add command to get the digest of a tagged manifest without parsing ou…

…tput

Signed-off-by: amands98 <amandeepsm.in@gmail.com>

cmd/oras/root/cmd.go

@@ -34,6 +34,7 @@ func New() *cobra.Command {
 		logoutCmd(),
 		versionCmd(),
 		discoverCmd(),
+		resolveCmd(),
 		copyCmd(),
 		tagCmd(),
 		attachCmd(),

cmd/oras/root/resolve.go

@@ -0,0 +1,81 @@
+/*
+Copyright The ORAS Authors.
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package root
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/spf13/cobra"
+	"oras.land/oras-go/v2"
+	"oras.land/oras/cmd/oras/internal/option"
+)
+
+type resolveOptions struct {
+	option.Common
+	option.Platform
+	option.Target
+
+	FullRef bool
+}
+
+func resolveCmd() *cobra.Command {
+	var opts resolveOptions
+
+	cmd := &cobra.Command{
+		Use:     "resolve [flags] <name>:<tag>",
+		Short:   "Resolves digest of the target artifact",
+		Args:    cobra.ExactArgs(1),
+		Aliases: []string{"digest"},
+		PreRunE: func(cmd *cobra.Command, args []string) error {
+			opts.RawReference = args[0]
+			return option.Parse(&opts)
+		},
+		RunE: func(cmd *cobra.Command, args []string) error {
+			return runResolve(cmd.Context(), opts)
+		},
+	}
+
+	cmd.Flags().BoolVar(&opts.FullRef, "full-ref", false, "print the full artifact reference with digest")
+	option.ApplyFlags(&opts, cmd.Flags())
+	return cmd
+}
+
+func runResolve(ctx context.Context, opts resolveOptions) error {
+	ctx, _ = opts.WithContext(ctx)
+	repo, err := opts.NewReadonlyTarget(ctx, opts.Common)
+	if err != nil {
+		return err
+	}
+	if err := opts.EnsureReferenceNotEmpty(); err != nil {
+		return err
+	}
+	resolveOpts := oras.DefaultResolveOptions
+	resolveOpts.TargetPlatform = opts.Platform.Platform
+	desc, err := oras.Resolve(ctx, repo, opts.Reference, resolveOpts)
+
+	if err != nil {
+		return fmt.Errorf("failed to resolve the digest: %w", err)
+	}
+
+	if opts.FullRef {
+		fmt.Printf("%s@%s\n", opts.RawReference, desc.Digest.String())
+	} else {
+		fmt.Println(desc.Digest.String())
+	}
+
+	return nil
+}