alias: add new backend to create aliases for remote names #1049

The alias backend is a wrapper for an existing remote.
It allows you to name a "remote:path" as an "alias:".

backend/alias/alias.go

@@ -0,0 +1,45 @@
+package alias
+
+import (
+	"errors"
+	"path"
+	"path/filepath"
+	"strings"
+
+	"github.com/ncw/rclone/fs"
+	"github.com/ncw/rclone/fs/config"
+)
+
+// Register with Fs
+func init() {
+	fsi := &fs.RegInfo{
+		Name:        "alias",
+		Description: "Alias for a existing remote",
+		NewFs:       NewFs,
+		Options: []fs.Option{{
+			Name: "remote",
+			Help: "Remote or path to alias.\nCan be \"myremote:path/to/dir\", \"myremote:bucket\", \"myremote:\" or \"/local/path\".",
+		}},
+	}
+	fs.Register(fsi)
+}
+
+// NewFs contstructs an Fs from the path.
+//
+// The returned Fs is the actual Fs, referenced by remote in the config
+func NewFs(name, root string) (fs.Fs, error) {
+	remote := config.FileGet(name, "remote")
+	if remote == "" {
+		return nil, errors.New("alias can't point to an empty remote - check the value of the remote setting")
+	}
+	if strings.HasPrefix(remote, name+":") {
+		return nil, errors.New("can't point alias remote at itself - check the value of the remote setting")
+	}
+	fsInfo, configName, fsPath, err := fs.ParseRemote(remote)
+	if err != nil {
+		return nil, err
+	}
+
+	root = filepath.ToSlash(root)
+	return fsInfo.NewFs(configName, path.Join(fsPath, root))
+}