command/fmt: Accept input from STDIN

So that you can do automatic formatting from an editor. You probably want to
disable the `-write` and `-list` options so that you just get the
re-formatted content, e.g.

    cat main.tf | terraform fmt -write=false -list=false -

I've added a non-exported field called `input` so that we can override this
for the tests. If not specified, like in `commands.go`, then it will default
to `os.Stdin` which works on the command line.

command/fmt.go

@@ -3,6 +3,8 @@ package command
 import (
 	"flag"
 	"fmt"
+	"io"
+	"os"
 	"strings"
 
 	"github.com/hashicorp/hcl/hcl/fmtcmd"
@@ -10,6 +12,7 @@ import (
 )
 
 const (
+	stdinArg      = "-"
 	fileExtension = "tf"
 )
 
@@ -17,10 +20,15 @@ const (
 // files to a canonical format and style.
 type FmtCommand struct {
 	Meta
-	opts fmtcmd.Options
+	opts  fmtcmd.Options
+	input io.Reader // STDIN if nil
 }
 
 func (c *FmtCommand) Run(args []string) int {
+	if c.input == nil {
+		c.input = os.Stdin
+	}
+
 	args = c.Meta.process(args, false)
 
 	cmdFlags := flag.NewFlagSet("fmt", flag.ContinueOnError)
@@ -40,15 +48,15 @@ func (c *FmtCommand) Run(args []string) int {
 		return 1
 	}
 
-	var dir string
+	var dirs []string
 	if len(args) == 0 {
-		dir = "."
-	} else {
-		dir = args[0]
+		dirs = []string{"."}
+	} else if args[0] != stdinArg {
+		dirs = []string{args[0]}
 	}
 
 	output := &cli.UiWriter{Ui: c.Ui}
-	err := fmtcmd.Run([]string{dir}, []string{fileExtension}, nil, output, c.opts)
+	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
@@ -64,6 +72,7 @@ Usage: terraform fmt [options] [DIR]
 	Rewrites all Terraform configuration files to a canonical format.
 
 	If DIR is not specified then the current working directory will be used.
+	If DIR is "-" then content will be read from STDIN.
 
 Options:
 

command/fmt_test.go

@@ -1,6 +1,7 @@
 package command
 
 import (
+	"bytes"
 	"fmt"
 	"io/ioutil"
 	"os"
@@ -123,6 +124,34 @@ func TestFmt_directoryArg(t *testing.T) {
 	}
 }
 
+func TestFmt_stdinArg(t *testing.T) {
+	input := new(bytes.Buffer)
+	input.Write(fmtFixture.input)
+
+	ui := new(cli.MockUi)
+	c := &FmtCommand{
+		Meta: Meta{
+			ContextOpts: testCtxConfig(testProvider()),
+			Ui:          ui,
+		},
+		input: input,
+	}
+
+	args := []string{
+		"-write=false",
+		"-list=false",
+		"-",
+	}
+	if code := c.Run(args); code != 0 {
+		t.Fatalf("wrong exit code. errors: \n%s", ui.ErrorWriter.String())
+	}
+
+	expected := fmtFixture.golden
+	if actual := ui.OutputWriter.Bytes(); !bytes.Equal(actual, expected) {
+		t.Fatalf("got: %q\nexpected: %q", actual, expected)
+	}
+}
+
 var fmtFixture = struct {
 	filename      string
 	input, golden []byte

website/source/docs/commands/fmt.html.markdown

@@ -17,7 +17,8 @@ Usage: `terraform fmt [options] [DIR]`
 
 By default, `fmt` scans the current directory for configuration files. If
 the `dir` argument is provided then it will scan that given directory
-instead.
+instead. If `dir` is a single dash (`-`) then `fmt` will read from standard
+input (STDIN).
 
 The command-line flags are all optional. The list of available flags are: