gen-driftignore defaults to writing to a file

Users can still print to stdout by passing `-o -`.

pkg/analyser/analysis.go

@@ -69,6 +69,7 @@ type GenDriftIgnoreOptions struct {
 	ExcludeDeleted   bool
 	ExcludeDrifted   bool
 	InputPath        string
+	OutputPath       string
 }
 
 func (a Analysis) MarshalJSON() ([]byte, error) {

pkg/cmd/gen_driftignore.go

@@ -5,8 +5,10 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"time"
 
 	"github.com/cloudskiff/driftctl/pkg/analyser"
+	"github.com/pkg/errors"
 	"github.com/spf13/cobra"
 )
 
@@ -16,25 +18,25 @@ func NewGenDriftIgnoreCmd() *cobra.Command {
 	cmd := &cobra.Command{
 		Use:   "gen-driftignore",
 		Short: "Generate a .driftignore file based on your scan result",
-		Long:  "This command will generate a new .driftignore file containing your current drifts and send output to /dev/stdout\n\nExample: driftctl scan -o json://stdout | driftctl gen-driftignore > .driftignore",
+		Long:  "This command will generate a new .driftignore file containing your current drifts\n\nExample: driftctl scan -o json://stdout | driftctl gen-driftignore",
 		Args:  cobra.NoArgs,
 		RunE: func(cmd *cobra.Command, args []string) error {
-			driftFile := os.Stdin
-			if opts.InputPath != "-" {
-				var err error
-				driftFile, err = os.Open(opts.InputPath)
-				if err != nil {
-					return err
-				}
-				defer driftFile.Close()
-			}
-
-			_, list, err := genDriftIgnore(opts, driftFile)
+			_, list, err := genDriftIgnore(opts)
 			if err != nil {
 				return err
 			}
 
-			fmt.Println(list)
+			ignoreFile := os.Stdout
+			if opts.OutputPath != "-" {
+				var err error
+				ignoreFile, err = os.OpenFile(opts.OutputPath, os.O_CREATE|os.O_APPEND|os.O_WRONLY, 0644)
+				if err != nil {
+					return errors.Errorf("error opening output file: %s", err)
+				}
+				defer ignoreFile.Close()
+				fmt.Fprintf(os.Stderr, "Appending ignore rules to %s\n", opts.OutputPath)
+			}
+			fmt.Fprintf(ignoreFile, "# Generated by gen-driftignore cmd @ %s\n%s\n", time.Now().Format(time.RFC1123), list)
 
 			return nil
 		},
@@ -46,12 +48,23 @@ func NewGenDriftIgnoreCmd() *cobra.Command {
 	fl.BoolVar(&opts.ExcludeDeleted, "exclude-missing", false, "Exclude missing resources")
 	fl.BoolVar(&opts.ExcludeDrifted, "exclude-changed", false, "Exclude resources that changed on cloud provider")
 	fl.StringVarP(&opts.InputPath, "input", "i", "-", "Input where the JSON should be parsed from. Defaults to stdin.")
+	fl.StringVarP(&opts.OutputPath, "output", "o", ".driftignore", "Output file path to write the driftignore to.")
 
 	return cmd
 }
 
-func genDriftIgnore(opts *analyser.GenDriftIgnoreOptions, drift *os.File) (int, string, error) {
-	input, err := io.ReadAll(drift)
+func genDriftIgnore(opts *analyser.GenDriftIgnoreOptions) (int, string, error) {
+	driftFile := os.Stdin
+	if opts.InputPath != "-" {
+		var err error
+		driftFile, err = os.Open(opts.InputPath)
+		if err != nil {
+			return 0, "", err
+		}
+		defer driftFile.Close()
+	}
+
+	input, err := io.ReadAll(driftFile)
 	if err != nil {
 		return 0, "", err
 	}

pkg/cmd/gen_driftignore_test.go

@@ -1,15 +1,15 @@
 package cmd
 
 import (
-	"bytes"
 	"errors"
-	"io"
 	"os"
+	"strings"
 	"testing"
 
 	"github.com/cloudskiff/driftctl/test"
 	"github.com/spf13/cobra"
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func TestGenDriftIgnoreCmd_Input(t *testing.T) {
@@ -62,13 +62,16 @@ func TestGenDriftIgnoreCmd_Input(t *testing.T) {
 			rootCmd := &cobra.Command{Use: "root"}
 			rootCmd.AddCommand(NewGenDriftIgnoreCmd())
 
-			stdout := os.Stdout // keep backup of the real stdout
-			r, w, _ := os.Pipe()
-			os.Stdout = w
+			f, err := os.CreateTemp("", "TestGenDriftIgnoreCmd_Input")
+			require.Nil(t, err)
+			defer func() {
+				f.Close()
+				os.Remove(f.Name())
+			}()
 
-			args := append([]string{"gen-driftignore"}, c.args...)
+			args := append([]string{"gen-driftignore", "-o", f.Name()}, c.args...)
 
-			_, err := test.Execute(rootCmd, args...)
+			_, err = test.Execute(rootCmd, args...)
 			if c.err != nil {
 				assert.EqualError(t, err, c.err.Error())
 				return
@@ -76,26 +79,13 @@ func TestGenDriftIgnoreCmd_Input(t *testing.T) {
 				assert.Equal(t, c.err, err)
 			}
 
-			outC := make(chan []byte)
-			// copy the output in a separate goroutine so printing can't block indefinitely
-			go func() {
-				var buf bytes.Buffer
-				_, _ = io.Copy(&buf, r)
-				outC <- buf.Bytes()
-			}()
-
-			// back to normal state
-			w.Close()
-			os.Stdout = stdout // restoring the real stdout
-			result := <-outC
+			output, err := os.ReadFile(f.Name())
+			require.Nil(t, err)
 
 			if c.output != "" {
-				output, err := os.ReadFile(c.output)
-				if err != nil {
-					t.Fatal(err)
-				}
-
-				assert.Equal(t, string(output), string(result))
+				expectedOutput, err := os.ReadFile(c.output)
+				require.Nil(t, err)
+				assert.Equal(t, string(expectedOutput), trimLeadingComment(string(output)))
 			}
 		})
 	}
@@ -110,6 +100,7 @@ func TestGenDriftIgnoreCmd_ValidFlags(t *testing.T) {
 	cases := []struct {
 		args []string
 	}{
+		{args: []string{"gen-driftignore"}},
 		{args: []string{"gen-driftignore", "--exclude-unmanaged"}},
 		{args: []string{"gen-driftignore", "--exclude-missing"}},
 		{args: []string{"gen-driftignore", "--exclude-changed"}},
@@ -152,3 +143,10 @@ func TestGenDriftIgnoreCmd_InvalidFlags(t *testing.T) {
 		assert.EqualError(t, err, tt.err.Error())
 	}
 }
+
+// The leading comment, "Generated by gen-driftignore..." contains a timestamp,
+// that we don't care to assert on.
+func trimLeadingComment(content string) string {
+	lines := strings.Split(content, "\n")
+	return strings.Join(lines[1:], "\n")
+}