Linux System Initialization
Let's take a look at the sample Slackware inittab in Listing 1. I've numbered the lines for easy reference. The numbers don't appear in your inittab—your inittab will begin two spaces to the right of the line numbers. Within the inittab file, lines beginning with a “#” sign are disabled and left as explanatory remarks or examples for possible future use. Be sure to read all the comments throughout; they were inserted to help you and may give you a hint on how to better customize your own inittab. Most programs, such as mgetty or efax, that were meant to run from inittab come with examples of how to implement them.
Since you already know how to read a line (id:runlevel(s):action:process), I'm going to cover only those few lines of special interest.
As I've already mentioned, Slackware isn't a true BSD system in the old style. Rather than having just a single-user mode and multi-user mode, it actually uses runlevel 3 as its default runlevel. It runs a system initialization script first, rc.S. This script is designed to be run only once at bootup. Then it runs rc.M. It skips the line with rc.K unless a system operator intervenes and deliberately changes to that state. When changing states between single-user and multi-user modes, the appropriate script is called. (See Listing 1, lines 15, 18 and 21.)
rc.0 and rc.6 are each files that are also run when the system is brought down. (See Listing 1, lines 27 and 30.)
You will see power management (UPS power management) handled in the script as well as the ctrl-alt-del key sequence. (See Listing 1, lines 24, 33, 36 and 39.)
Something odd you should notice about this inittab (which was lifted straight from a distribution CD): while the default init runlevel is 3, if a power daemon signals the system to shut down, then power is restored, the shutdown is canceled, and the system is brought back up at runlevel 5. However, since runlevels 3 and 5 are essentially identical (they run the same rc scripts), there is no difference in this case.
Now we come to the standard part which all inittabs were specifically designed to handle: initializing and respawning gettys. When UNIX was young, dumb terminals hung off serial ports. These dumb terminals were called teletype terminals or simply TTYs. So, the program that sent a login screen to the tty was called getty for “get TTY”. Today's getty performs the same basic function, although the TTY today is not likely to be quite so dumb. Adding and subtracting virtual terminals is as easy as adding or subtracting lines in the inittab; you can have up to 255.
Next, you'll see a line that allows the X Display Manager (XDM) to be respawned in runlevel 4.
About the only thing I haven't mentioned is that the scripts which do all the work on the Slackware system are all located in /etc/rc.d. Look them over. Slackware uses a minimal number of scripts to start background processes. Specifically referenced by inittab are rc.S, rc.K, rc.M, rc.0 and rc.6. Called by scripts (such as rc.M), but not by init, are rc.inet, rc.inet2, rc.local, rc.serial and others.
Take a look at the Red Hat inittab (Listing 2). In this file are some good explanations of what Red Hat does with runlevels. I won't belabor it further here. Note that the runlevels chosen for use by Red Hat are just one convention and not indicative of all System V UNIX systems, not even other Linux System V initializations.
As you can see, Red Hat defaults to runlevel 3, but you can change this to 5 once you have the X server properly configured. (See Listing 2, lines 18 and 56.) Given the number of graphical tools Red Hat has put together, you'd think they'd encourage the use of runlevel 5, but using that as the out-of-the-box default would cause trouble if X was not properly configured first.
Just below the default runlevel, you'll see the system initialization script (Listing 2, line 21). This is run once when the system boots. Then init jumps down to (in this case) line 13 (Listing 2, line 26). The lines for 10 through 12 and 14 through 16 are skipped because our default runlevel is 3.
Notice that ud, ca, pf and pr run regardless of the runlevel. When the runlevel column is null, the process is run in every runlevel.
The getty lines should look familiar to you. Don't be bothered by the fact that Red Hat chose mingetty over getty. They both do the same thing: send a login banner to the tty.
Finally, runlevel 5 spawns XDM (X Display Manager).
Under Red Hat, you'll find all the system initialization scripts in /etc/rc.d. This subdirectory has even more subdirectories—one for each runlevel: rc0.d to rc6.d and init.d. Within the /etc/rc.d/rc#.d subdirectories (where the # is replaced by a single digit number) are links to the master scripts stored in /etc/rc.d/init.d. The scripts in init.d take an argument of start or stop, and occasionally reload or restart.
The links in the /etc/rc.d/rc#.d directories all begin with either an S or a K for start or kill respectively, a number which indicates a relative order for the scripts and the script name—commonly the same name as the master script found in init.d to which it is linked. For example, S20lpd will run the script lpd in init.d with the argument start which starts up the line-printer daemon. The scripts can also be called from the command line:
The nice part about System V initialization is that it is easy for root to start, stop, restart or reload a daemon or process subsystem from the command line simply by calling the appropriate script in init.d with the argument start, stop, reload or restart.
When not called from a command line with an argument, the rc script parses the command line. If it is running K20lpd, it runs the lpd init script with a stop argument. When init has followed the link in inittab to rc.d/rc3.d, it begins by running all scripts that start with a K in numerical order from lowest to highest, then likewise for the S scripts. This ensures that the correct daemons are running in each runlevel, and are stopped and started in the correct order. For example, you can't start sendmail or bind/named (Berkeley DNS or Domain Name Service daemon) before you start networking. The BSD-style script Slackware uses will start networking early in the rc.M script, but you must always be cognizant of order whenever you modify Slackware startup scripts. Remember when we changed to runlevel 8 above and nothing happened? Since no subdirectory rc8.d exists and consequently no kill or start scripts, no scripts were run when we changed states. Had we come from boot directly to runlevel 8, we would have had a problem. Only the kernel, init and those daemons started via the sysinit, boot or bootwait commands in the inittab would have been running. I'll let you look at the scripts in the ../init.d/ directory for yourself, but an example for those with Slackware systems is shown in Listing 3.
For those who find editing links to add or delete scripts in any particular runlevel a tedious task or who are just not comfortable doing this, Red Hat distributes a program called tksysv. This program uses a graphical interface (using Tcl/Tk) to read the script names in /etc/rc.d/init.d and displays them on the far left side of the application box. If you have a system with init.d in a different location, you can install symbolic links (for each of the rc#.d directories) and it will function just fine, or hack the script and customize it to your system. The system also reads the links in each of the rc#.d subdirectories and displays them for each runlevel from left to right with start scripts above and kill scripts below. (See Figure 1.) You can add, delete and even change the order of execution as you see fit.
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