BOOTUP(7)
💖 BOOTUP(7) — bootup 💖
okay bestie, buckle UP, because this is bootup — the manual page about, like, the entire journey your Linux box goes on from "power button pressed" to "you have a desktop." it's giving ✨main character energy✨ for your whole system. no cap.
✨ NAME ✨
bootup — System bootup process
🌸 DESCRIPTION 🌸
so like. a lot of different components are involved in booting a Linux system, and they all have to pass the vibe check in order. here's the tea:
Immediately after power-up, the system firmware does a lil minimal hardware initialization, then hands control over to a boot loader (e.g. systemd-boot(7) or GRUB[1]) that's chilling on a persistent storage device. That boot loader then invokes an OS kernel from disk (or the network). On systems using EFI or other types of firmware, the firmware itself may also load the kernel directly. iconic of it tbh.
The kernel then (optionally) mounts an in-memory file system, which goes looking 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. (yes bestie, terminology is messy, we've all been there 🙈). It's the boot loader or the firmware that loads both the kernel and the initrd/initramfs images into memory, but it's the kernel that 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. slay.
On shutdown, the system manager stops all services, unmounts all non-busy file systems (detaching the storage technologies backing them), and then (optionally) jumps into the exitrd. The exitrd is backed by tmpfs and unmounts/detaches the remaining file systems, including the real root. As a last step, the system is powered down. 💀 goodbye queen.
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 into various discrete steps which are exposed as target units. (See systemd.target(5) for detailed information about target units — iykyk.)
The boot-up process is highly parallelized, so the order in which specific target units are reached is not deterministic, but it still adheres to a limited amount of ordering structure. it's chaos but make it organized 💅
When systemd starts up the system, it will activate all units that are dependencies of default.target (as well as recursively all dependencies of these dependencies). Usually, default.target is simply an alias of graphical.target or multi-user.target, 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. 🦋
cryptsetup-pre.target veritysetup-pre.target
|
(various low-level v
API VFS mounts: (various cryptsetup/veritysetup devices...)
mqueue, configfs, | |
debugfs, ...) v |
| cryptsetup.target |
| (various swap | | remote-fs-pre.target
| devices...) | | | |
| | | | | v
| v local-fs-pre.target | | | (network file systems)
| swap.target | | v v |
| | v | remote-cryptsetup.target |
| | (various low-level (various mounts and | remote-veritysetup.target |
| | services: udevd, fsck services...) | | |
| | tmpfiles, random | | | remote-fs.target
| | seed, sysctl, ...) v | | |
| | | local-fs.target | | _____________/
| | | | | |/
\____|______|_______________ ______|___________/ |
\ / |
v |
sysinit.target |
| |
______________________/|\_____________________ |
/ | | | \ |
| | | | | |
v v | v | |
(various (various | (various | |
timers...) paths...) | sockets...) | |
| | | | | |
v v | v | |
timers.target paths.target | sockets.target | |
| | | | v |
v \_______ | _____/ rescue.service |
\|/ | |
v v |
basic.target rescue.target |
| |
________v____________________ |
/ | \ |
| | | |
v v v |
display- (various system (various system |
manager.service services services) |
| required for | |
| graphical UIs) v v
| | multi-user.target
emergency.service | | |
| \_____________ | _____________/
v \|/
emergency.target v
graphical.targetTarget 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 default.target to them.
timers.target is pulled-in by basic.target asynchronously. This allows timers units to depend on services which become only available later in boot. (it's the "I'll get there when I get there" of boot targets 🫶)
🎀 USER MANAGER STARTUP 🎀
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 default.target.
The following chart is a structural overview of the well-known user units. For non-graphical sessions, default.target is used. Whenever the user logs into a graphical session, the login manager will start the graphical-session.target 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
timers.target paths.target sockets.target sound.target
| | |
\______________ _|_________________/ (bluetooth devices)
\ / |
V v
basic.target bluetooth.target
|
__________/ \_______ (smartcard devices)
/ \ |
| | v
| v smartcard.target
v graphical-session-pre.target
(various user services) | (printers)
| v |
| (services for the graphical session) v
| | printer.target
v v
default.target graphical-session.target🦋 BOOTUP IN THE INITRD 🦋
okay so plot twist bestie — systemd can hang out in the initrd too 💅 It figures out it's in an initrd by checking for the /etc/initrd-release file. The default target there is initrd.target.
