> ## Documentation Index
> Fetch the complete documentation index at: https://docs.quadrillion.io/llms.txt
> Use this file to discover all available pages before exploring further.

# Kernel Toolbar

> Control code execution with the kernel picker and toolbar actions.

Every notebook has a kernel — the runtime that executes your code. The kernel toolbar at the top of each notebook lets you select, control, and monitor the kernel.

## The kernel picker

Click the **kernel button** (shows the current kernel name) to open the picker.

### Kernel information

The button displays:

* **Status indicator**: Colored dot showing kernel state
* **Display name**: Language and environment (e.g., "Python 3.11 | /usr/local/bin/python")

### Selecting a kernel

The picker groups available kernels by language:

* **Python**: All Python installations and virtual environments
* **R**: R installations
* **Julia**: Julia installations

Each entry shows the language version and environment path. Click to switch kernels.

<Info>
  Up to five kernels show per group. Click **See more kernels** to view all available options.
</Info>

### Active kernels

If a kernel is already running, it appears separately under **Active** — you can attach to an existing kernel session instead of starting a new one.

## Status indicators

The kernel status dot changes color:

| Color     | State   | Meaning                  |
| --------- | ------- | ------------------------ |
| **Gray**  | Idle    | Ready to execute         |
| **Blue**  | Info    | Starting up              |
| **Green** | Success | Execution completed      |
| **Red**   | Error   | Kernel crashed or failed |

When code is running, a spinner appears in the toolbar.

## Toolbar actions

The toolbar provides buttons for common kernel operations:

### Execution control

| Action                | Description                                  |
| --------------------- | -------------------------------------------- |
| **Interrupt**         | Stop the currently running cell              |
| **Run all**           | Run all cells from top to bottom             |
| **Run up to current** | Run cells from top through the selected cell |
| **Clear outputs**     | Remove all cell outputs                      |

### Kernel management

| Action       | Description                       |
| ------------ | --------------------------------- |
| **Restart**  | Restart the kernel (clears state) |
| **Shutdown** | Stop the kernel completely        |

### Combined actions

Some actions combine restart with execution:

* **Restart and run all**: Fresh kernel, then run everything
* **Restart and run up to current**: Fresh kernel, then run to current cell
* **Restart and clear**: Fresh kernel, clear all outputs

## Execution indicator

When cells are running:

* A **spinner** appears in the toolbar
* Click the spinner to **scroll to the executing cell**
* The spinner persists until execution completes

This helps you track long-running computations — click to jump back to where the action is.

## Default kernel

Set your preferred default kernel in [Settings > General](/settings/general):

1. Go to **Settings**
2. Under **Default Python Environment**, select your preferred Python
3. New notebooks use this kernel automatically

Qualia validates that the selected kernel exists. If it's missing (e.g., a virtual environment was deleted), you'll see a warning.

## Responsive layout

When the window is narrow, toolbar actions collapse into a **More** menu (⋮). All the same actions are available — just fewer visible buttons.

## Keyboard shortcuts

| Shortcut                 | Action           |
| ------------------------ | ---------------- |
| **Cmd/Ctrl+I**           | Interrupt kernel |
| **Cmd/Ctrl+.**           | Restart kernel   |
| **Cmd/Ctrl+Shift+Enter** | Run all cells    |

## Troubleshooting

### Kernel not starting

* Check that the Python/R/Julia installation exists
* Verify virtual environments are activated correctly
* Look at the error message in the status area

### Kernel keeps dying

* Memory issues can crash kernels — check your system resources
* Some packages have installation problems — try in a fresh environment
* The kernel log may have more details

### Wrong environment

* Verify the kernel picker shows the expected path
* Check that packages are installed in that environment
* The environment shown in the picker should match where you installed dependencies