The runlevels are defined in a traditional Linux system in
/etc/init.d. Some distributions use the traditional
/etc/inittab file to manage boot services. Ubuntu has not used this for several years since Upstart has been created and is used in the boot process, so by default the file does not exist. However, if you have used one in the past and want to create one in Ubuntu, it will be read and honored. Because it is not standard in Ubuntu, this book does not cover
Each runlevel tells the
init command what services to start or stop. Although runlevels might all have custom definitions, Ubuntu has adopted some standards:
Runlevel 0—Known as “halt,” this runlevel shuts down the system.
Runlevel 1—This is a special ...