Java™ Application Development on Linux - Dator

452Chapter 20Open Source Web <strong>Application</strong> Serverscontrols. Let’s take a very brief look at /etc/inittab and how it works; thenwe’ll move on to the extended scripts that manage server processes. That iswhere we will integrate JBoss.A key concept in the System V init system is the runlevel. The idea is thata system can have a number of “standard” configurations, numbered from 0 to6, where 0 is shutdown, 1 is single-user, 2 to 5 are up to the system administrator,and 6 is reboot. The init 9 command can be used (by the root user) tochange the system from its current runlevel to another:# init 1What happens when you issue such a command is determined by the/etc/inittab file. Let’s take a look at the out-of-the-box /etc/inittab filefrom a Fedora Core 1 10 system (Example 20.1).This is a pretty complex file, and we don’t want to bog down in it toomuch, since most of what interests us occurs outside this file.The basic format of a line in /etc/inittab is:id:runlevels:action:processThe id is a unique 1–4 character identifier. The runlevels is a list of therunlevel numbers to which the record applies. The action specifies what actionis to be taken. The process is the program to run. The respawn action, forexample, tells init that when the process exits, it should be run again. The onceaction says it should be run once on transition to the runlevel. We won’t gointo too much more here. See the man inittab page for details.The part that concerns us are the l0 through l6 entries. Note that thesecause the /etc/rc.d/rc script to be run once, with the runlevel passed as anargument. This is the key to System V init system.9. telinit is a common alias from other UNIX implementations. Linux symlinks this to init.10. During the writing of this book, RedHat decided to put their completely Free SoftwareOS out to a public-controlled project and to stop calling it “RedHat.” The name RedHat isreserved for Fedora-based Linux systems that must be purchased with support contracts. It isstill the same system with a different name maintained by basically the same people. The keydifference is that you cannot purchase support for Fedora (at least from RedHat, we expectsome enterprising folks to offer Fedora support for a fee at some point).