feat(cmd) add --non-zero-exit-code flag to diff command

This makes it easy to integrate decK into a CI pipeline.
A script can easily determine if there is a diff present by examining
the exit code of decK.

Fix #98

cmd/common.go

@@ -2,6 +2,7 @@ package cmd
 
 import (
 	"net/http"
+	"os"
 
 	"github.com/fatih/color"
 	"github.com/hbagdi/deck/diff"
@@ -109,5 +110,9 @@ func syncMain(filename string, dry bool, parallelism int) error {
 	printFn("  Created: %v\n", stats.CreateOps)
 	printFn("  Updated: %v\n", stats.UpdateOps)
 	printFn("  Deleted: %v\n", stats.DeleteOps)
+	if diffCmdNonZeroExitCode &&
+		stats.CreateOps+stats.UpdateOps+stats.DeleteOps != 0 {
+		os.Exit(2)
+	}
 	return nil
 }

cmd/diff.go

@@ -8,8 +8,9 @@ import (
 )
 
 var (
-	diffCmdKongStateFile string
-	diffCmdParallelism   int
+	diffCmdKongStateFile   string
+	diffCmdParallelism     int
+	diffCmdNonZeroExitCode bool
 )
 
 // diffCmd represents the diff command
@@ -48,4 +49,8 @@ func init() {
 		"select-tag", []string{},
 		"only entities matching tags specified via this flag are diffed.\n"+
 			"Multiple tags are ANDed together.")
+	diffCmd.Flags().BoolVar(&diffCmdNonZeroExitCode, "non-zero-exit-code",
+		false, "return exit code 2 if there is a diff present,\n"+
+			"exit code 0 if no diff is found,\n"+
+			"and exit code 1 if an error occurs.")
 }