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 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

       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               |

       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 252                                                                                                               BOOTUP(7)