload.LoadBuildList()
-		}
-		if *modSync {
-			// LoadALL already added missing modules.
-			// Remove unused modules.
-			used := map[module.Version]bool{modload.Target: true}
-			for _, pkg := range pkgs {
-				used[modload.PackageModule(pkg)] = true
-			}
-
-			inGoMod := make(map[string]bool)
-			for _, r := range modload.ModFile().Require {
-				inGoMod[r.Mod.Path] = true
-			}
-
-			var keep []module.Version
-			for _, m := range modload.BuildList() {
-				if used[m] {
-					keep = append(keep, m)
-				} else if *modV && inGoMod[m.Path] {
-					fmt.Fprintf(os.Stderr, "unused %s\n", m.Path)
-				}
-			}
-			modload.SetBuildList(keep)
-			modSyncGoSum() // updates memory copy; WriteGoMod on next line flushes it out
-		}
-		modload.WriteGoMod()
-		if *modVendor {
-			runVendor()
-		}
-	}
-
-	// Read-only queries, processed only after updating go.mod.
-
-	if *modJSON {
-		modPrintJSON()
-	}
-
-	if *modGraph {
-		modPrintGraph()
-	}
-
-	if *modPackages {
-		for _, pkg := range modload.TargetPackages() {
-			fmt.Printf("%s\n", pkg)
-		}
-	}
-
-	if *modVerify {
-		runVerify()
-	}
-}
-
-// parsePathVersion parses -flag=arg expecting arg to be path@version.
-func parsePathVersion(flag, arg string) (path, version string) {
-	i := strings.Index(arg, "@")
-	if i < 0 {
-		base.Fatalf("go mod: -%s=%s: need path@version", flag, arg)
-	}
-	path, version = strings.TrimSpace(arg[:i]), strings.TrimSpace(arg[i+1:])
-	if err := module.CheckPath(path); err != nil {
-		base.Fatalf("go mod: -%s=%s: invalid path: %v", flag, arg, err)
-	}
-
-	// We don't call modfile.CheckPathVersion, because that insists
-	// on versions being in semver form, but here we want to allow
-	// versions like "master" or "1234abcdef", which the go command will resolve
-	// the next time it runs (or during -fix).
-	// Even so, we need to make sure the version is a valid token.
-	if modfile.MustQuote(version) {
-		base.Fatalf("go mod: -%s=%s: invalid version %q", flag, arg, version)
-	}
-
-	return path, version
-}
-
-// parsePath parses -flag=arg expecting arg to be path (not path@version).
-func parsePath(flag, arg string) (path string) {
-	if strings.Contains(arg, "@") {
-		base.Fatalf("go mod: -%s=%s: need just path, not path@version", flag, arg)
-	}
-	path = arg
-	if err := module.CheckPath(path); err != nil {
-		base.Fatalf("go mod: -%s=%s: invalid path: %v", flag, arg, err)
-	}
-	return path
-}
-
-// parsePathVersionOptional parses path[@version], using adj to
-// describe any errors.
-func parsePathVersionOptional(adj, arg string, allowDirPath bool) (path, version string, err error) {
-	if i := strings.Index(arg, "@"); i < 0 {
-		path = arg
-	} else {
-		path, version = strings.TrimSpace(arg[:i]), strings.TrimSpace(arg[i+1:])
-	}
-	if err := module.CheckPath(path); err != nil {
-		if !allowDirPath || !modfile.IsDirectoryPath(path) {
-			return path, version, fmt.Errorf("invalid %s path: %v", adj, err)
-		}
-	}
-	if path != arg && modfile.MustQuote(version) {
-		return path, version, fmt.Errorf("invalid %s version: %q", adj, version)
-	}
-	return path, version, nil
-}
-
-// flagRequire implements the -require flag.
-func flagRequire(arg string) {
-	path, version := parsePathVersion("require", arg)
-	modEdits = append(modEdits, func(f *modfile.File) {
-		if err := f.AddRequire(path, version); err != nil {
-			base.Fatalf("go mod: -require=%s: %v", arg, err)
-		}
-	})
-}
-
-// flagDropRequire implements the -droprequire flag.
-func flagDropRequire(arg string) {
-	path := parsePath("droprequire", arg)
-	modEdits = append(modEdits, func(f *modfile.File) {
-		if err := f.DropRequire(path); err != nil {
-			base.Fatalf("go mod: -droprequire=%s: %v", arg, err)
-		}
-	})
-}
-
-// flagExclude implements the -exclude flag.
-func flagExclude(arg string) {
-	path, version := parsePathVersion("exclude", arg)
-	modEdits = append(modEdits, func(f *modfile.File) {
-		if err := f.AddExclude(path, version); err != nil {
-			base.Fatalf("go mod: -exclude=%s: %v", arg, err)
-		}
-	})
-}
-
-// flagDropExclude implements the -dropexclude flag.
-func flagDropExclude(arg string) {
-	path, version := parsePathVersion("dropexclude", arg)
-	modEdits = append(modEdits, func(f *modfile.File) {
-		if err := f.DropExclude(path, version); err != nil {
-			base.Fatalf("go mod: -dropexclude=%s: %v", arg, err)
-		}
-	})
-}
-
-// flagReplace implements the -replace flag.
-func flagReplace(arg string) {
-	var i int
-	if i = strings.Index(arg, "="); i < 0 {
-		base.Fatalf("go mod: -replace=%s: need old[@v]=new[@w] (missing =)", arg)
-	}
-	old, new := strings.TrimSpace(arg[:i]), strings.TrimSpace(arg[i+1:])
-	if strings.HasPrefix(new, ">") {
-		base.Fatalf("go mod: -replace=%s: separator between old and new is =, not =>", arg)
-	}
-	oldPath, oldVersion, err := parsePathVersionOptional("old", old, false)
-	if err != nil {
-		base.Fatalf("go mod: -replace=%s: %v", arg, err)
-	}
-	newPath, newVersion, err := parsePathVersionOptional("new", new, true)
-	if err != nil {
-		base.Fatalf("go mod: -replace=%s: %v", arg, err)
-	}
-	if newPath == new && !modfile.IsDirectoryPath(new) {
-		base.Fatalf("go mod: -replace=%s: unversioned new path must be local directory", arg)
-	}
-
-	modEdits = append(modEdits, func(f *modfile.File) {
-		if err := f.AddReplace(oldPath, oldVersion, newPath, newVersion); err != nil {
-			base.Fatalf("go mod: -replace=%s: %v", arg, err)
-		}
-	})
-}
-
-// flagDropReplace implements the -dropreplace flag.
-func flagDropReplace(arg string) {
-	path, version, err := parsePathVersionOptional("old", arg, true)
-	if err != nil {
-		base.Fatalf("go mod: -dropreplace=%s: %v", arg, err)
-	}
-	modEdits = append(modEdits, func(f *modfile.File) {
-		if err := f.DropReplace(path, version); err != nil {
-			base.Fatalf("go mod: -dropreplace=%s: %v", arg, err)
-		}
-	})
-}
-
-// fileJSON is the -json output data structure.
-type fileJSON struct {
-	Module  module.Version
-	Require []requireJSON
-	Exclude []module.Version
-	Replace []replaceJSON
-}
-
-type requireJSON struct {
-	Path     string
-	Version  string `json:",omitempty"`
-	Indirect bool   `json:",omitempty"`
-}
-
-type replaceJSON struct {
-	Old module.Version
-	New module.Version
-}
-
-// modPrintJSON prints the -json output.
-func modPrintJSON() {
-	modFile := modload.ModFile()
-
-	var f fileJSON
-	f.Module = modFile.Module.Mod
-	for _, r := range modFile.Require {
-		f.Require = append(f.Require, requireJSON{Path: r.Mod.Path, Version: r.Mod.Version, Indirect: r.Indirect})
-	}
-	for _, x := range modFile.Exclude {
-		f.Exclude = append(f.Exclude, x.Mod)
-	}
-	for _, r := range modFile.Replace {
-		f.Replace = append(f.Replace, replaceJSON{r.Old, r.New})
-	}
-	data, err := json.MarshalIndent(&f, "", "\t")
-	if err != nil {
-		base.Fatalf("go mod -json: internal error: %v", err)
-	}
-	data = append(data, '\n')
-	os.Stdout.Write(data)
-}
-
-// modPrintGraph prints the -graph output.
-func modPrintGraph() {
-	reqs := modload.MinReqs()
-
-	format := func(m module.Version) string {
-		if m.Version == "" {
-			return m.Path
-		}
-		return m.Path + "@" + m.Version
-	}
-
-	// Note: using par.Work only to manage work queue.
-	// No parallelism here, so no locking.
-	var out []string
-	var deps int // index in out where deps start
-	var work par.Work
-	work.Add(modload.Target)
-	work.Do(1, func(item interface{}) {
-		m := item.(module.Version)
-		list, _ := reqs.Required(m)
-		for _, r := range list {
-			work.Add(r)
-			out = append(out, format(m)+" "+format(r)+"\n")
-		}
-		if m == modload.Target {
-			deps = len(out)
-		}
-	})
-
-	sort.Slice(out[deps:], func(i, j int) bool {
-		return out[deps+i][0] < out[deps+j][0]
-	})
-
-	w := bufio.NewWriter(os.Stdout)
-	for _, line := range out {
-		w.WriteString(line)
-	}
-	w.Flush()
-}
 
