“Linux is not like other operating systems such as MS-DOS and
Windows 9x. Linux is its own beast. Quite a few would call Linux a
UNIX clone, but I don’t think that is fair to Linux. Linux is much
more open and capable than most UNIX systems.”
“When Linux starts, it calls a program named init, which runs
all subsequent startup tasks. Those tasks include the multiuser
environment, networking, and services such as sendmail.”
“The run level is just that — the level of functionality in
which the operating system is running. The levels range from level
1 to level 6, and each level has a different degree of
functionality. The run levels are specified within the /etc/inittab
file. /etc/inittab is the master file in which the init program
looks to execute its subsequent services. The very first services
it executes are those located in the /etc/rc.d directory tree.”