BOOTUP(7)                           bootup                           BOOTUP(7)

       bootup - System bootup process

       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 usually implemented as an initramfs -- a compressed archive
       which is extracted when the kernel boots up into a lightweight
       in-memory file system based on tmpfs, but 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

       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 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

       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 grahpical 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 sesion)     v
                        |                    |             
                        v                    v

       The initial RAM disk implementation (initrd) can be set up using
       systemd as well. In this case, boot up inside the initrd follows the
       following structure.

       systemd detects that it is run within an initrd by checking for the
       file /etc/initrd-release. The default target in the initrd is The bootup process begins identical to the system
       manager bootup (see above) until it reaches From there,
       systemd approaches the special target Before any file
       systems are mounted, it must be determined whether the system will
       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
                                                    |          devices, ...)
                                                    |                |
                                                    v                v
                                                    |                |
                                                    \_______   ______/
                                                            \ /
                                                    (various low-level

                       _____________________________________/ \_________________________________
                      /                         |                        |                      \
                      |                         |                        |                      |
                      v                         v                        v                      v
           systemd-reboot.service   systemd-poweroff.service   systemd-halt.service   systemd-kexec.service
                      |                         |                        |                      |
                      v                         v                        v                      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.

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

        1. GRUB

systemd 245                                                          BOOTUP(7)
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