-// modSyncGoSum resets the go.sum file content
-// to be exactly what's needed for the current go.mod.
-func modSyncGoSum() {
-	// Assuming go.sum already has at least enough from the successful load,
-	// we only have to tell modfetch what needs keeping.
-	reqs := modload.Reqs()
-	keep := make(map[module.Version]bool)
-	var walk func(module.Version)
-	walk = func(m module.Version) {
-		keep[m] = true
-		list, _ := reqs.Required(m)
-		for _, r := range list {
-			if !keep[r] {
-				walk(r)
-			}
-		}
-	}
-	walk(modload.Target)
-	modfetch.TrimGoSum(keep)
+	Commands: []*base.Command{
+		cmdEdit,
+		cmdFix,
+		cmdGraph,
+		cmdInit,
+		cmdTidy,
+		cmdVendor,
+		cmdVerify,
+	},
 }

src/cmd/go/internal/modcmd/tidy.go

+// 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.
+
+// go mod tidy
+
+package modcmd
+
+import (
+	"fmt"
+	"os"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/modfetch"
+	"cmd/go/internal/modload"
+	"cmd/go/internal/module"
+)
+
+var cmdTidy = &base.Command{
+	UsageLine: "go mod tidy [-v]",
+	Short:     "add missing and remove unused modules",
+	Long: `
+Tidy makes sure go.mod matches the source code in the module.
+It adds any missing modules necessary to build the current module's
+packages and dependencies, and it removes unused modules that
+don't provide any relevant packages. It also adds any missing entries
+to go.sum and removes any unnecessary ones.
+
+The -v flag causes tidy to print information about removed modules
+to standard error.
+	`,
+}
+
+func init() {
+	cmdTidy.Run = runTidy // break init cycle
+	cmdTidy.Flag.BoolVar(&cfg.BuildV, "v", false, "")
+}
+
+func runTidy(cmd *base.Command, args []string) {
+	if len(args) > 0 {
+		base.Fatalf("go mod tidy: no arguments allowed")
+	}
+
+	// LoadALL adds missing modules.
+	// Remove unused modules.
+	used := map[module.Version]bool{modload.Target: true}
+	for _, pkg := range modload.LoadALL() {
+		used[modload.PackageModule(pkg)] = true
+	}
+
+	inGoMod := make(map[string]bool)
+	for _, r := range modload.ModFile().Require {
+		inGoMod[r.Mod.Path] = true
+	}
+
+	var keep []module.Version
+	for _, m := range modload.BuildList() {
+		if used[m] {
+			keep = append(keep, m)
+		} else if cfg.BuildV && inGoMod[m.Path] {
+			fmt.Fprintf(os.Stderr, "unused %s\n", m.Path)
+		}
+	}
+	modload.SetBuildList(keep)
+	modTidyGoSum() // updates memory copy; WriteGoMod on next line flushes it out
+	modload.WriteGoMod()
+}
+
+// modTidyGoSum resets the go.sum file content
+// to be exactly what's needed for the current go.mod.
+func modTidyGoSum() {
+	// Assuming go.sum already has at least enough from the successful load,
+	// we only have to tell modfetch what needs keeping.
+	reqs := modload.Reqs()
+	keep := make(map[module.Version]bool)
+	var walk func(module.Version)
+	walk = func(m module.Version) {
+		keep[m] = true
+		list, _ := reqs.Required(m)
+		for _, r := range list {
+			if !keep[r] {
+				walk(r)
+			}
+		}
+	}
+	walk(modload.Target)
+	modfetch.TrimGoSum(keep)
+}

src/cmd/go/internal/modcmd/vendor.go

@@ -14,11 +14,33 @@ import (
 	"strings"
 
 	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
 	"cmd/go/internal/modload"
 	"cmd/go/internal/module"
 )
 
