There are four steps to linux boot.

1. BIOS-UEFI Boot
2. Bootloader
3. Kernel Boot
4. Userspace Startup

The BIOS is not part of Linux. It is a chip on the motherboard of the computer. UEFI is the name for the new improved BIOS. As of 2020, all new computers are manufactured with UEFI instead of BIOS, but people still tend to use the term BIOS out of habit. Here we will use the term BIOS-UEFI.

The BIOS-UEFI is a chip with firmware burned into it. As soon as your computer receives power, it loads the code from the BIOS-UEFI into the CPU and passes control to it.

The BIOS-UEFI program will do the following.

1. Power-On System Test (POST)
2. BIOS-UEFI Settings, read and change if requested
3. Find and launch the bootloader.

Check for the CPU, check for the L1 Cache memory.

These are simple tests. It cannot do more thorough tests for disks and network cards because those things are driven by the operating system, and the operating system isn't running yet.

Hold down keys while booting

• F2 - open BIOS-UEFI settings
• Alt+F10 - reset PC
• F12 - show menu of bootloader

Check the keyboard to see if the user is holding down the F2 key. If so, run the BIOS-UEFI Settings code. This is menu of seven or eight forms where the user can view and change the settings.

The settings are stored in the CMOS.

Complimentary Metal Oxide Semiconductor (CMOS). The CMOS is powered by a battery so the settings are maintained regardeless of whether the computer is plugged in or not.

Among the settings:

• boot disk order.

The final step for the BIOS-UEFI is to launch the bootloader program. Before it can do that it must find it. The bootloader is a program file somewhere on a disk. It might be on a hard disk, a cdrom, or a usb-stick. So the BIOS-UEFI must have enough knowledge of the disk, partition, and file system to find the program. And that brings us to the difference between BIOS and UEFI.

In the original BIOS, a boot disk would have a Master Boot Record (MBR) in sector 0. The MBR was 512 bytes long and it contained some bootstrap code and a partition table that could delineate up to four primary partitions on the disk. The was enough file system information that the bootstrap code could find the bootloader program on the disk and execute it.

Disks larger than 2 TB cannot be described by such a small partition table. Modern disks have a GUID Partition Table (GPT).

Manufacturers looked for ways to extend the capabilities to support larger disks and larger bootloaders. Intel developed Extensible Firmware Interface (EFI). This was take over by an industry group and renamed Universial EFI (UEFI).

In Linux, the bootloader could be called the kernel loader, because that's what it does. It loads the kernel into memory and passes control to it.

The bootloader shipped with Ubuntu and installed by default is a program named Grub. Like everything in Linux, there are a dozen other alternative bootloader programs, such as coreboot, Das U-Boot, and LILO.

The bootloader is shipped and installed with the operating system, but technically it's not part of the operating system. A bootloader from one operating system can load a different operating system. Oftentimes a Linux bootloader can start Windows, and a Windows bootloader can start Linux. This makes dual-boot installations possible.

The central part of the kernel is stored as a single image file. The bootloader copies this image into memory where it will stay permanently.

Additionally, there are kernel modules which can be loaded into memory as needed. The bootloader loads a subset of these kernel modules.

Now the bootloader program passes control to the kernel, and Linux is up and running. The kernel starts up his processes and when ready it passes control to the init process also known as userspace startup.

The init process starts all of the processes running in userspace.

The complete story is told here: Startup.

GUI - settings - about, shows Gnome version 3.36.3

How is gnome started at startup? How can it be optional? GRUB, rcconf, lightdm

https://askubuntu.com/questions/378046/how-to-run-ubuntu-xubuntu-in-a-complete-non-gui-mode

For Jetson Nano

Boot to console by default

Optionally boot to graphical

The disable didn’t work because the Debian /etc/X11/default-display-manager logic is winding up overriding it.

In order to make text boot the default under systemd (regardless of which distro, really):

systemctl set-default multi-user.target

To change back to booting to the GUI,

systemctl set-default graphical.target

Goals

X boot judson without gui.

X Start gui on judson at will.

X Shutdown gui on judson at will.

Start python webserver on judson at boot.

X Connect to judson webserver from phone.

Start skateboard program at boot.

Connect to Tello at boot.

Let webmon talk to skateboard. Can they share data files?

Judson is normally running in multi-user.target, no GUI

$ sudo systemctl isolate graphical.target    ===== open the GUI =====
$ sudo systemctl isolate multi-user.target   ===== close the GUI =====
Ctrl+Alt+F1   ===== get a tty login screen =====