bootup(7) — Linux manual page


BOOTUP(7)                        bootup                        BOOTUP(7)

NAME         top

       bootup - System bootup process

DESCRIPTION         top

       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

       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
       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).


       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 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 (as well as recursively
       all dependencies of these dependencies). Usually,
       is simply an alias of or,
       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.

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

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


       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 The following chart is a structural overview of
       the well-known user units. For non-graphical sessions, is used. Whenever the user logs into a graphical
       session, the login manager will start the 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
                   |                  |                 |
                   \______________   _|_________________/         (bluetooth devices)
                                  \ /                                   |
                                   V                                    v
                        __________/ \_______                      (smartcard devices)
                       /                    \                           |
                       |                    |                           v
                       |                    v            
           (various user services)          |                       (printers)
                       |                    v                           |
                       |       (services for the graphical session)     v
                       |                    |             
                       v                    v


       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 The bootup process
       is identical to the system manager bootup until the target After that, systemd executes the special 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, so no filesystems can be mounted before the
       check is complete. When the root device becomes available, is reached. If the root device can be
       mounted at /sysroot, the sysroot.mount unit becomes active and 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 is reached. The service
       initrd-cleanup.service isolates to the,
       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)
                                          |                       emergency.service
                   ______________________/|                               |
                  /                       |                               v
                  |                       |
                  |                       v
                  |                  sysroot.mount
                  |                       |
                  |                       v
                  |                       |
                  |                       v
                  v            initrd-parse-etc.service
           (custom initrd                 |
            services...)                  v
                  |            (sysroot-usr.mount and
                  |             various mounts marked
                  |               with fstab option
                  |              x-initrd.mount...)
                  |                       |
                  |                       v
                  \______________________ |
                                     isolates to
                  /                       v
                  |        initrd-udevadm-cleanup-db.service
                  v                       |
           (custom initrd                 |
            services...)                  |
                  \______________________ |
                                Transition to Host OS


       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
                                          |                |
                                          \_______   ______/
                                                  \ /
                                          (various low-level
                       ___________________________/ \_________________________________
                      /               |               |               |               \
                      |               |               |               |               |
                      v               |               |               |               |
           systemd-reboot.service     |               |               |               |
                      |               v               |               |               |
                      |    systemd-poweroff.service   |               |               |
                      v               |               v               |               |
               |      systemd-halt.service     |               |
                                      v               |               v               |
                             |    systemd-kexec.service      |
                                                      v               |               |
                                                 |  systemd-soft-reboot.service
                                                                      v               |

       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         top

       systemd(1), boot(7), systemd.special(7),,
       systemd-halt.service(8), systemd-soft-reboot.service(8)

NOTES         top

        1. GRUB

COLOPHON         top

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systemd 257~devel                                              BOOTUP(7)

Pages that refer to this page: systemd(1)org.freedesktop.systemd1(5)systemd-system.conf(5)boot(7)dracut.bootup(7)systemd.directives(7)systemd.index(7)systemd.special(7)kernel-install(8)systemd-gpt-auto-generator(8)systemd-poweroff.service(8)systemd-soft-reboot.service(8)