-func runVendor() {
+var cmdVendor = &base.Command{
+	UsageLine: "go mod vendor [-v]",
+	Short:     "make vendored copy of dependencies",
+	Long: `
+Vendor resets the main module's vendor directory to include all packages
+needed to build and test all the main module's packages.
+It does not include test code for vendored packages.
+
+The -v flag causes vendor to print the names of vendored
+modules and packages to standard error.
+	`,
+	Run: runVendor,
+}
+
+func init() {
+	cmdVendor.Flag.BoolVar(&cfg.BuildV, "v", false, "")
+}
+
+func runVendor(cmd *base.Command, args []string) {
+	if len(args) != 0 {
+		base.Fatalf("go mod vendor: vendor takes no arguments")
+	}
 	pkgs := modload.LoadVendor()
 
 	vdir := filepath.Join(modload.ModRoot, "vendor")
@@ -46,12 +68,12 @@ func runVendor() {
 				}
 			}
 			fmt.Fprintf(&buf, "# %s %s%s\n", m.Path, m.Version, repl)
-			if *modV {
+			if cfg.BuildV {
 				fmt.Fprintf(os.Stderr, "# %s %s%s\n", m.Path, m.Version, repl)
 			}
 			for _, pkg := range pkgs {
 				fmt.Fprintf(&buf, "%s\n", pkg)
-				if *modV {
+				if cfg.BuildV {
 					fmt.Fprintf(os.Stderr, "%s\n", pkg)
 				}
 				vendorPkg(vdir, pkg)

src/cmd/go/internal/modcmd/verify.go

@@ -17,7 +17,25 @@ import (
 	"cmd/go/internal/module"
 )
 
-func runVerify() {
+var cmdVerify = &base.Command{
+	UsageLine: "go mod verify",
+	Short:     "verify dependencies have expected content",
+	Long: `
+Verify checks that the dependencies of the current module,
+which are stored in a local downloaded source cache, have not been
+modified since being downloaded. If all the modules are unmodified,
+verify prints "all modules verified." Otherwise it reports which
+modules have been changed and causes 'go mod' to exit with a
+non-zero status.
+	`,
+	Run: runVerify,
+}
+
+func runVerify(cmd *base.Command, args []string) {
+	if len(args) != 0 {
+		// NOTE(rsc): Could take a module pattern.
+		base.Fatalf("go mod verify: verify takes no arguments")
+	}
 	ok := true
 	for _, mod := range modload.LoadBuildList()[1:] {
 		ok = verifyMod(mod) && ok

src/cmd/go/internal/modget/get.go

@@ -29,7 +29,7 @@ import (
 var CmdGet = &base.Command{
 	// Note: -d -m -u are listed explicitly because they are the most common get flags.
 	// Do not send CLs removing them because they're covered by [get flags].
-	UsageLine: "get [-d] [-m] [-u] [-v] [-insecure] [build flags] [packages]",
+	UsageLine: "go get [-d] [-m] [-u] [-v] [-insecure] [build flags] [packages]",
 	Short:     "add dependencies to current module and install them",
 	Long: `
 Get resolves and adds dependencies to the current development module

src/cmd/go/internal/modload/help.go

@@ -85,12 +85,12 @@ For more about the go.mod file, see https://research.swtch.com/vgo-module.
 
 To start a new module, simply create a go.mod file in the root of the
 module's directory tree, containing only a module statement.
-The 'go mod' command can be used to do this:
+The 'go mod init' command can be used to do this:
 
-	go mod -init -module example.com/m
+	go mod init example.com/m
 
 In a project already using an existing dependency management tool like
-godep, glide, or dep, 'go mod -init' will also add require statements
+godep, glide, or dep, 'go mod init' will also add require statements
 matching the existing configuration.
 
 Once the go.mod file exists, no additional steps are required:
@@ -147,7 +147,7 @@ package from the module. On the other hand, determining that a module requiremen
 is no longer necessary and can be deleted requires a full view of
 all packages in the module, across all possible build configurations
 (architectures, operating systems, build tags, and so on).
-The 'go mod -sync' command builds that view and then
+The 'go mod tidy' command builds that view and then
 adds any missing module requirements and removes unnecessary ones.
 
 As part of maintaining the require statements in go.mod, the go command
@@ -337,7 +337,7 @@ The go command maintains a cache of downloaded packages and computes
 and records the cryptographic checksum of each package at download time.
 In normal operation, the go command checks these pre-computed checksums
 against the main module's go.sum file, instead of recomputing them on
-each command invocation. The 'go mod -verify' command checks that
+each command invocation. The 'go mod verify' command checks that
 the cached copies of module downloads still match both their recorded
 checksums and the entries in go.sum.
 
@@ -356,7 +356,7 @@ By default, the go command satisfies dependencies by downloading modules
 from their sources and using those downloaded copies (after verification,
 as described in the previous section). To allow interoperation with older
 versions of Go, or to ensure that all files used for a build are stored
-together in a single file tree, 'go mod -vendor' creates a directory named
+together in a single file tree, 'go mod vendor' creates a directory named
 vendor in the root directory of the main module and stores there all the
 packages from dependency modules that are needed to support builds and
 tests of packages in the main module.

src/cmd/go/internal/modload/init.go

@@ -30,7 +30,6 @@ import (
 
 var (
 	cwd            string
-	enabled        = MustUseModules
 	MustUseModules = mustUseModules()
 	initialized    bool
 
@@ -41,8 +40,8 @@ var (
 
 	gopath string
 
-	CmdModInit   bool   // go mod -init flag
-	CmdModModule string // go mod -module flag
+	CmdModInit   bool   // running 'go mod init'
+	CmdModModule string // module argument for 'go mod init'
 )
 
 // ModFile returns the parsed go.mod file.
@@ -58,9 +57,7 @@ func ModFile() *modfile.File {
 }
 
 func BinDir() string {
-	if !Enabled() {
-		panic("modload.BinDir")
-	}
+	MustInit()
 	return filepath.Join(gopath, "bin")
 }
 
@@ -74,6 +71,8 @@ func mustUseModules() bool {
 	return strings.HasPrefix(name, "vgo")
 }
 
+var inGOPATH bool // running in GOPATH/src
+
 func Init() {
 	if initialized {
 		return
@@ -127,7 +126,7 @@ func Init() {
 		base.Fatalf("go: %v", err)
 	}
 
-	inGOPATH := false
+	inGOPATH = false
 	for _, gopath := range filepath.SplitList(cfg.BuildContext.GOPATH) {
 		if gopath == "" {
 			continue
@@ -137,14 +136,7 @@ func Init() {
 			break
 		}
 	}
-	if inGOPATH && !MustUseModules && cfg.CmdName == "mod" {
-		base.Fatalf("go: modules disabled inside GOPATH/src by GO111MODULE=auto; see 'go help modules'")
-	}
 
-	if CmdModInit {
-		// Running 'go mod -init': go.mod will be created in current directory.
-		ModRoot = cwd
-	} else {
 	if inGOPATH && !MustUseModules {
 		// No automatic enabling in GOPATH.
 		if root, _ := FindModuleRoot(cwd, "", false); root != "" {
@@ -152,26 +144,18 @@ func Init() {
 		}
 		return
 	}
-		root, _ := FindModuleRoot(cwd, "", MustUseModules)
-		if root == "" {
-			// If invoked as vgo, insist on a mod file.
-			if MustUseModules {
-				base.Fatalf("go: cannot find main module root; see 'go help modules'")
-			}
-			return
-		}
 
