# Modifying data

{% hint style="success" %}
Use `--root` whenever you are **modifying data and intend to save it back to a file**.\
This ensures you get the complete updated document rather than just the changed value.
{% endhint %}

### Output Behaviour

By default, **dasel outputs the result of the final selector** in your query.\
This is convenient when you’re simply retrieving a value, but can be less helpful when you’re modifying data, since you often want to see the full document instead.

***

#### Retrieving a Value

```bash
$ echo '{
  "foo": {
    "bar": "baz"
  }
}' | dasel -i json 'foo.bar'
"baz"
```

Here, the final selector is `foo.bar`, so dasel outputs its value: `"baz"`.

***

#### Modifying a Value

```bash
$ echo '{
  "foo": {
    "bar": "baz"
  }
}' | dasel -i json 'foo.bar = "bong"'
"bong"
```

When updating a value, dasel still outputs the result of the final selector — in this case, the new value `"bong"`.

***

#### Outputting the Entire Document

To print the entire modified document, use the `--root` flag.\
This changes the output to always return the full root node, regardless of the final selector.

```bash
$ echo '{
  "foo": {
    "bar": "baz"
  }
}' | dasel -i json --root 'foo.bar = "bong"'
{
  "foo": {
    "bar": "bong"
  }
}
```

***

#### Comparison

| Mode     | Example Command                           | Output                                 |
| -------- | ----------------------------------------- | -------------------------------------- |
| Default  | `dasel -i json 'foo.bar = "bong"'`        | `"bong"` (final selector value)        |
| `--root` | `dasel -i json --root 'foo.bar = "bong"'` | Full document with updated `"foo.bar"` |


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://daseldocs.tomwright.me/input-output/modifying-data.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.