This line starts and when necessary, restarts the X application used for the X-based console logon required by runlevel 5. Every line in the inittab file handles some important task. However, the real heart of the inittab file consists of the seven lines that follow the comment “System initialization” near the beginning of the inittab file (refer to Listing 1.4.) They are the lines that invoke the startup scripts. The first of these is the si entry: si::sysinit:/etc/rc.d/rc.sysinit This entry tells init to initialize the system by running the boot script located at /etc/rc.d/rc.sysinit. This script, like all startup scripts, is an executable file that contains Linux shell commands. Notice that the entry shows the full path to the startup script. One of the most common complaints about different Linux distributions is that the key files are stored in different locations in the filesystem. Don’t worry about memorizing these differences just look in the /etc/inittab file. It tells you exactly where the startup scripts are located. The six lines that follow the si entry in inittab are used to invoke the startup scripts for each runlevel. Except for the runlevel involved, each line is identical: l5:5:wait:/etc/rc.d/rc 5 This line starts all of the processes and services needed to provide the full multiuser support defined by runlevel 5. The label is l5, which is symbolic of level 5. The runlevel is, of course, 5. init is directed to wait until the startup script terminates before going on to any other entries in the inittab file that relate to runlevel 5. init executes the script /etc/rc.d/rc, and passes that script the command-line argument 5. Startup Scripts Anything that can be run from a shell prompt can be stored in a file and run as a shell script. System administrators use this capability to automate all kinds of processes; Linux uses this capability to automate the startup of system services. Two main types of scripts are used: the system initialization script and the runlevel initialization script. System Initialization The system initialization script runs first. On a Red Hat system, this is a single script named /etc/ rc.d/rc.sysinit. Other Linux distributions might use a different filename, but all versions of Linux use script files to initialize the system. The rc.sysinit script performs many essential system initialization tasks, such as preparing the network and the filesystems for use. The rc.sysinit script begins the network initialization by reading the /etc/sysconfig/network file, which contains several network configuration values set during the initial installation. If the file is not found, networking is disabled. If it is found, the script assigns the system the hostname stored there. The initialization script performs many small but important tasks, such as setting the system clock, applying any keyboard maps, and starting USB and PnP support. The bulk of the script, however, is used to prepare the filesystem for use. The script activates the swap file, which is necessary before the swap space is used. The rc.sysinit script also runs the filesystem check, using the fsck command to check the structure and integrity of the Linux filesystems. If a filesystem error is 29
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