-		ModRoot = root
+	if CmdModInit {
+		// Running 'go mod init': go.mod will be created in current directory.
+		ModRoot = cwd
+	} else {
+		ModRoot, _ = FindModuleRoot(cwd, "", MustUseModules)
+		if ModRoot == "" && !MustUseModules {
+			return
 		}
-
-	if c := cache.Default(); c == nil {
-		// With modules, there are no install locations for packages
-		// other than the build cache.
-		base.Fatalf("go: cannot use modules with build cache disabled")
 	}
 
 	cfg.ModulesEnabled = true
-	enabled = true
 	load.ModBinDir = BinDir
 	load.ModLookup = Lookup
 	load.ModPackageModuleInfo = PackageModuleInfo
@@ -183,15 +167,60 @@ func Init() {
 	search.SetModRoot(ModRoot)
 }
 
+func init() {
+	load.ModInit = Init
+
+	// Set modfetch.SrcMod unconditionally, so that go clean -modcache can run even without modules enabled.
+	if list := filepath.SplitList(cfg.BuildContext.GOPATH); len(list) > 0 && list[0] != "" {
+		modfetch.SrcMod = filepath.Join(list[0], "src/mod")
+	}
+}
+
+// Enabled reports whether modules are (or must be) enabled.
+// If modules must be enabled but are not, Enabled returns true
+// and then the first use of module information will call die
+// (usually through InitMod and MustInit).
 func Enabled() bool {
 	if !initialized {
 		panic("go: Enabled called before Init")
 	}
-	return enabled
+	return ModRoot != "" || MustUseModules
+}
+
+// MustInit calls Init if needed and checks that
+// modules are enabled and the main module has been found.
+// If not, MustInit calls base.Fatalf with an appropriate message.
+func MustInit() {
+	if Init(); ModRoot == "" {
+		die()
+	}
+	if c := cache.Default(); c == nil {
+		// With modules, there are no install locations for packages
+		// other than the build cache.
+		base.Fatalf("go: cannot use modules with build cache disabled")
+	}
+}
+
+// Failed reports whether module loading failed.
+// If Failed returns true, then any use of module information will call die.
+func Failed() bool {
+	Init()
+	return cfg.ModulesEnabled && ModRoot == ""
+}
+
+func die() {
+	if os.Getenv("GO111MODULE") == "off" {
+		base.Fatalf("go: modules disabled by GO111MODULE=off; see 'go help modules'")
+	}
+	if inGOPATH && !MustUseModules {
+		base.Fatalf("go: modules disabled inside GOPATH/src by GO111MODULE=auto; see 'go help modules'")
+	}
+	base.Fatalf("go: cannot find main module; see 'go help modules'")
 }
 
 func InitMod() {
-	if Init(); !Enabled() || modFile != nil {
+	MustInit()
+	if modFile != nil {
 		return
 	}
 
@@ -217,10 +246,10 @@ func InitMod() {
 	codehost.WorkRoot = filepath.Join(srcMod, "cache/vcs")
 
 	if CmdModInit {
-		// Running go mod -init: do legacy module conversion
-		// (go.mod does not exist yet, and it's not our job to write it).
+		// Running go mod init: do legacy module conversion
 		legacyModInit()
 		modFileToBuildList()
+		WriteGoMod()
 		return
 	}
 
@@ -376,7 +405,7 @@ func FindModuleRoot(dir, limit string, legacyConfigOK bool) (root, file string)
 // Exported only for testing.
 func FindModulePath(dir string) (string, error) {
 	if CmdModModule != "" {
-		// Running go mod -init -module=x/y/z; return x/y/z.
+		// Running go mod init x/y/z; return x/y/z.
 		return CmdModModule, nil
 	}
 

src/cmd/go/internal/modload/load.go

@@ -50,9 +50,6 @@ var loaded *loader
 // ImportPaths returns the set of packages matching the args (patterns),
 // adding modules to the build list as needed to satisfy new imports.
 func ImportPaths(args []string) []string {
-	if Init(); !Enabled() {
-		return search.ImportPaths(args)
-	}
 	InitMod()
 
 	cleaned := search.CleanImportPaths(args)
@@ -172,9 +169,6 @@ func warnPattern(pattern string, list []string) []string {
 // ImportFromFiles adds modules to the build list as needed
 // to satisfy the imports in the named Go source files.
 func ImportFromFiles(gofiles []string) {
-	if Init(); !Enabled() {
-		return
-	}
 	InitMod()
 
 	imports, testImports, err := imports.ScanFiles(gofiles, imports.Tags())
@@ -198,9 +192,6 @@ func ImportFromFiles(gofiles []string) {
 // (typically in commands that care about the module but
 // no particular package).
 func LoadBuildList() []module.Version {
-	if Init(); !Enabled() {
-		base.Fatalf("go: LoadBuildList called but modules not enabled")
-	}
 	InitMod()
 	ReloadBuildList()
 	WriteGoMod()
@@ -232,9 +223,6 @@ func LoadVendor() []string {
 }
 
 func loadAll(testAll bool) []string {
-	if Init(); !Enabled() {
-		panic("go: misuse of LoadALL/LoadVendor")
-	}
 	InitMod()
 
 	loaded = newLoader()

src/cmd/go/internal/run/run.go

@@ -18,7 +18,7 @@ import (
 )
 
 var CmdRun = &base.Command{
-	UsageLine: "run [build flags] [-exec xprog] package [arguments...]",
+	UsageLine: "go run [build flags] [-exec xprog] package [arguments...]",
 	Short:     "compile and run Go program",
 	Long: `
 Run compiles and runs the named main Go package.

src/cmd/go/internal/test/test.go

@@ -37,7 +37,7 @@ func init() {
 	CmdTest.Run = runTest
 }
 
-const testUsage = "test [build/test flags] [packages] [build/test flags & test binary flags]"
+const testUsage = "go test [build/test flags] [packages] [build/test flags & test binary flags]"
 
 var CmdTest = &base.Command{
 	CustomFlags: true,
@@ -168,7 +168,7 @@ flags are also accessible by 'go test'.
 
