Provided by: systemd_252.5-2ubuntu3_amd64 bug


       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

       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

       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

           (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),


        1. GRUB