Added a small utility method to display a warnings.

changelog/16441.txt

@@ -0,0 +1,3 @@
+```release-note:improvement
+base: Added a small utility method to display a warning if flags are passed after arguments in a command.
+```

command/base.go

@@ -549,6 +549,7 @@ type FlagSets struct {
 	mainSet     *flag.FlagSet
 	hiddens     map[string]struct{}
 	completions complete.Flags
+	ui          cli.Ui
 }
 
 // NewFlagSets creates a new flag sets.
@@ -564,6 +565,7 @@ func NewFlagSets(ui cli.Ui) *FlagSets {
 		mainSet:     mainSet,
 		hiddens:     make(map[string]struct{}),
 		completions: complete.Flags{},
+		ui:          ui,
 	}
 }
 
@@ -582,8 +584,17 @@ func (f *FlagSets) Completions() complete.Flags {
 }
 
 // Parse parses the given flags, returning any errors.
+// Warnings, if any, regarding the arguments format are sent to stdout
 func (f *FlagSets) Parse(args []string) error {
-	return f.mainSet.Parse(args)
+	err := f.mainSet.Parse(args)
+
+	var out bytes.Buffer
+	printArgsWarningIfAny(&out, f.Args())
+	if out.String() != "" {
+		f.ui.Warn(out.String())
+	}
+
+	return err
 }
 
 // Parsed reports whether the command-line flags have been parsed.
@@ -603,10 +614,10 @@ func (f *FlagSets) Visit(fn func(*flag.Flag)) {
 }
 
 // Help builds custom help for this command, grouping by flag set.
-func (fs *FlagSets) Help() string {
+func (f *FlagSets) Help() string {
 	var out bytes.Buffer
 
-	for _, set := range fs.flagSets {
+	for _, set := range f.flagSets {
 		printFlagTitle(&out, set.name+":")
 		set.VisitAll(func(f *flag.Flag) {
 			// Skip any hidden flags

command/base_helpers.go

@@ -292,3 +292,16 @@ func parseFlagFile(raw string) (string, error) {
 
 	return raw, nil
 }
+
+func printArgsWarningIfAny(w io.Writer, args []string) {
+	var trailingFlags []string
+	for _, arg := range args {
+		if strings.HasPrefix(arg, "-") {
+			trailingFlags = append(trailingFlags, arg)
+		}
+	}
+
+	if len(trailingFlags) > 0 {
+		fmt.Fprintf(w, "Flag(s) must be provided before positional argument(s): %s", trailingFlags)
+	}
+}

command/base_helpers_test.go

@@ -1,10 +1,12 @@
 package command
 
 import (
+	"bytes"
 	"fmt"
 	"io"
 	"io/ioutil"
 	"os"
+	"strings"
 	"testing"
 	"time"
 )
@@ -209,3 +211,50 @@ func TestParseFlagFile(t *testing.T) {
 		})
 	}
 }
+
+func TestArgWarnings(t *testing.T) {
+	t.Parallel()
+
+	cases := []struct {
+		args     []string
+		expected string
+	}{
+		{
+			[]string{"a", "b", "c"},
+			"",
+		},
+		{
+			[]string{"a", "-b"},
+			"-b",
+		},
+		{
+			[]string{"a", "--b"},
+			"--b",
+		},
+		{
+			[]string{"a-b", "-c"},
+			"-c",
+		},
+		{
+			[]string{"a", "-b-c"},
+			"-b-c",
+		},
+		{
+			[]string{"-a", "b"},
+			"-a",
+		},
+	}
+
+	for _, tc := range cases {
+		tc := tc
+
+		t.Run(tc.expected, func(t *testing.T) {
+			var out bytes.Buffer
+
+			printArgsWarningIfAny(&out, tc.args)
+			if !strings.Contains(out.String(), tc.expected) {
+				t.Fatalf("expected %s to contain %s", out.String(), tc.expected)
+			}
+		})
+	}
+}