The first step is booting the kernel. As described in the previous section, this can be done from floppy or hard drive. As the kernel loads into memory, it will print messages to the system console, but usually also saves them in the system log files as well. As root, you can always check the file /var/log/messages (which contains kernel messages emitted during runtime as well). The command dmesg prints out the last lines of the kernel message ring buffer; directly after booting, naturally, you will get the boot messages.

Next, the kernel reports which processors it has found and, because this output is from a system with two processors, how the processors will work together:

Then, it tells us which console font it has picked and which console type it has detected:

The kernel gathers information about the PCI bus and checks for any PCI cards present in the system:

tells you how much swap space the kernel has found. Among the further tasks performed during a typical boot are finding and configuring a parallel port (lp1), detecting and configuring the network card, and finally setting up the USB subsystem.

Once the device drivers are initialized, the kernel executes the program init, which is found in /etc, /bin, or /sbin (it's /sbin/init on most systems). init is a general-purpose program that spawns new processes and restarts certain programs when they exit. For example, each virtual console has a getty process running on it, started by init. Upon login, the getty process is replaced with another. After logging out, init starts a new getty process, allowing you to log in again.

A runlevel is a number or letter that specifies the current system state, as far as init is concerned. For example, when the system runlevel is changed to 3, all entries in /etc/inittab containing 3 in the column specifying the runlevels will be executed. Runlevels are a useful way to group entries in /etc/inittab together. For example, you might want to say that runlevel 1 executes only the bare minimum of configuration scripts, runlevel 2 executes everything in runlevel 1 plus networking configuration, runlevel 3 executes everything in levels 1 and 2 plus dial-in login access, and so on. Today, the Red Hat distribution is set up so that runlevel 5 automatically starts the X Window System graphical interface. The SuSE distribution does it at runlevel 3, and the Debian distribution does so at runlevel 2 — provided you have installed X.

Let's take a look at a sample /etc/inittab file:

In our sample file, we see first that the default runlevel is set to 3. The action field for this entry is initdefault, which causes the given runlevel to be set to the default. That's the runlevel normally used whenever the system boots. You can override the default with any level you want by running init manually (which you might do when debugging your configuration) and passing in the desired runlevel as an argument. For instance, the following command shuts down all current processes and starts runlevel 5 (warn all your users to log off before doing this!):

LILO can also boot in single-user mode (usually runlevel 1) — see Section 5.2.2.3 earlier in this chapter.

The next entry tells init to execute the script /etc/rc.d/rc.sysinit when the system boots. (The action field is sysinit, which specifies that this entry should be executed when init is first started at system boot.) This file is simply a shell script containing commands to handle basic system initialization; for example, swapping is enabled, filesystems are checked and mounted, and the system clock is synchronized with the CMOS clock. You can take a look at this file on your system; we'll be talking more about the commands contained therein in Chapter 6; sees Section 6.1 and Section 6.2. On other distributions, this file might be elsewhere. For example, on SuSE it is /etc/init.d/boot, which is also where it should be according to the Linux Filesystem Hierarchy Standard (FHS).

Next, we see that the system executes the script /etc/rc.d/rc when it enters any of the runlevels through 6, with the appropriate runlevel as an argument. rc is a generic startup script that executes other scripts as appropriate for that runlevel. The action field here is wait, which tells init to execute the given command, and to wait for it to complete execution before doing anything else.

Linux stores startup commands in files with rc in the name, using an old Unix convention. The commands do all the things necessary to have a fully functioning system, like starting the servers or daemons mentioned in Chapter 4. Thanks to these commands, the system comes up ready with logging facilities, mail, a web server, or whatever you installed and asked it to run. As explained in the previous section, the files are invoked from /etc/inittab. The commands are standard shell commands, and you can simply read the various rc files to see what they do.

In this section, we describe the structure of the rc files so that you can understand where everything starts, and so that you can start or stop servers manually in the rare case that they don't do what you want them to do. We'll use Red Hat as our model, but once you get the idea of what to look for, you can find the corresponding files on any Linux distribution. Red Hat is both a good and a bad example because it violates the FHS. The Linux FHS is a distribution-neutral initiative to define standard directory names and filenames for important system files. Any Linux distribution that wants to be a good Linux citizen should follow this standard. Red Hat has decided — not for the first time — not to be a good citizen, so the path- and file-names given here for Red Hat give you an example of the variety that you may encounter when looking for system files. Examples for distributions following the FHS are SuSE and Debian.

