# bootup

## 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, 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
           non-busy file systems (detaching the storage technologies backing
           them), and then (optionally) jumps into the exitrd. The exitrd is
           backed by tmpfs and unmounts/detaches the remaining file systems,
           including the real root. 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_          |  systemd-soft-reboot.service
                                                                          v               |
                                                                    _kexec.target_          |
                                                                                          v
                                                                                  _soft-reboot.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
           could not be stopped or released for some reason during the
           primary, unit-based shutdown phase described above.
    


## THE EXITRD



           The "exitrd" is a concept symmetrical to the initrd. When the
           system manager is shutting down and /run/initramfs/shutdown
           exists, it will switch root to /run/initramfs/ and execute
           /shutdown. This program runs from the tmpfs mounted on /run/, so
           it can unmount the old root file system and perform additional
           steps, for example dismantle complex storage or perform additional
           logging about the shutdown.
    


## NOTES



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


* * *