The bootup process is literally identical to the system manager bootup up until the target basic.target. After that, systemd runs the special target initrd.target.
Before any file systems get mounted, the manager decides whether the system is gonna resume from hibernation or just proceed with a normal boot. This is handled by systemd-hibernate-resume.service, which must be finished before local-fs-pre.target — so no filesystems can be mounted before the check is complete. No cap. 🫶
When the root device becomes available, initrd-root-device.target is reached. If the root device can be mounted at /sysroot, the sysroot.mount unit becomes active and initrd-root-fs.target is reached.
Then initrd-parse-etc.service scans /sysroot/etc/fstab for a possible /usr/ mount point and any extra entries marked with the x-initrd.mount option. Everything it finds gets mounted below /sysroot, and initrd-fs.target is reached.
Then initrd-cleanup.service isolates to initrd-switch-root.target, where cleanup services can run. And as the very last step, initrd-switch-root.service is activated, which causes the system to switch its root to /sysroot. slay. ✨
: (beginning identical to above)
:
v
basic.target
| emergency.service
______________________/| |
/ | v
| initrd-root-device.target emergency.target
| |
| v
| sysroot.mount
| |
| v
| initrd-root-fs.target
| |
| v
v initrd-parse-etc.service
(custom initrd |
services...) v
| (sysroot-usr.mount and
| various mounts marked
| with fstab option
| x-initrd.mount...)
| |
| v
| initrd-fs.target
\______________________ |
\|
v
initrd.target
|
v
initrd-cleanup.service
isolates to
initrd-switch-root.target
|
v
______________________/|
/ v
| initrd-udevadm-cleanup-db.service
v |
(custom initrd |
services...) |
\______________________ |
\|
v
initrd-switch-root.target
|
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. It's giving ✨organized chaos✨
(conflicts with (conflicts with
all system all file system
services) mounts, swaps,
| cryptsetup/
| veritysetup
| devices, ...)
| |
v v
shutdown.target umount.target
| |
\_______ ______/
\ /
v
(various low-level
services)
|
v
final.target
|
___________________________/ \_________________________________
/ | | | \
| | | | |
v | | | |
systemd-reboot.service | | | |
| v | | |
| systemd-poweroff.service | | |
v | v | |
reboot.target | systemd-halt.service | |
v | v |
poweroff.target | systemd-kexec.service |
v | |
halt.target | systemd-soft-reboot.service
v |
kexec.target |
v
soft-reboot.targetThe system shutdown targets you'll actually see in the wild are the emphasized ones, iykyk. 👑
Note that systemd-halt.service(8), systemd-reboot.service, systemd-poweroff.service and systemd-kexec.service will transition the system and the server manager (PID 1) into the second phase of system shutdown (implemented in the systemd-shutdown binary). That phase unmounts any remaining file systems, kills any remaining processes, and releases 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 already terminated and released, so the second phase just operates as a safety net for everything that couldn't be stopped or released for some reason during the primary, unit-based shutdown phase described above. She's the backup plan, fr fr. 🫶
🌈 THE EXITRD 🌈
The "exitrd" is a concept symmetrical to the initrd — like its mirror twin. 🪞
When the system manager is shutting down and /run/initramfs/shutdown exists, it will switch root to /run/initramfs/ and execute /shutdown.
That program runs from the tmpfs mounted on /run/, so it can unmount the old root file system and perform additional steps — for example dismantle complex storage or perform additional logging about the shutdown. iconic. 💅
🎀 SEE ALSO 🎀
systemd(1), boot(7), systemd.special(7), systemd.target(5), systemd-halt.service(8), systemd-soft-reboot.service(8)
⭐ NOTES ⭐
and that's the whole page bestie, we ate and left no crumbs 💖 this was systemd 262.1's BOOTUP(7) — go forth and shut down your system correctly, you absolute main character 🌸🦋
📦 where this page comes from
- arch rolling: systemd 262.1-1 · LGPL-2.1-or-later AND CC0-1.0 AND GPL-2.0-or-later AND MIT-0
- 💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on : a language model's rewrite of the original, under its terms