On Red Hat, the top-level rc script is /etc/rc.d/rc. The path is slightly different in other distributions (/etc/init.d/rc on SuSE, for instance), but the contents are similar. In the previous section, you saw how the /etc/inittab invokes the script under a variety of circumstances with different numbers from 0 to 6 as arguments. The numbers correspond to runlevels, and each one causes the rc files to invoke a different set of scripts. So our next step is to find the scripts corresponding to each runlevel.

On Red Hat, scripts for each runlevel are stored in the directory /etc/rc.d/rcN.d where N is the runlevel being started. Thus, for runlevel 3, scripts in /etc/rc.d/rc3.d would be used. Again, slightly different conventions are the rule in other distributions. On Debian, for instance, the directory for each runlevel is /etc/rcN.d/.

Take a look in one of those directories; you will see a number of filenames of the form Snnxxxx or Knnxxxx where nn is a number from 00 to 99, and xxxx is the name of some system service. The scripts whose names begin with K are executed by /etc/rc.d/rc first to kill any existing services, and then the scripts whose names begin with S are executed to start new services.

The numbers nn in the names are used to enforce an ordering on the scripts as they are executed: scripts with lower numbers are executed before those with higher numbers. The name xxxx is simply used to help you identify to which system service the script corresponds. This naming convention might seem odd, but it makes it easy to add or remove scripts from these directories and have them automatically executed at the appropriate time by /etc/rc.d/rc. For customizing startup scripts, you'll find it convenient to use a graphical runlevel editor, such as ksysv in KDE.

For example, the script to initialize networking might be called S10network, while the script to stop the system logging daemon might be called K70syslog. If these files are placed in the appropriate /etc/rc.d/rcN.d directories, /etc/rc.d/rc will run them, in numerical order, at system startup or shutdown time. If the default runlevel of your system is 3, look in /etc/rc.d/rc3.d to see which scripts are executed when the system boots normally.

Because the same services are started or stopped at different runlevels, the Red Hat distribution uses symbolic links instead of repeating the same script in multiple places. Thus, each S or K file is a symbolic link that points to a central directory that stores startup or shutdown scripts for all services. On Red Hat, this central directory is /etc/rc.d/init.d, while on SuSE and Debian, it is /etc/init.d. On Debian and SuSE, the directory contains a script called skeleton that you can adapt to start and stop any new daemons you might write.

Knowing the location of a startup or shutdown script is useful in case you don't want to completely reboot or enter a different runlevel, but need to start or stop a particular service. Look in the init.d directory for a script of the appropriate name and execute it, passing the parameter start or stop. For example, on SuSE, if you want the Apache web server to be running but your system is in a runlevel that does not include Apache, just enter the following:

tigger# /sbin/init.d/apache start

Another important system configuration script is /etc/rc.d/rc.local, which is executed after the other system initialization scripts are run. (How is this accomplished? Generally, a symbolic link to rc.local is made in each /etc/rc.d/rcN.d directory with the name S99local. Because 99 is the largest numerical order any of the S scripts can have, it is executed last. Voilà!) You can edit rc.local to accomplish any peculiar or otherwise out-of-place system commands at boot time, or if you're not sure where else they should be executed. Debian doesn't have an equivalent of the rc.local script, but nothing stops you from adding it and invoking it from rc if you're used to having it.

The next entry, labeled ca, is executed when the key combination Ctrl-Alt-Delete is pressed on the console. This key combination produces an interrupt that usually reboots the system. Under Linux, this interrupt is caught and sent to init, which executes the entry with the action field of ctrlaltdel. The command shown here, /sbin/shutdown -t3 -rf now, will do a "safe" reboot of the system. (See Section 5.5 later in this chapter.) This way we protect the system from sudden reboot when Ctrl-Alt-Delete is pressed.

Finally, the inittab file includes entries that execute /sbin/agetty for the first six virtual consoles. agetty is one of the several getty variants available for Linux. These programs permit logins on terminals; without them the terminal would be effectively dead and would not respond when a user walked up and pressed a key or mouse button. The various getty commands open a terminal device (such as a virtual console or a serial line), set various parameters for the terminal driver, and execute /bin/login to initiate a login session on that terminal. Therefore, to allow logins on a given virtual console, you must be running getty or agetty on it. agetty is the version used on a number of Linux systems, but others use getty, which has a slightly different syntax. See the manual pages for getty and agetty on your system.

Now you should be familiar with init, but the various files and commands in /etc/rc.d, which do all the work, remain a mystery. We can't delve into these files without more background on other system administration tasks, such as managing filesystems. We'll lead you through these tasks in the next few chapters, and eventually all should be clear.