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# 1  
Old 08-04-2009
bootup sequence

What is the boot up sequence in UNIX?
# 2  
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.
# 3  
Old 08-05-2009
Adding on to what Corona said, many systems' init process will include a line to start a shell script. In Linux, this is:
Code:
si::sysinit:/etc/rc.d/rc.sysinit

In Solaris 10 this is:
Code:
smf::sysinit:/lib/svc/bin/svc.startd

Then there is the concept of "init level". This allows one to start the operating system with a different startup-configuration. If you start with init level 3 in Linux, the inittab checks for the line:
Code:
l3:3:wait:/etc/rc.d/rc 3

and then runs the script /etc/rc.d/rc with parameter "3". This rc script then in turn looks into the /etc/rc.d/rc3.d directory and runs all the scripts (in lexicographical order) starting with the letter S. (There are more steps here, but I'm giving you the basics).
 

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