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Full Discussion: bootup sequence
Top Forums UNIX for Dummies Questions & Answers bootup sequence Post 302340894 by Corona688 on Tuesday 4th of August 2009 03:32:20 PM
Old 08-04-2009
Since UNIX covers so many platforms and systems and varieties its hard to give a better than vague answer. But I will bite the bullet and be vague for you.
  • The system automatically loads the bootloader. This is a small program with only one job, to find and load the operating system kernel. On PC's its always in the first sector of the hard drive, or the 'boot' sector.
  • The bootloader loads the kernel into memory and runs it.
  • In linux at least, the first thing the kernel does is detect a whole slew of system parameters and devices. This is similar to the process of loading a driver, except the drivers are built in and just need to be operated.
  • Next, it attempts to mount the root filesystem, since it can't do anything without one.
  • Lastly the kernel creates process number one, init, which runs the processes to prepare the rest of the system and start up all the services its is configured to for its particular runlevel, including all terminal logins etc.
This is the simple way. Sometimes instead of a root filesystem, the bootloader gives the kernel a glob of memory containing files loaded from disk, which it may use to load extra drivers etc. before it mounts the real root filesystem overtop of it.

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SYSTEMD-FSTAB-GENERATOR(8)                                    systemd-fstab-generator                                   SYSTEMD-FSTAB-GENERATOR(8)

systemd-fstab-generator - Unit generator for /etc/fstab SYNOPSIS
/lib/systemd/system-generators/systemd-fstab-generator DESCRIPTION
systemd-fstab-generator is a generator that translates /etc/fstab (see fstab(5) for details) into native systemd units early at boot and when configuration of the system manager is reloaded. This will instantiate mount and swap units as necessary. The passno field is treated like a simple boolean, and the ordering information is discarded. However, if the root file system is checked, it is checked before all the other file systems. See systemd.mount(5) and systemd.swap(5) for more information about special /etc/fstab mount options this generator understands. One special topic is handling of symbolic links. Historical init implementations supported symlinks in /etc/fstab. Because mount units will refuse mounts where the target is a symbolic link, this generator will resolve any symlinks as far as possible when processing /etc/fstab in order to enhance backwards compatibility. If a symlink target does not exist at the time that this generator runs, it is assumed that the symlink target is the final target of the mount. systemd-fstab-generator implements systemd.generator(7). KERNEL COMMAND LINE
systemd-fstab-generator understands the following kernel command line parameters: fstab=, rd.fstab= Takes a boolean argument. Defaults to "yes". If "no", causes the generator to ignore any mounts or swap devices configured in /etc/fstab. rd.fstab= is honored only by the initial RAM disk (initrd) while fstab= is honored by both the main system and the initrd. root= Takes the root filesystem to mount in the initrd. root= is honored by the initrd. rootfstype= Takes the root filesystem type that will be passed to the mount command. rootfstype= is honored by the initrd. rootflags= Takes the root filesystem mount options to use. rootflags= is honored by the initrd. mount.usr= Takes the /usr filesystem to be mounted by the initrd. If mount.usrfstype= or mount.usrflags= is set, then mount.usr= will default to the value set in root=. Otherwise, this parameter defaults to the /usr entry found in /etc/fstab on the root filesystem. mount.usr= is honored by the initrd. mount.usrfstype= Takes the /usr filesystem type that will be passed to the mount command. If mount.usr= or mount.usrflags= is set, then mount.usrfstype= will default to the value set in rootfstype=. Otherwise, this value will be read from the /usr entry in /etc/fstab on the root filesystem. mount.usrfstype= is honored by the initrd. mount.usrflags= Takes the /usr filesystem mount options to use. If mount.usr= or mount.usrfstype= is set, then mount.usrflags= will default to the value set in rootflags=. Otherwise, this value will be read from the /usr entry in /etc/fstab on the root filesystem. mount.usrflags= is honored by the initrd. systemd.volatile= Controls whether the system shall boot up in volatile mode. Takes a boolean argument or the special value state. If false (the default), this generator makes no changes to the mount tree and the system is booted up in normal mode. If true the generator ensures systemd-volatile-root.service(8) is run as part of the initial RAM disk ("initrd"). This service changes the mount table before transitioning to the host system, so that a volatile memory file system ("tmpfs") is used as root directory, with only /usr mounted into it from the configured root file system, in read-only mode. This way the system operates in fully stateless mode, with all configuration and state reset at boot and lost at shutdown, as /etc and /var will be served from the (initially unpopulated) volatile memory file system. If set to state the generator will leave the root directory mount point unaltered, however will mount a "tmpfs" file system to /var. In this mode the normal system configuration (i.e. the contents of "/etc") is in effect (and may be modified during system runtime), however the system state (i.e. the contents of "/var") is reset at boot and lost at shutdown. Note that in none of these modes the root directory, /etc, /var or any other resources stored in the root file system are physically removed. It's thus safe to boot a system that is normally operated in non-volatile mode temporarily into volatile mode, without losing data. Note that enabling this setting will only work correctly on operating systems that can boot up with only /usr mounted, and are able to automatically populate /etc, and also /var in case of "systemd.volatile=yes". SEE ALSO
systemd(1), fstab(5), systemd.mount(5), systemd.swap(5), systemd-cryptsetup-generator(8), kernel-command-line(7) systemd 237 SYSTEMD-FSTAB-GENERATOR(8)

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