command: Add -check flag to fmt

command/fmt.go

@@ -1,6 +1,7 @@
 package command
 
 import (
+	"bytes"
 	"flag"
 	"fmt"
 	"io"
@@ -21,6 +22,7 @@ const (
 type FmtCommand struct {
 	Meta
 	opts  fmtcmd.Options
+	check bool
 	input io.Reader // STDIN if nil
 }
 
@@ -38,6 +40,7 @@ func (c *FmtCommand) Run(args []string) int {
 	cmdFlags.BoolVar(&c.opts.List, "list", true, "list")
 	cmdFlags.BoolVar(&c.opts.Write, "write", true, "write")
 	cmdFlags.BoolVar(&c.opts.Diff, "diff", false, "diff")
+	cmdFlags.BoolVar(&c.check, "check", false, "check")
 	cmdFlags.Usage = func() { c.Ui.Error(c.Help()) }
 
 	if err := cmdFlags.Parse(args); err != nil {
@@ -61,13 +64,37 @@ func (c *FmtCommand) Run(args []string) int {
 		dirs = []string{args[0]}
 	}
 
-	output := &cli.UiWriter{Ui: c.Ui}
+	var output io.Writer
+	list := c.opts.List // preserve the original value of -list
+	if c.check {
+		// set to true so we can use the list output to check
+		// if the input needs formatting
+		c.opts.List = true
+		c.opts.Write = false
+		output = &bytes.Buffer{}
+	} else {
+		output = &cli.UiWriter{Ui: c.Ui}
+	}
+
 	err = fmtcmd.Run(dirs, []string{fileExtension}, c.input, output, c.opts)
 	if err != nil {
 		c.Ui.Error(fmt.Sprintf("Error running fmt: %s", err))
 		return 2
 	}
 
+	if c.check {
+		buf := output.(*bytes.Buffer)
+		ok := buf.Len() == 0
+		if list {
+			io.Copy(&cli.UiWriter{Ui: c.Ui}, buf)
+		}
+		if ok {
+			return 0
+		} else {
+			return 3
+		}
+	}
+
 	return 0
 }
 
@@ -84,10 +111,12 @@ Options:
 
   -list=true       List files whose formatting differs (always false if using STDIN)
 
-  -write=true      Write result to source file instead of STDOUT (always false if using STDIN)
+  -write=true      Write result to source file instead of STDOUT (always false if using STDIN or -check)
 
   -diff=false      Display diffs of formatting changes
 
+  -check=false     Check if the input is formatted. Exit status will be 0 if all input is properly formatted and non-zero otherwise.
+
 `
 	return strings.TrimSpace(helpText)
 }

command/fmt_test.go

@@ -179,6 +179,60 @@ func TestFmt_nonDefaultOptions(t *testing.T) {
 	}
 }
 
+func TestFmt_check(t *testing.T) {
+	tempDir, err := fmtFixtureWriteDir()
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+	defer os.RemoveAll(tempDir)
+
+	ui := new(cli.MockUi)
+	c := &FmtCommand{
+		Meta: Meta{
+			testingOverrides: metaOverridesForProvider(testProvider()),
+			Ui:               ui,
+		},
+	}
+
+	args := []string{
+		"-check",
+		tempDir,
+	}
+	if code := c.Run(args); code != 3 {
+		t.Fatalf("wrong exit code. expected 3")
+	}
+
+	if actual := ui.OutputWriter.String(); !strings.Contains(actual, tempDir) {
+		t.Fatalf("expected:\n%s\n\nto include: %q", actual, tempDir)
+	}
+}
+
+func TestFmt_checkStdin(t *testing.T) {
+	input := new(bytes.Buffer)
+	input.Write(fmtFixture.input)
+
+	ui := new(cli.MockUi)
+	c := &FmtCommand{
+		Meta: Meta{
+			testingOverrides: metaOverridesForProvider(testProvider()),
+			Ui:               ui,
+		},
+		input: input,
+	}
+
+	args := []string{
+		"-check",
+		"-",
+	}
+	if code := c.Run(args); code != 3 {
+		t.Fatalf("wrong exit code. expected 3, got %d", code)
+	}
+
+	if ui.OutputWriter != nil {
+		t.Fatalf("expected no output, got: %q", ui.OutputWriter.String())
+	}
+}
+
 var fmtFixture = struct {
 	filename      string
 	input, golden []byte

website/docs/commands/fmt.html.markdown

@@ -24,5 +24,7 @@ The command-line flags are all optional. The list of available flags are:
 
 * `-list=true` - List files whose formatting differs (disabled if using STDIN)
 * `-write=true` - Write result to source file instead of STDOUT (disabled if
-    using STDIN)
+    using STDIN or -check)
 * `-diff=false` - Display diffs of formatting changes
+* `-check=false` - Check if the input is formatted. Exit status will be 0 if
+    all input is properly formatted and non-zero otherwise.