 // Usage prints the usage message for 'go test -h' and exits.
 func Usage() {
-	os.Stderr.WriteString(testUsage + "\n\n" +
+	os.Stderr.WriteString("usage: " + testUsage + "\n\n" +
 		strings.TrimSpace(testFlag1) + "\n\n\t" +
 		strings.TrimSpace(testFlag2) + "\n")
 	os.Exit(2)

src/cmd/go/internal/tool/tool.go

@@ -18,7 +18,7 @@ import (
 
 var CmdTool = &base.Command{
 	Run:       runTool,
-	UsageLine: "tool [-n] command [args...]",
+	UsageLine: "go tool [-n] command [args...]",
 	Short:     "run specified go tool",
 	Long: `
 Tool runs the go tool command identified by the arguments.

src/cmd/go/internal/version/version.go

@@ -14,7 +14,7 @@ import (
 
 var CmdVersion = &base.Command{
 	Run:       runVersion,
-	UsageLine: "version",
+	UsageLine: "go version",
 	Short:     "print Go version",
 	Long:      `Version prints the Go version, as reported by runtime.Version.`,
 }

src/cmd/go/internal/vet/vet.go

@@ -15,7 +15,7 @@ import (
 var CmdVet = &base.Command{
 	Run:         runVet,
 	CustomFlags: true,
-	UsageLine:   "vet [-n] [-x] [build flags] [vet flags] [packages]",
+	UsageLine:   "go vet [-n] [-x] [build flags] [vet flags] [packages]",
 	Short:       "report likely mistakes in packages",
 	Long: `
 Vet runs the Go vet command on the packages named by the import paths.

src/cmd/go/internal/work/build.go

@@ -22,7 +22,7 @@ import (
 )
 
 var CmdBuild = &base.Command{
-	UsageLine: "build [-o output] [-i] [build flags] [packages]",
+	UsageLine: "go build [-o output] [-i] [build flags] [packages]",
 	Short:     "compile packages and dependencies",
 	Long: `
 Build compiles the packages named by the import paths,
@@ -341,7 +341,7 @@ func runBuild(cmd *base.Command, args []string) {
 }
 
 var CmdInstall = &base.Command{
-	UsageLine: "install [-i] [build flags] [packages]",
+	UsageLine: "go install [-i] [build flags] [packages]",
 	Short:     "compile and install packages and dependencies",
 	Long: `
 Install compiles and installs the packages named by the import paths.

src/cmd/go/internal/work/init.go

@@ -17,6 +17,7 @@ import (
 )
 
 func BuildInit() {
+	load.ModInit()
 	instrumentInit()
 	buildModeInit()
 

src/cmd/go/main.go

@@ -40,7 +40,7 @@ import (
 )
 
 func init() {
-	base.Commands = []*base.Command{
+	base.Go.Commands = []*base.Command{
 		bug.CmdBug,
 		work.CmdBuild,
 		clean.CmdClean,
@@ -129,26 +129,42 @@ func main() {
 		os.Exit(2)
 	}
 
+	// TODO(rsc): Remove all these helper prints in Go 1.12.
 	switch args[0] {
 	case "mod":
-		// Skip modload.Init (which may insist on go.mod existing)
-		// so that go mod -init has a chance to write the file.
-	case "version":
-		// Skip modload.Init so that users can report bugs against
-		// go mod -init.
+		if len(args) >= 2 {
+			flag := args[1]
+			if strings.HasPrefix(flag, "--") {
+				flag = flag[1:]
+			}
+			if i := strings.Index(flag, "="); i >= 0 {
+				flag = flag[:i]
+			}
+			switch flag {
+			case "-sync":
+				fmt.Fprintf(os.Stderr, "go: go mod -sync is now go mod tidy\n")
+				os.Exit(2)
+			case "-init", "-fix", "-graph", "-vendor", "-verify":
+				fmt.Fprintf(os.Stderr, "go: go mod %s is now go mod %s\n", flag, flag[1:])
+				os.Exit(2)
+			case "-fmt", "-json", "-module", "-require", "-droprequire", "-replace", "-dropreplace", "-exclude", "-dropexclude":
+				fmt.Fprintf(os.Stderr, "go: go mod %s is now go mod edit %s\n", flag, flag)
+				os.Exit(2)
+			}
+		}
 	case "vendor":
-		fmt.Fprintf(os.Stderr, "go vendor is now go mod -vendor\n")
+		fmt.Fprintf(os.Stderr, "go: vgo vendor is now go mod vendor\n")
 		os.Exit(2)
 	case "verify":
-		fmt.Fprintf(os.Stderr, "go verify is now go mod -verify\n")
+		fmt.Fprintf(os.Stderr, "go: vgo verify is now go mod verify\n")
 		os.Exit(2)
-	default:
-		// Run modload.Init so that each subcommand doesn't have to worry about it.
-		// Also install the module get command instead of the GOPATH go get command
-		// if modules are enabled.
-		modload.Init()
-		if modload.Enabled() {
-			// Might not have done this above, so do it now.
+	}
+
+	if args[0] == "get" {
+		// Replace get with module-aware get if appropriate.
+		// Note that if MustUseModules is true, this happened already above,
+		// but no harm in doing it again.
+		if modload.Init(); modload.Enabled() {
 			*get.CmdGet = *modget.CmdGet
 		}
 	}
@@ -166,8 +182,29 @@ func main() {
 		}
 	}
 
-	for _, cmd := range base.Commands {
-		if cmd.Name() == args[0] && cmd.Runnable() {
+BigCmdLoop:
+	for bigCmd := base.Go; ; {
+		for _, cmd := range bigCmd.Commands {
+			if cmd.Name() != args[0] {
+				continue
+			}
+			if len(cmd.Commands) > 0 {
+				bigCmd = cmd
+				args = args[1:]
+				if len(args) == 0 {
+					help.PrintUsage(os.Stderr, bigCmd)
+				}
+				if args[0] == "help" {
+					// Accept 'go mod help' and 'go mod help foo' for 'go help mod' and 'go help mod foo'.
+					help.Help(append(strings.Split(cfg.CmdName, " "), args[1:]...))
+					return
+				}
+				cfg.CmdName += " " + args[0]
+				continue BigCmdLoop
+			}
+			if !cmd.Runnable() {
+				continue
+			}
 			cmd.Flag.Usage = func() { cmd.Usage() }
 			if cmd.CustomFlags {
 				args = args[1:]
@@ -179,11 +216,14 @@ func main() {
 			base.Exit()
 			return
 		}
+		helpArg := ""
+		if i := strings.LastIndex(cfg.CmdName, " "); i >= 0 {
+			helpArg = " " + cfg.CmdName[:i]
 		}
-
-	fmt.Fprintf(os.Stderr, "go: unknown subcommand %q\nRun 'go help' for usage.\n", args[0])
+		fmt.Fprintf(os.Stderr, "go %s: unknown command\nRun 'go help%s' for usage.\n", cfg.CmdName, helpArg)
 		base.SetExitStatus(2)
 		base.Exit()
+	}
 }
 
 func init() {
@@ -195,6 +235,6 @@ func mainUsage() {
 	if len(os.Args) > 1 && os.Args[1] == "test" {
 		test.Usage()
 	}
-	help.PrintUsage(os.Stderr)
+	help.PrintUsage(os.Stderr, base.Go)
 	os.Exit(2)
 }

