How Linux Boots Part-2

There are several different actions in addition to initdefault and wait, especially pertaining to power management, and the inittab(5) manual page tells you all about them. The ones that you're most likely to encounter are explained in the following sections.

respawn:The respawn action causes init to run the command that follows, and if the command finishes executing, to run it again. You're likely to see something similar to this line in your inittab file:

1:2345:respawn:/sbin/mingetty tty1

The getty programs provide login prompts. The preceding line is for the first virtual console (/dev/tty1), the one you see when you press ALT-F1 or CONTROL-ALT-F1. The respawn action brings the login prompt back after you log out.

ctrlaltdel:The ctrlaltdel action controls what the system does when you press CONTROL-ALT-DELETE on a virtual console. On most systems, this is some sort of reboot command using the shutdown command.

sysinit:The sysinit action is the very first thing that init should run when it starts up, before entering any runlevels.

How processes in runlevels start

You are now ready to learn how init starts the system services, just before it lets you log in. Recall this inittab line from earlier:

l5:5:wait:/etc/rc.d/rc 5

This small line triggers many other programs. rc stands for run commands, and you will hear people refer to the commands as scripts, programs, or services. So, where are these commands, anyway?

For runlevel 5, in this example, the commands are probably either in /etc/rc.d/rc5.d or /etc/rc5.d. Runlevel 1 uses rc1.d, runlevel 2 uses rc2.d, and so on. You might find the following items in the rc5.d directory:

S10sysklogd S20ppp S99gpm
S12kerneld S25netstd_nfs S99httpd
S15netstd_init S30netstd_misc S99rmnologin
S18netbase S45pcmcia S99sshd
S20acct S89atd
S20logoutd S89cron

The rc 5 command starts programs in this runlevel directory by running the following commands:

S10sysklogd start
S12kerneld start
S15netstd_init start
S18netbase start
...
S99sshd start

Notice the start argument in each command. The S in a command name means that the command should run in start mode, and the number (00 through 99) determines where in the sequence rc starts the command.

The rc*.d commands are usually shell scripts that start programs in /sbin or /usr/sbin. Normally, you can figure out what one of the commands actually does by looking at the script with less or another pager program.

You can start one of these services by hand. For example, if you want to start the httpd Web server program manually, run S99httpd start. Similarly, if you ever need to kill one of the services when the machine is on, you can run the command in the rc*.d directory with the stop argument (S99httpd stop, for instance).

Some rc*.d directories contain commands that start with K (for "kill," or stop mode). In this case, rc runs the command with the stop argument instead of start. You are most likely to encounter K commands in runlevels that shut the system down.

Adding and removing services

If you want to add, delete, or modify services in the rc*.d directories, you need to take a closer look at the files inside. A long listing reveals a structure like this:

lrwxrwxrwx . . . S10sysklogd -> ../init.d/sysklogd
lrwxrwxrwx . . . S12kerneld -> ../init.d/kerneld
lrwxrwxrwx . . . S15netstd_init -> ../init.d/netstd_init
lrwxrwxrwx . . . S18netbase -> ../init.d/netbase
...

The commands in an rc*.d directory are actually symbolic links to files in an init.d directory, usually in /etc or /etc/rc.d. Linux distributions contain these links so that they can use the same startup scripts for all runlevels. This convention is by no means a requirement, but it often makes organization a little easier.