I just read in Linux for Dummies that the runlevel used for booting into the GUI is 5 and that used for booting to a command line is 3. But then I looked at my inittab file and saw some remarks indicating that the runlevel number controls halting and rebooting and whether the session (wrong word?) is multiuser or single-user. Is the book wrong?

Runlevels were SYSVs Unix's way of providing different services in a somewhat layered fashion. Consider run level 1 being single user, level 2 being run level 1 + some networking, run level 3 being full multi-user, and often run level 6 being reboot. Run levels 4, and 5 were rarely used.

Today, run level 5 is generally the same as 3, except the TTY login is replaced by a GUI login (eg. GDM). Run level 4 still isn't used much. Run level 0 was halt. Some distros use a single run level (3 or 5 only) for multiuser.

When the kernel finishes booting it runs the init program and passes the run level as a parameter to init. All of the possible things that init can do are described in a file called /etc/inittab. init looks up the run level it has been passed in /etc/inittab and runs the scripts that correspond to that run level.

The scripts can be anything. Each distribution creates whatever init scripts it feels like and describes them as run levels in /etc/inittab. What the book describes as run levels 3 and 5 is how they are defined in Red Hat, Fedora, and CentOS. Almost all distributions also have run levels for halting and rebooting and whether the session is multiuser or single-user. Debian also has a runlevel for power failures.

The book is not wrong. It is just describing 2 of the several run levels found in Red Hat.

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Steve Stites

Thank you, Jailbait. That answered my question.

Slackware still uses runlevel 4 as the default to start X instead of 5. 5 is treated like runlevel 3 (multiuser).