noble (7) bootup.7.gz

Provided by: systemd_255.4-1ubuntu8.5_amd64 bug

NAME

       bootup - System bootup process

DESCRIPTION

       A number of different components are involved in the boot of a Linux system. Immediately after power-up,
       the system firmware will do minimal hardware initialization, and hand control over to a boot loader (e.g.
       systemd-boot(7) or GRUB[1]) stored on a persistent storage device. This boot loader will then invoke an
       OS kernel from disk (or the network). On systems using EFI or other types of firmware, this firmware may
       also load the kernel directly.

       The kernel (optionally) mounts an in-memory file system, often generated by dracut(8), which looks for
       the root file system. Nowadays this is implemented as an "initramfs" — a compressed CPIO archive that the
       kernel extracts into a tmpfs. In the past normal file systems using an in-memory block device (ramdisk)
       were used, and the name "initrd" is still used to describe both concepts. It's the boot loader or the
       firmware that loads both the kernel and initrd/initramfs images into memory, but the kernel which
       interprets it as a file system.  systemd(1) may be used to manage services in the initrd, similarly to
       the real system.

       After the root file system is found and mounted, the initrd hands over control to the host's system
       manager (such as systemd(1)) stored in the root file system, which is then responsible for probing all
       remaining hardware, mounting all necessary file systems and spawning all configured services.

       On shutdown, the system manager stops all services, unmounts all file systems (detaching the storage
       technologies backing them), and then (optionally) jumps back into the initrd code which unmounts/detaches
       the root file system and the storage it resides on. As a last step, the system is powered down.

       Additional information about the system boot process may be found in boot(7).

SYSTEM MANAGER BOOTUP

       At boot, the system manager on the OS image is responsible for initializing the required file systems,
       services and drivers that are necessary for operation of the system. On systemd(1) systems, this process
       is split up in various discrete steps which are exposed as target units. (See systemd.target(5) for
       detailed information about target units.) The boot-up process is highly parallelized so that the order in
       which specific target units are reached is not deterministic, but still adheres to a limited amount of
       ordering structure.

       When systemd starts up the system, it will activate all units that are dependencies of default.target (as
       well as recursively all dependencies of these dependencies). Usually, default.target is simply an alias
       of graphical.target or multi-user.target, depending on whether the system is configured for a graphical
       UI or only for a text console. To enforce minimal ordering between the units pulled in, a number of
       well-known target units are available, as listed on systemd.special(7).

       The following chart is a structural overview of these well-known units and their position in the boot-up
       logic. The arrows describe which units are pulled in and ordered before which other units. Units near the
       top are started before units nearer to the bottom of the chart.

                                        cryptsetup-pre.target veritysetup-pre.target
                                                             |
           (various low-level                                v
            API VFS mounts:             (various cryptsetup/veritysetup devices...)
            mqueue, configfs,                                |    |
            debugfs, ...)                                    v    |
            |                                  cryptsetup.target  |
            |  (various swap                                 |    |    remote-fs-pre.target
            |   devices...)                                  |    |     |        |
            |    |                                           |    |     |        v
            |    v                       local-fs-pre.target |    |     |  (network file systems)
            |  swap.target                       |           |    v     v                 |
            |    |                               v           |  remote-cryptsetup.target  |
            |    |  (various low-level  (various mounts and  |  remote-veritysetup.target |
            |    |   services: udevd,    fsck services...)   |             |              |
            |    |   tmpfiles, random            |           |             |    remote-fs.target
            |    |   seed, sysctl, ...)          v           |             |              |
            |    |      |                 local-fs.target    |             | _____________/
            |    |      |                        |           |             |/
            \____|______|_______________   ______|___________/             |
                                        \ /                                |
                                         v                                 |
                                  sysinit.target                           |
                                         |                                 |
                  ______________________/|\_____________________           |
                 /              |        |      |               \          |
                 |              |        |      |               |          |
                 v              v        |      v               |          |
            (various       (various      |  (various            |          |
             timers...)      paths...)   |   sockets...)        |          |
                 |              |        |      |               |          |
                 v              v        |      v               |          |
           timers.target  paths.target   |  sockets.target      |          |
                 |              |        |      |               v          |
                 v              \_______ | _____/         rescue.service   |
                                        \|/                     |          |
                                         v                      v          |
                                     basic.target         rescue.target    |
                                         |                                 |
                                 ________v____________________             |
                                /              |              \            |
                                |              |              |            |
                                v              v              v            |
                            display-    (various system   (various system  |
                        manager.service     services        services)      |
                                |         required for        |            |
                                |        graphical UIs)       v            v
                                |              |            multi-user.target
           emergency.service    |              |              |
                   |            \_____________ | _____________/
                   v                          \|/
           emergency.target                    v
                                         graphical.target

       Target units that are commonly used as boot targets are emphasized. These units are good choices as goal
       targets, for example by passing them to the systemd.unit= kernel command line option (see systemd(1)) or
       by symlinking default.target to them.

       timers.target is pulled-in by basic.target asynchronously. This allows timers units to depend on services
       which become only available later in boot.