src/cmd/go/testdata/mod/mod_sync.txt → src/cmd/go/testdata/mod/mod_tidy.txt

 env GO111MODULE=on
 
-# sync removes unused y, but everything else is used
-go mod -sync -v
+# tidy removes unused y, but everything else is used
+go mod tidy -v
 stderr '^unused y.1'
 ! stderr '^unused [^y]'
 

src/cmd/go/testdata/script/help.txt

+# go help shows overview.
+go help
+stdout 'Go is a tool'
+stdout 'bug.*start a bug report'
+
+# go help bug shows usage for bug
+go help bug
+stdout 'usage: go bug'
+stdout 'bug report'
+
+# go bug help is an error (bug takes no arguments)
+! go bug help
+stderr 'bug takes no arguments'
+
+# go help mod shows mod subcommands
+go help mod
+stdout 'go mod <command>'
+stdout tidy
+
+# go help mod tidy explains tidy
+go help mod tidy
+stdout 'usage: go mod tidy'
+
+# go mod help tidy does too
+go mod help tidy
+stdout 'usage: go mod tidy'
+
+# go mod --help doesn't print help but at least suggests it.
+! go mod --help
+stderr 'Run ''go help mod'' for usage.'

src/cmd/go/testdata/script/mod_edit.txt

@@ -3,43 +3,43 @@ env GO111MODULE=on
 # Test that go mod edits and related mod flags work.
 # Also test that they can use a dummy name that isn't resolvable. golang.org/issue/24100
 
-# go mod -init
-! go mod -init
+# go mod init
+! go mod init
 stderr 'cannot determine module path'
 ! exists go.mod
 
-go mod -init -module x.x/y/z
+go mod init x.x/y/z
 stderr 'creating new go.mod: module x.x/y/z'
 cmp go.mod $WORK/go.mod.init
 
-! go mod -init
+! go mod init
 cmp go.mod $WORK/go.mod.init
 
 # go mod edits
-go mod -droprequire=x.1 -require=x.1@v1.0.0 -require=x.2@v1.1.0 -droprequire=x.2 -exclude='x.1 @ v1.2.0' -exclude=x.1@v1.2.1 -replace=x.1@v1.3.0=y.1@v1.4.0 -replace='x.1@v1.4.0 = ../z'
+go mod edit -droprequire=x.1 -require=x.1@v1.0.0 -require=x.2@v1.1.0 -droprequire=x.2 -exclude='x.1 @ v1.2.0' -exclude=x.1@v1.2.1 -replace=x.1@v1.3.0=y.1@v1.4.0 -replace='x.1@v1.4.0 = ../z'
 cmp go.mod $WORK/go.mod.edit1
-go mod -droprequire=x.1 -dropexclude=x.1@v1.2.1 -dropreplace=x.1@v1.3.0 -require=x.3@v1.99.0
+go mod edit -droprequire=x.1 -dropexclude=x.1@v1.2.1 -dropreplace=x.1@v1.3.0 -require=x.3@v1.99.0
 cmp go.mod $WORK/go.mod.edit2
 
-# go mod -json
-go mod -json
+# go mod edit -json
+go mod edit -json
 cmp stdout $WORK/go.mod.json
 
-# go mod -replace
-go mod -replace=x.1@v1.3.0=y.1/v2@v2.3.5 -replace=x.1@v1.4.0=y.1/v2@v2.3.5
+# go mod edit -replace
+go mod edit -replace=x.1@v1.3.0=y.1/v2@v2.3.5 -replace=x.1@v1.4.0=y.1/v2@v2.3.5
 cmp go.mod $WORK/go.mod.edit3
-go mod -replace=x.1=y.1/v2@v2.3.6
+go mod edit -replace=x.1=y.1/v2@v2.3.6
 cmp go.mod $WORK/go.mod.edit4
-go mod -dropreplace=x.1
+go mod edit -dropreplace=x.1
 cmp go.mod $WORK/go.mod.edit5
 
-# go mod -packages
-go mod -packages
-cmp stdout $WORK/go.mod.packages
-
-# go mod -fmt
+# go mod edit -fmt
 cp $WORK/go.mod.badfmt go.mod
-go mod -fmt
+go mod edit -fmt -print # -print should avoid writing file
+cmp stdout $WORK/go.mod.edit4
+cmp go.mod $WORK/go.mod.badfmt
+go mod edit -fmt # without -print, should write file (and nothing to stdout)
+! stdout .
 cmp go.mod $WORK/go.mod.edit4
 
 -- x.go --
@@ -126,9 +126,6 @@ module x.x/y/z
 exclude x.1 v1.2.0
 
 require x.3 v1.99.0
--- $WORK/go.mod.packages --
-x.x/y/z
-x.x/y/z/w
 -- $WORK/go.mod.badfmt --
 module     x.x/y/z
 

src/cmd/go/testdata/script/mod_enabled.txt

@@ -37,8 +37,10 @@ go env GOMOD
 stdout z[/\\]go.mod
 
 cd $GOPATH/src/x/y
-! go env GOMOD
-stderr 'cannot find main module root'
+go env GOMOD
+! stdout .
+! go list -m
+stderr 'cannot find main module'
 
 cd $GOPATH/foo
 go env GOMOD

src/cmd/go/testdata/script/mod_find.txt

 # Derive module path from import comment.
 cd $WORK/x
 exists x.go
-go mod -init
+go mod init
 stderr 'module x'
 
 # Import comment works even with CRLF line endings.
 rm go.mod
 addcrlf x.go
-go mod -init
+go mod init
 stderr 'module x'
 
 # go mod should die in GOPATH if modules are not enabled for GOPATH
 cd $GOPATH/src/example.com/x/y
-! go mod -init
+! go mod init
 stderr 'go: modules disabled inside GOPATH/src by GO111MODULE=auto; see ''go help modules'''
 
 env GO111MODULE=on
 
 # Derive module path from location inside GOPATH.
 cd $GOPATH/src/example.com/x/y
-go mod -init
+go mod init
 stderr 'module example.com/x/y$'
 rm go.mod
 
 # Module path from Godeps/Godeps.json overrides GOPATH.
 cd $GOPATH/src/example.com/x/y/z
-go mod -init
+go mod init
 stderr 'unexpected.com/z'
 rm go.mod
 
 # Empty directory outside GOPATH fails.
 mkdir $WORK/empty
 cd $WORK/empty
-! go mod -init
+! go mod init
 stderr 'cannot determine module path for source directory'
 rm go.mod
 
 # Empty directory inside GOPATH/src uses location inside GOPATH.
 mkdir $GOPATH/src/empty
 cd $GOPATH/src/empty
-go mod -init
+go mod init
 stderr 'empty'
 rm go.mod
 
@@ -48,37 +48,37 @@ rm go.mod
 # gplink1/src/empty where gopathlink -> GOPATH
 symlink $WORK/gopathlink -> gopath
 cd $WORK/gopathlink/src/empty
-go mod -init
+go mod init
 rm go.mod
 
