.: Advanced Linux Networking :.

The preceding discussion has referred to the runlevel, but with only a minimal description of what it is. The runlevel and SysV startup scripts are tied together quite intimately. When the computer boots, it enters a specific runlevel. Ultimately, what this means is that the computer executes the startup scripts to which the links in the SysV script link directories point. More specifically, Linux passes the start parameter to the scripts whose links start with S, and passes the stop parameter to those whose links begin with K.

How does Linux know what runlevel to enter when it boots, though? That's set in the /etc/inittab file, which is the configuration file for init, which is the first process that Linux runs and the progenitor of all other processes. Specifically, /etc/inittab contains a line such as the following:

id:5:initdefault:

The leading id is the key to identifying this line, but the following number (5 in this example) is what you'll use to permanently set the runlevel. If you change this value and reboot, the computer will come up in a different runlevel. Runlevels 0, 1, and 6 have special meanings—they correspond to a system shutdown, a single-user mode, and a system reboot, respectively. Runlevels 2 through 5 are normal runlevels, but their precise meanings vary from one distribution to another. For Caldera, Red Hat, Mandrake, SuSE 7.3, and TurboLinux, runlevel 3 is a normal text-mode startup (X is not started), and runlevel 5 is a GUI login mode (X is started, and a GUI login program is run). Earlier versions of SuSE use runlevels 2 and 3 instead of 3 and 5, and Slackware uses 3 and 4 for text-mode and GUI logins. By default, Debian doesn't run any SysV scripts differently in different normal operational runlevels, although it runs fewer text-mode login tools in runlevels above 3 (this detail is handled by subsequent lines of /etc/inittab). Most distributions include a series of comments in their /etc/inittab files describing the function of each runlevel, so look for that if you need more information or are using a distribution I don't discuss in this book.

If you want to change the runlevel temporarily, you can do so with the telinit command (init also works on most systems). The syntax for this command is as follows:

telinit [-t seconds] [runlevel]

When you change runlevels, it's possible that certain processes will have to be killed. Linux can do this through either a SIGTERM or a SIGKILL signal. The former is a more polite way of doing the job; it lets the program close its own files and so on. SIGKILL, by contrast, unceremoniously kills the program, which can lead to file corruption. When you change runlevels, telinit first tries to use a SIGTERM. Five seconds later, if a process hasn't quit, telinit sends a SIGKILL. If specified, the -t seconds parameter changes this interval. In most cases, the default five seconds is adequate.

The runlevel specification is a single-character code for the runlevel. Most obviously, you can provide a runlevel number. There are certain other characters you can pass that have special meaning, however:

• a, b, or c— Some entries in /etc/inittab use runlevels of a, b, or c for special purposes. If you specify this option to telinit, the program only handles those /etc/inittab entries; the system's runlevel is not changed.
• Q or q— If you pass one of these values as the runlevel, telinit reexamines the /etc/inittab file and initiates any changes.
• S or s— This option causes the system to change into a single-user mode.
• U or u— This option causes the init process to restart itself, without reexamining the /etc/inittab file.

Why would you want to change the runlevel? Changing the default runlevel allows you to control which servers start up by default. On most distributions, the most important of these is the X server. This is usually started by a special line towards the end of /etc/inittab, not through a SysV startup script, although some use a SysV startup script for XDM to do the job. Changing the runlevel while the computer is running allows you to quickly enable or disable a set of servers, or change out of GUI mode if you want to temporarily shut down X when your system is configured to start it automatically.