How do I restore ChromeOS after it has been removed?

The following material is sourced from a Chromebook Help article titled "Use the Chromebook Recovery Utility" and another Chromebook Help article titled "Recover your Chromebook". You may read these articles and follow the instructions output by the script, but a detailed guide is provided below for further clarity.

Summary

1. Download the Recovery Tool specified at this link: https://dl.google.com/dl/edgedl/chromeos/recovery/linux_recovery.sh
2. Change the script permissions to allow execution with the following command: $ sudo chmod 755 linux_recovery.sh
3. Run the script with root privileges with the following command: $ sudo bash linux_recovery.sh
4. Follow the on-screen instructions.
5. Reboot as detailed here.

Detailed Guide

A ChromeOS recovery drive must be created, which is then used to restore ChromeOS on boot. Google provides a script to download the disk image and write it to a drive (this is a destructive process that will wipe the drive being used). The disk image can be written to a flash drive connected via USB or an SD card connected via the SD card reader.

The following commands exemplify the process of using the recovery script:

$ wget https://dl.google.com/dl/edgedl/chromeos/recovery/linux_recovery.sh
$ chmod 755 ./linux_recovery.sh
$ sudo ./linux_recovery.sh

You will see a prompt as such:

======================================================================
This tool is in maintenance mode.
Try the new Chromebook Recovery Utility on Chrome OS, Windows, or Mac.
For more information, visit http://www.google.com/chromeos/recovery.
======================================================================

Working in /tmp/tmp.crosrec/
Downloading config file from https://dl.google.com/dl/edgedl/chromeos/recovery/recovery.conf

If you know the Model string displayed at the recovery screen,
type some or all of it; otherwise just press Enter:

You can enter your model string at the prompt, or simply type "samus" (for the Pixel 2) and press Enter.

If you know the Model string displayed at the recovery screen,
type some or all of it; otherwise just press Enter: samus

This may take a few minutes to print the full list...

There are up to 84 recovery images to choose from:

0 - 
49 - Google Chromebook Pixel (2015)
  channel:  stable-channel
  pattern:   ^SAMUS .*

Please select a recovery image to download: 

Enter the number listed for the channel you want. The script will download the image and ask to which drive the image should be written:

Please select a recovery image to download: 49

Downloading image zipfile from https://dl.google.com/dl/edgedl/chromeos/recovery/chromeos_7647.84.0_samus_recovery_stable-channel_mp.bin.zip

  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  553M  100  553M    0     0  4650k      0  0:02:01  0:02:01 --:--:-- 4524k
Unpacking the zipfile
Archive:  chromeos_7647.84.0_samus_recovery_stable-channel_mp.bin.zip
  inflating: chromeos_7647.84.0_samus_recovery_stable-channel_mp.bin  


I found 2 USB drives.  We need one with at least 1462MB capacity.

0 - 

1 - Use /dev/sdb 31914MB Generic  Power Saving USB

2 - Use /dev/sdc 8103MB          Rocketfish      


Tell me what to do (or just press Enter to scan again): 

Enter the number of the chosen detected drive, press Enter, and after two confirmation queries, the image will be copied:

Tell me what to do (or just press Enter to scan again): 2

Is this the device you want to put the recovery image on?

  /dev/sdc 8103MB          Rocketfish      

You must enter 'YES' (all uppercase) to continue: YES


I'm really going to erase this device. This will permanently ERASE
whatever you may have on that drive. You won't be able to undo it.

  /dev/sdc 8103MB          Rocketfish      

If you're sure that's correct, enter 'DoIt' now (case is important): DoIt


Installing the recovery image


unmounting...
copying... (this may take several minutes)
365+1 records in
366+0 records out
1535115264 bytes (1.5 GB) copied, 120.582 s, 12.7 MB/s


Done. Remove the USB drive and insert it in your Chrome notebook.


Shall I remove the temporary files now? [y/n] y

Reboot to restore

With the recovery media inserted, follow these steps to recover/restore ChromeOS:

1. Press and hold the Esc + Refresh keyboard icon keys.
2. Press the Power button, then let go.
3. Continue holding the Esc + Refresh keyboard icon keys until the “Chrome OS is missing or damaged” message appears, then let go.