 # GOPATH/src/link where link -> out of GOPATH
 symlink $GOPATH/src/link -> $WORK/empty
 cd $WORK/empty
-! go mod -init
+! go mod init
 cd $GOPATH/src/link
-go mod -init
+go mod init
 stderr link
 rm go.mod
 
 # GOPATH/src/empty where GOPATH itself is a symlink
 env GOPATH=$WORK/gopathlink
 cd $GOPATH/src/empty
-go mod -init
+go mod init
 rm go.mod
 cd $WORK/gopath/src/empty
-go mod -init
+go mod init
 rm go.mod
 
 # GOPATH/src/link where GOPATH and link are both symlinks
 cd $GOPATH/src/link
-go mod -init
+go mod init
 stderr link
 rm go.mod
 
 # Too hard: doesn't match unevaluated nor completely evaluated. (Only partially evaluated.)
 # Whether this works depends on which OS we are running on.
 # cd $WORK/gopath/src/link
-# ! go mod -init
+# ! go mod init
 
 -- $WORK/x/x.go --
 package x // import "x"

src/cmd/go/testdata/script/mod_get_commit.txt

@@ -32,12 +32,12 @@ grep 'rsc.io/quote v0.0.0-20180214005840-23179ee8a569' go.mod
 go get rsc.io/quote@23179ee8
 grep 'rsc.io/quote v1.5.1' go.mod
 
-# go mod -require does not interpret commits
-go mod -require rsc.io/quote@23179ee
+# go mod edit -require does not interpret commits
+go mod edit -require rsc.io/quote@23179ee
 grep 'rsc.io/quote 23179ee' go.mod
 
-# but go mod -fix fixes them
-go mod -fix
+# but go mod fix fixes them
+go mod fix
 grep 'rsc.io/quote v1.5.1' go.mod
 
 -- go.mod --

src/cmd/go/testdata/script/mod_get_indirect.txt

@@ -17,15 +17,15 @@ go list
 grep 'rsc.io/quote v1.5.2$' go.mod
 grep 'golang.org/x/text [v0-9a-f\.-]+$' go.mod
 
-# indirect tag should be added by go mod -sync
+# indirect tag should be added by go mod tidy
 cp $WORK/tmp/usequote.go x.go
-go mod -sync
+go mod tidy
 grep 'rsc.io/quote v1.5.2$' go.mod
 grep 'golang.org/x/text [v0-9a-f\.-]+ // indirect' go.mod
 
 # requirement should be dropped entirely if not needed
 cp $WORK/tmp/usetext.go x.go
-go mod -sync
+go mod tidy
 ! grep rsc.io/quote go.mod
 grep 'golang.org/x/text [v0-9a-f\.-]+$' go.mod
 

src/cmd/go/testdata/script/mod_getmode_vendor.txt

 env GO111MODULE=on
 
 go get -m rsc.io/quote@v1.5.1
-go mod -vendor
+go mod vendor
 env GOPATH=$WORK/empty
 env GOPROXY=file:///nonexist
 

src/cmd/go/testdata/script/mod_go_version.txt

@@ -10,7 +10,7 @@ go build sub.1
 stderr 'module requires Go 1.11111'
 
 go build versioned.1
-go mod -require versioned.1@v1.1.0
+go mod edit -require versioned.1@v1.1.0
 ! go build versioned.1
 stderr 'module requires Go 1.99999'
 

src/cmd/go/testdata/script/mod_graph.txt

 env GO111MODULE=on
 
-go mod -graph
+go mod graph
 stdout '^m rsc.io/quote@v1.5.2$'
 stdout '^rsc.io/quote@v1.5.2 rsc.io/sampler@v1.3.0$'
 ! stdout '^m rsc.io/sampler@v1.3.0$'

src/cmd/go/testdata/script/mod_internal.txt

@@ -2,7 +2,7 @@ env GO111MODULE=on
 
 # golang.org/x/internal should be importable from other golang.org/x modules.
 rm go.mod
-go mod -init -module golang.org/x/anything
+go mod init golang.org/x/anything
 go build .
 
 # ...but that should not leak into other modules.
@@ -20,7 +20,7 @@ stderr 'use of vendored package golang_org/x/net/http/httpguts not allowed$'
 
 # Dependencies should be able to use their own internal modules...
 rm go.mod
-go mod -init -module golang.org/notx
+go mod init golang.org/notx
 go build ./throughdep
 
 # ... but other modules should not, even if they have transitive dependencies.
@@ -34,20 +34,19 @@ stderr 'use of internal package golang.org/x/.* not allowed in golang.org/notx/u
 
 # Replacing an internal module should keep it internal to the same paths.
 rm go.mod
-go mod -init -module golang.org/notx
-go mod -replace golang.org/x/internal=./replace/golang.org/notx/internal
+go mod init golang.org/notx
+go mod edit -replace golang.org/x/internal=./replace/golang.org/notx/internal
 go build ./throughdep
 
 ! go build ./baddep
 stderr 'use of internal package golang.org/x/.* not allowed in golang.org/notx/useinternal$'
 
-go mod -replace golang.org/x/internal=./vendor/golang.org/x/internal
+go mod edit -replace golang.org/x/internal=./vendor/golang.org/x/internal
 go build ./throughdep
 
 ! go build ./baddep
 stderr 'use of internal package golang.org/x/.* not allowed in golang.org/notx/useinternal$'
 
-
 -- useinternal.go --
 package useinternal
 import _ "golang.org/x/internal/subtle"

src/cmd/go/testdata/script/mod_nomod.txt

+# Test go commands with no module.
+env GO111MODULE=on
+
+# go mod edit fails unless given explicit mod file argument
+! go mod edit -json
+go mod edit -json x.mod
+
+# bug succeeds
+[exec:echo] env BROWSER=echo
+[exec:echo] go bug
+
+# commands that load the package in the current directory fail
+! go build
+! go fmt
+! go generate
+! go get
+! go install
+! go list
+! go run x.go
+! go test
+! go vet
+
+# clean succeeds, even with -modcache
+go clean -modcache
+
+# doc succeeds for standard library
+go doc unsafe
+
+# env succeeds
+go env
+
+# tool succeeds
+go tool -n test2json
+
+# version succeeds
+go version
+
+-- x.mod --
+module m
+
+-- x.go --
+package main
+func main() {}

src/cmd/go/testdata/script/mod_replace.txt

@@ -5,22 +5,20 @@ exec ./a1.exe
 stdout 'Don''t communicate by sharing memory'
 
 # Modules can be replaced by local packages.
-go mod -replace=rsc.io/quote/v3=./local/rsc.io/quote/v3
+go mod edit -replace=rsc.io/quote/v3=./local/rsc.io/quote/v3
 go build -o a2.exe .
 exec ./a2.exe
 stdout 'Concurrency is not parallelism.'
 
