Can supply value for write from file

cmd/commands_test.go

@@ -1201,6 +1201,29 @@ func TestWriteCmd(t *testing.T) {
 	test.AssertResult(t, expectedOutput, result.Output)
 }
 
+func TestWriteFromFileCmd(t *testing.T) {
+	content := `b:
+  c: 3
+`
+	filename := test.WriteTempYamlFile(content)
+	defer test.RemoveTempYamlFile(filename)
+
+	source := `kittens: are cute # sure are!`
+	fromFilename := test.WriteTempYamlFile(source)
+	defer test.RemoveTempYamlFile(fromFilename)
+
+	cmd := getRootCommand()
+	result := test.RunCmd(cmd, fmt.Sprintf("write %s b.c -f %s", filename, fromFilename))
+	if result.Error != nil {
+		t.Error(result.Error)
+	}
+	expectedOutput := `b:
+  c:
+    kittens: are cute # sure are!
+`
+	test.AssertResult(t, expectedOutput, result.Output)
+}
+
 func TestWriteEmptyCmd(t *testing.T) {
 	content := ``
 	filename := test.WriteTempYamlFile(content)

cmd/constant.go

@@ -9,6 +9,7 @@ var customTag = ""
 var printMode = "v"
 var writeInplace = false
 var writeScript = ""
+var sourceYamlFile = ""
 var outputToJSON = false
 var prettyPrint = false
 var explodeAnchors = false

cmd/utils.go

@@ -367,6 +367,17 @@ func readUpdateCommands(args []string, expectedArgs int, badArgsMessage string)
 		}
 
 		log.Debugf("Read write commands file '%v'", updateCommands)
+	} else if sourceYamlFile != "" && len(args) == expectedArgs-1 {
+		log.Debugf("Reading value from %v", sourceYamlFile)
+		var value yaml.Node
+		err := readData(sourceYamlFile, 0, &value)
+		if err != nil && err != io.EOF {
+			return nil, err
+		}
+		log.Debug("args %v", args[expectedArgs-2])
+		updateCommands = make([]yqlib.UpdateCommand, 1)
+		updateCommands[0] = yqlib.UpdateCommand{Command: "update", Path: args[expectedArgs-2], Value: value.Content[0], Overwrite: true}
+
 	} else if len(args) < expectedArgs {
 		return nil, errors.New(badArgsMessage)
 	} else {

cmd/write.go

@@ -43,6 +43,7 @@ format is list of update commands (update or delete) like so:
 	}
 	cmdWrite.PersistentFlags().BoolVarP(&writeInplace, "inplace", "i", false, "update the yaml file inplace")
 	cmdWrite.PersistentFlags().StringVarP(&writeScript, "script", "s", "", "yaml script for updating yaml")
+	cmdWrite.PersistentFlags().StringVarP(&sourceYamlFile, "from", "f", "", "yaml file for updating yaml (as-is)")
 	cmdWrite.PersistentFlags().StringVarP(&customTag, "tag", "t", "", "set yaml tag (e.g. !!int)")
 	cmdWrite.PersistentFlags().StringVarP(&docIndex, "doc", "d", "0", "process document index number (0 based, * for all documents)")
 	return cmdWrite