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bootup(7) [opendarwin man page]

BOOTUP(7)							      bootup								 BOOTUP(7)

bootup - System bootup process DESCRIPTION
A number of different components are involved in the system boot. Immediately after power-up, the system BIOS will do minimal hardware initialization, and hand control over to a boot loader stored on a persistent storage device. This boot loader will then invoke an OS kernel from disk (or the network). In the Linux case, this kernel (optionally) extracts and executes an initial RAM disk image (initrd), such as generated by dracut(8), which looks for the root file system (possibly using systemd(1) for this). 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 on the OS image, 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). 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 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. | v (various mounts and (various swap (various cryptsetup fsck services...) devices...) devices...) (various low-level (various low-level | | | services: udevd, API VFS mounts: v v v tmpfiles, random mqueue, configfs, seed, sysctl, ...) debugfs, ...) | | | | | \__________________|_________________ | ___________________|____________________/ |/ v | ____________________________________/|\________________________________________ / | | | | | | | | v v | v v (various (various | (various rescue.service timers...) paths...) | sockets...) | | | | | v v v | v | | | | | v \_________________ | ___________________/ |/ v | ____________________________________/| emergency.service / | | | | | | v v v v display- (various system (various system manager.service services services) | required for | | graphical UIs) v | | | | | \_________________ | _________________/ |/ 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. BOOTUP IN THE INITIAL RAM DISK (INITRD) 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. 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) : v | 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 | \______________________ | | v | v initrd-cleanup.service isolates to | v ______________________/| / v | initrd-udevadm-cleanup-db.service v | (custom initrd | services...) | \______________________ | | v | 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 | devices, ...) | | v v | | \_______ ______/ / v (various low-level services) | v | _____________________________________/ \_________________________________ / | | | | | | 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. SEE ALSO
systemd(1), boot(7), systemd.special(7),, dracut(8) systemd 237 BOOTUP(7)
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