UNIX for Dummies Questions & Answers

What is the boot up sequence in UNIX?

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.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.

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).