chore: improve error message when attaching with no file or annotation (

#1253)

Signed-off-by: Xiaoxuan Wang <[email protected]>

cmd/oras/root/attach.go

@@ -106,7 +106,13 @@ func runAttach(cmd *cobra.Command, opts *attachOptions) error {
 		return err
 	}
 	if len(opts.FileRefs) == 0 && len(annotations[option.AnnotationManifest]) == 0 {
-		return errors.New("no blob or manifest annotation are provided")
+		return &oerrors.Error{
+			Err:   errors.New(`neither file nor annotation provided in the command`),
+			Usage: fmt.Sprintf("%s %s", cmd.Parent().CommandPath(), cmd.Use),
+			Recommendation: `To attach to an existing artifact, please provide files 
+			via argument or annotations via flag "--annotation". Run "oras attach -h" 
+			for more options and examples`,
+		}
 	}
 
 	// prepare manifest

test/e2e/suite/command/attach.go

@@ -51,13 +51,13 @@ var _ = Describe("ORAS beginners:", func() {
 
 		It("should fail if no file reference or manifest annotation provided for registry", func() {
 			ORAS("attach", "--artifact-type", "oras/test", RegistryRef(ZOTHost, ImageRepo, foobar.Tag)).
-				ExpectFailure().MatchErrKeyWords("Error: no blob or manifest annotation are provided").Exec()
+				ExpectFailure().MatchErrKeyWords("Error: neither file nor annotation", "Usage:").Exec()
 		})
 
 		It("should fail if no file reference or manifest annotation provided for OCI layout", func() {
 			root := GinkgoT().TempDir()
 			ORAS("attach", "--artifact-type", "oras/test", LayoutRef(root, foobar.Tag)).
-				ExpectFailure().MatchErrKeyWords("Error: no blob or manifest annotation are provided").Exec()
+				ExpectFailure().MatchErrKeyWords("Error: neither file nor annotation", "Usage:").Exec()
 		})
 
 		It("should fail if distribution spec is unknown", func() {