USER MANAGER STARTUP

       The system manager starts the user@uid.service unit for each user, which launches a separate unprivileged
       instance of systemd for each user — the user manager. Similarly to the system manager, the user manager
       starts units which are pulled in by default.target. The following chart is a structural overview of the
       well-known user units. For non-graphical sessions, default.target is used. Whenever the user logs into a
       graphical session, the login manager will start the graphical-session.target target that is used to pull
       in units required for the graphical session. A number of targets (shown on the right side) are started
       when specific hardware is available to the user.

              (various           (various         (various
               timers...)         paths...)        sockets...)    (sound devices)
                   |                  |                 |               |
                   v                  v                 v               v
             timers.target      paths.target     sockets.target    sound.target
                   |                  |                 |
                   \______________   _|_________________/         (bluetooth devices)
                                  \ /                                   |
                                   V                                    v
                             basic.target                          bluetooth.target
                                   |
                        __________/ \_______                      (smartcard devices)
                       /                    \                           |
                       |                    |                           v
                       |                    v                      smartcard.target
                       v            graphical-session-pre.target
           (various user services)          |                       (printers)
                       |                    v                           |
                       |       (services for the graphical session)     v
                       |                    |                       printer.target
                       v                    v
                default.target      graphical-session.target

BOOTUP IN THE INITRD

       Systemd can be used in the initrd as well. It detects the initrd environment by checking for the
       /etc/initrd-release file. The default target in the initrd is initrd.target. The bootup process is
       identical to the system manager bootup until the target basic.target. After that, systemd executes the
       special target initrd.target. Before any file systems are mounted, the manager will determine whether the
       system shall resume from hibernation or proceed with normal boot. This is accomplished by
       systemd-hibernate-resume.service which must be finished before local-fs-pre.target, so no filesystems can
       be mounted before the check is complete. When the root device becomes available,
       initrd-root-device.target is reached. If the root device can be mounted at /sysroot, the sysroot.mount
       unit becomes active and initrd-root-fs.target is reached. The service initrd-parse-etc.service scans
       /sysroot/etc/fstab for a possible /usr/ mount point and additional entries marked with the x-initrd.mount
       option. All entries found are mounted below /sysroot, and initrd-fs.target is reached. The service
       initrd-cleanup.service isolates to the initrd-switch-root.target, where cleanup services can run. As the
       very last step, the initrd-switch-root.service is activated, which will cause the system to switch its
       root to /sysroot.

                                          : (beginning identical to above)
                                          :
                                          v
                                    basic.target
                                          |                       emergency.service
                   ______________________/|                               |
                  /                       |                               v
                  |            initrd-root-device.target          emergency.target
                  |                       |
                  |                       v
                  |                  sysroot.mount
                  |                       |
                  |                       v
                  |             initrd-root-fs.target
                  |                       |
                  |                       v
                  v            initrd-parse-etc.service
           (custom initrd                 |
            services...)                  v
                  |            (sysroot-usr.mount and
                  |             various mounts marked
                  |               with fstab option
                  |              x-initrd.mount...)
                  |                       |
                  |                       v
                  |                initrd-fs.target
                  \______________________ |
                                         \|
                                          v
                                     initrd.target
                                          |
                                          v
                                initrd-cleanup.service
                                     isolates to
                               initrd-switch-root.target
                                          |
                                          v
                   ______________________/|
                  /                       v
                  |        initrd-udevadm-cleanup-db.service
                  v                       |
           (custom initrd                 |
            services...)                  |
                  \______________________ |
                                         \|
                                          v
                              initrd-switch-root.target
                                          |
                                          v
                              initrd-switch-root.service
                                          |
                                          v
                                Transition to Host OS

SYSTEM MANAGER SHUTDOWN

       System shutdown with systemd also consists of various target units with some minimal ordering structure
       applied:

                                  (conflicts with  (conflicts with
                                     all system     all file system
                                      services)     mounts, swaps,
                                          |           cryptsetup/
                                          |           veritysetup
                                          |          devices, ...)
                                          |                |
                                          v                v
                                   shutdown.target    umount.target
                                          |                |
                                          \_______   ______/
                                                  \ /
                                                   v
                                          (various low-level
                                               services)
                                                   |
                                                   v
                                             final.target
                                                   |
                       ___________________________/ \_________________
                      /               |               |               \
                      |               |               |               |
                      v               |               |               |
           systemd-reboot.service     |               |               |
                      |               v               |               |
                      |    systemd-poweroff.service   |               |
                      v               |               v               |
                reboot.target         |      systemd-halt.service     |
                                      v               |               v
                              poweroff.target         |    systemd-kexec.service
                                                      v               |
                                                 halt.target          |
                                                                      v
                                                                kexec.target

       Commonly used system shutdown targets are emphasized.

       Note that systemd-halt.service(8), systemd-reboot.service, systemd-poweroff.service and
       systemd-kexec.service will transition the system and server manager (PID 1) into the second phase of
       system shutdown (implemented in the systemd-shutdown binary), which will unmount any remaining file
       systems, kill any remaining processes and release any other remaining resources, in a simple and robust
       fashion, without taking any service or unit concept into account anymore. At that point, regular
       applications and resources are generally terminated and released already, the second phase hence operates
       only as safety net for everything that couldn't be stopped or released for some reason during the
       primary, unit-based shutdown phase described above.

SEE ALSO

       systemd(1), boot(7), systemd.special(7), systemd.target(5), systemd-halt.service(8), dracut(8)

NOTES

        1. GRUB
           https://www.gnu.org/software/grub/