command: allow module depth to be set via env var

Another convienence env var here with TF_MODULE_DEPTH.

Works like you'd expect it to!

command/graph.go

@@ -23,7 +23,7 @@ func (c *GraphCommand) Run(args []string) int {
 	args = c.Meta.process(args, false)
 
 	cmdFlags := flag.NewFlagSet("graph", flag.ContinueOnError)
-	cmdFlags.IntVar(&moduleDepth, "module-depth", 0, "module-depth")
+	c.addModuleDepthFlag(cmdFlags, &moduleDepth)
 	cmdFlags.BoolVar(&verbose, "verbose", false, "verbose")
 	cmdFlags.BoolVar(&drawCycles, "draw-cycles", false, "draw-cycles")
 	cmdFlags.Usage = func() { c.Ui.Error(c.Help()) }

command/meta.go

@@ -399,6 +399,20 @@ func (m *Meta) uiHook() *UiHook {
 	}
 }
 
+const (
+	// The name of the environment variable that can be used to set module depth.
+	ModuleDepthEnvVar = "TF_MODULE_DEPTH"
+)
+
+func (m *Meta) addModuleDepthFlag(flags *flag.FlagSet, moduleDepth *int) {
+	flags.IntVar(moduleDepth, "module-depth", 0, "module-depth")
+	if envVar := os.Getenv(ModuleDepthEnvVar); envVar != "" {
+		if md, err := strconv.Atoi(envVar); err == nil {
+			*moduleDepth = md
+		}
+	}
+}
+
 // contextOpts are the options used to load a context from a command.
 type contextOpts struct {
 	// Path to the directory where the root module is.

command/meta_test.go

@@ -1,6 +1,7 @@
 package command
 
 import (
+	"flag"
 	"io/ioutil"
 	"os"
 	"path/filepath"
@@ -215,3 +216,55 @@ func TestMeta_initStatePaths(t *testing.T) {
 		t.Fatalf("bad: %#v", m)
 	}
 }
+
+func TestMeta_addModuleDepthFlag(t *testing.T) {
+	old := os.Getenv(ModuleDepthEnvVar)
+	defer os.Setenv(ModuleDepthEnvVar, old)
+
+	cases := map[string]struct {
+		EnvVar   string
+		Args     []string
+		Expected int
+	}{
+		"env var sets value when no flag present": {
+			EnvVar:   "4",
+			Args:     []string{},
+			Expected: 4,
+		},
+		"flag overrides envvar": {
+			EnvVar:   "4",
+			Args:     []string{"-module-depth=-1"},
+			Expected: -1,
+		},
+		"negative envvar works": {
+			EnvVar:   "-1",
+			Args:     []string{},
+			Expected: -1,
+		},
+		"invalid envvar is ignored": {
+			EnvVar:   "-#",
+			Args:     []string{},
+			Expected: 0,
+		},
+		"empty envvar is okay too": {
+			EnvVar:   "",
+			Args:     []string{},
+			Expected: 0,
+		},
+	}
+
+	for tn, tc := range cases {
+		m := new(Meta)
+		var moduleDepth int
+		flags := flag.NewFlagSet("test", flag.ContinueOnError)
+		os.Setenv(ModuleDepthEnvVar, tc.EnvVar)
+		m.addModuleDepthFlag(flags, &moduleDepth)
+		err := flags.Parse(tc.Args)
+		if err != nil {
+			t.Fatalf("%s: err: %#v", tn, err)
+		}
+		if moduleDepth != tc.Expected {
+			t.Fatalf("%s: expected: %#v, got: %#v", tn, tc.Expected, moduleDepth)
+		}
+	}
+}

command/plan.go

@@ -25,7 +25,7 @@ func (c *PlanCommand) Run(args []string) int {
 	cmdFlags := c.Meta.flagSet("plan")
 	cmdFlags.BoolVar(&destroy, "destroy", false, "destroy")
 	cmdFlags.BoolVar(&refresh, "refresh", true, "refresh")
-	cmdFlags.IntVar(&moduleDepth, "module-depth", 0, "module-depth")
+	c.addModuleDepthFlag(cmdFlags, &moduleDepth)
 	cmdFlags.StringVar(&outPath, "out", "", "path")
 	cmdFlags.StringVar(&c.Meta.statePath, "state", DefaultStateFilename, "path")
 	cmdFlags.StringVar(&c.Meta.backupPath, "backup", "", "path")

command/show.go

@@ -22,7 +22,7 @@ func (c *ShowCommand) Run(args []string) int {
 	args = c.Meta.process(args, false)
 
 	cmdFlags := flag.NewFlagSet("show", flag.ContinueOnError)
-	cmdFlags.IntVar(&moduleDepth, "module-depth", 0, "module-depth")
+	c.addModuleDepthFlag(cmdFlags, &moduleDepth)
 	cmdFlags.Usage = func() { c.Ui.Error(c.Help()) }
 	if err := cmdFlags.Parse(args); err != nil {
 		return 1