 # The module path of the replacement doesn't need to match.
 # (For example, it could be a long-running fork with its own import path.)
-go mod -replace=rsc.io/quote/v3=./local/not-rsc.io/quote/v3
+go mod edit -replace=rsc.io/quote/v3=./local/not-rsc.io/quote/v3
 go build -o a3.exe .
 exec ./a3.exe
 stdout 'Clear is better than clever.'
 
 # However, the same module can't be used as two different paths.
-go mod -dropreplace=rsc.io/quote/v3
-go mod -replace=not-rsc.io/quote/v3@v3.0.0=rsc.io/quote/v3@v3.0.0
-go mod -require=not-rsc.io/quote/v3@v3.0.0
+go mod edit -dropreplace=rsc.io/quote/v3 -replace=not-rsc.io/quote/v3@v3.0.0=rsc.io/quote/v3@v3.0.0 -require=not-rsc.io/quote/v3@v3.0.0
 ! go build -o a4.exe .
 
 

src/cmd/go/testdata/script/mod_sync_quote.txt → src/cmd/go/testdata/script/mod_tidy_quote.txt

-# Check that mod -sync does not introduce repeated
+# Check that mod tidy does not introduce repeated
 # require statements when input go.mod has quoted requirements.
 env GO111MODULE=on
 
-go mod -sync -json
-stdout -count=1 '"Path": "rsc.io/quote"'
+go mod tidy
+grep -count=1 rsc.io/quote go.mod
 
 cp go.mod2 go.mod
-go mod -sync -json
-stdout -count=1 '"Path": "rsc.io/quote"'
+go mod tidy
+grep -count=1 rsc.io/quote go.mod
 
 
 -- go.mod --

src/cmd/go/testdata/script/mod_sync_sum.txt → src/cmd/go/testdata/script/mod_tidy_sum.txt

@@ -2,7 +2,7 @@ env GO111MODULE=on
 
 # go.sum should list directly used modules and dependencies
 go get rsc.io/quote@v1.5.2
-go mod -sync
+go mod tidy
 grep rsc.io/sampler go.sum
 
 # go.sum should not normally lose old entries
@@ -11,8 +11,8 @@ grep 'rsc.io/quote v1.0.0' go.sum
 grep 'rsc.io/quote v1.5.2' go.sum
 grep rsc.io/sampler go.sum
 
-# go mod -sync should clear dead entries from go.sum
-go mod -sync
+# go mod tidy should clear dead entries from go.sum
+go mod tidy
 grep 'rsc.io/quote v1.0.0' go.sum
 ! grep 'rsc.io/quote v1.5.2' go.sum
 ! grep rsc.io/sampler go.sum
@@ -20,7 +20,7 @@ grep 'rsc.io/quote v1.0.0' go.sum
 # go.sum with no entries is OK to keep
 # (better for version control not to delete and recreate.)
 cp x.go.noimports x.go
-go mod -sync
+go mod tidy
 exists go.sum
 ! grep . go.sum
 

src/cmd/go/testdata/script/mod_vendor.txt

@@ -10,7 +10,7 @@ stdout '^w'
 go list -f {{.Dir}} x
 stdout 'src[\\/]x'
 
-go mod -vendor -v
+go mod vendor -v
 stderr '^# x v1.0.0 => ./x'
 stderr '^x'
 stderr '^# y v1.0.0 => ./y'

src/cmd/go/testdata/script/mod_vendor_nodeps.txt

 env GO111MODULE=on
 
-go mod -vendor
+go mod vendor
 stderr '^go: no dependencies to vendor'
 
 -- go.mod --

src/cmd/go/testdata/script/mod_verify.txt

@@ -2,26 +2,26 @@ env GO111MODULE=on
 
 # With good go.sum, verify succeeds by avoiding download.
 cp go.sum.good go.sum
-go mod -verify
+go mod verify
 ! exists $GOPATH/src/mod/cache/download/rsc.io/quote/@v/v1.1.0.zip
 
 # With bad go.sum, verify succeeds by avoiding download.
 cp go.sum.bad go.sum
-go mod -verify
+go mod verify
 ! exists $GOPATH/src/mod/cache/download/rsc.io/quote/@v/v1.1.0.zip
 
 # With bad go.sum, sync (which must download) fails.
 # Even if the bad sum is in the old legacy go.modverify file.
 rm go.sum
 cp go.sum.bad go.modverify
-! go mod -sync
+! go mod tidy
 stderr 'checksum mismatch'
 ! exists $GOPATH/src/mod/cache/download/rsc.io/quote/@v/v1.1.0.zip
 
 # With good go.sum, sync works (and moves go.modverify to go.sum).
 rm go.sum
 cp go.sum.good go.modverify
-go mod -sync
+go mod tidy
 exists $GOPATH/src/mod/cache/download/rsc.io/quote/@v/v1.1.0.zip
 exists $GOPATH/src/mod/rsc.io/quote@v1.1.0/quote.go
 ! exists go.modverify
@@ -30,39 +30,39 @@ exists $GOPATH/src/mod/rsc.io/quote@v1.1.0/quote.go
 grep '^rsc.io/quote v1.1.0/go.mod ' go.sum
 
 # verify should work
-go mod -verify
+go mod verify
 
 # basic loading of module graph should detect incorrect go.mod files.
-go mod -graph
+go mod graph
 cp go.sum.bad2 go.sum
-! go mod -graph
+! go mod graph
 stderr 'go.mod: checksum mismatch'
 
 # go.sum should be created and updated automatically.
 rm go.sum
-go mod -graph
+go mod graph
 exists go.sum
 grep '^rsc.io/quote v1.1.0/go.mod ' go.sum
 ! grep '^rsc.io/quote v1.1.0 ' go.sum
 
-go mod -sync
+go mod tidy
 grep '^rsc.io/quote v1.1.0/go.mod ' go.sum
 grep '^rsc.io/quote v1.1.0 ' go.sum
 
 # sync should ignore missing ziphash; verify should not
 rm $GOPATH/src/mod/cache/download/rsc.io/quote/@v/v1.1.0.ziphash
-go mod -sync
-! go mod -verify
+go mod tidy
+! go mod verify
 
 # Packages below module root should not be mentioned in go.sum.
 rm go.sum
-go mod -droprequire rsc.io/quote
+go mod edit -droprequire rsc.io/quote
 go list rsc.io/quote/buggy # re-resolves import path and updates go.mod
 grep '^rsc.io/quote v1.5.2/go.mod ' go.sum
 ! grep buggy go.sum
 
 # non-existent packages below module root should not be mentioned in go.sum
-go mod -droprequire rsc.io/quote
+go mod edit -droprequire rsc.io/quote
 ! go list rsc.io/quote/morebuggy
 grep '^rsc.io/quote v1.5.2/go.mod ' go.sum
 ! grep buggy go.sum

src/cmd/go/testdata/script/mod_version_nomod.txt

-# Test go version with no module.
-env GO111MODULE=on
-! go mod -json
-go version