dump: add --target option

changelog/unreleased/issue-4678

@@ -0,0 +1,7 @@
+Enhancement: Add --target flag to the dump command
+
+Restic `dump` always printed to the standard output. It now permits to select a
+`--target` file to print the output. 
+
+https://github.com/restic/restic/issues/4678
+https://github.com/restic/restic/pull/4682

cmd/restic/cmd_dump.go

@@ -46,6 +46,7 @@ Exit status is 0 if the command was successful, and non-zero if there was any er
 type DumpOptions struct {
 	restic.SnapshotFilter
 	Archive string
+	Target  string
 }
 
 var dumpOptions DumpOptions
@@ -56,6 +57,7 @@ func init() {
 	flags := cmdDump.Flags()
 	initSingleSnapshotFilter(flags, &dumpOptions.SnapshotFilter)
 	flags.StringVarP(&dumpOptions.Archive, "archive", "a", "tar", "set archive `format` as \"tar\" or \"zip\"")
+	flags.StringVarP(&dumpOptions.Target, "target", "t", "", "set the target path to dump the archive file")
 }
 
 func splitPath(p string) []string {
@@ -67,11 +69,11 @@ func splitPath(p string) []string {
 	return append(s, f)
 }
 
-func printFromTree(ctx context.Context, tree *restic.Tree, repo restic.BlobLoader, prefix string, pathComponents []string, d *dump.Dumper) error {
+func printFromTree(ctx context.Context, tree *restic.Tree, repo restic.BlobLoader, prefix string, pathComponents []string, d *dump.Dumper, checkStdoutArchiveFunc func() error) error {
 	// If we print / we need to assume that there are multiple nodes at that
 	// level in the tree.
 	if pathComponents[0] == "" {
-		if err := checkStdoutArchive(); err != nil {
+		if err := checkStdoutArchiveFunc(); err != nil {
 			return err
 		}
 		return d.DumpTree(ctx, tree, "/")
@@ -91,9 +93,9 @@ func printFromTree(ctx context.Context, tree *restic.Tree, repo restic.BlobLoade
 				if err != nil {
 					return errors.Wrapf(err, "cannot load subtree for %q", item)
 				}
-				return printFromTree(ctx, subtree, repo, item, pathComponents[1:], d)
+				return printFromTree(ctx, subtree, repo, item, pathComponents[1:], d, checkStdoutArchiveFunc)
 			case dump.IsDir(node):
-				if err := checkStdoutArchive(); err != nil {
+				if err := checkStdoutArchiveFunc(); err != nil {
 					return err
 				}
 				subtree, err := restic.LoadTree(ctx, repo, *node.Subtree)
@@ -168,8 +170,23 @@ func runDump(ctx context.Context, opts DumpOptions, gopts GlobalOptions, args []
 		return errors.Fatalf("loading tree for snapshot %q failed: %v", snapshotIDString, err)
 	}
 
-	d := dump.New(opts.Archive, repo, os.Stdout)
+	var outputFileWriter = os.Stdout
-	err = printFromTree(ctx, tree, repo, "/", splittedPath, d)
+	checkStdoutArchiveFunc := checkStdoutArchive
+	if opts.Target != "" {
+		file, err := os.OpenFile(opts.Target, os.O_RDWR|os.O_CREATE|os.O_EXCL, 0o666)
+		if err != nil {
+			return fmt.Errorf("cannot dump to file: %w", err)
+		}
+		defer func() {
+			_ = file.Close()
+		}()
+
+		outputFileWriter = file
+		checkStdoutArchiveFunc = func() error { return nil }
+	}
+
+	d := dump.New(opts.Archive, repo, outputFileWriter)
+	err = printFromTree(ctx, tree, repo, "/", splittedPath, d, checkStdoutArchiveFunc)
 	if err != nil {
 		return errors.Fatalf("cannot dump file: %v", err)
 	}

doc/050_restore.rst

@@ -174,3 +174,10 @@ To include the folder content at the root of the archive, you can use the ``<sna
 .. code-block:: console
 
     $ restic -r /srv/restic-repo dump latest:/home/other/work / > restore.tar
+
+It is also possible to ``dump`` the contents of a selected snapshot and folder
+structure to a file using the ``--target`` flag. The ``dump`` will fail if the 
+file exists.
+
+.. code-block:: console
+    $ restic -r /srv/restic-repo dump latest / --target /home/linux.user/output.tar -a tar