An introduction to services, runlevels, and rc.d scripts | Linux.com

An introduction to services, runlevels, and rc.d scripts

ALinux service is an application (or set of applications) that runs inthe background waiting to be used, or carrying out essential tasks.I've already mentioned a couple of typical ones (Apache and MySQL). Youwill generally be unaware of services until you need them.

How can you tell what services are running, and more importantly, how can you set up your own?

In this article I'm dealing with System V (derived from AT&T System V) based distributions. This is the most common Linux init system. Another is based on BSD (Berkeley Software Distribution). What's the difference between the two? Basically BSD doesn't have any runlevels. This means that System V gives a lot more flexibility to a system administrator.

Most Linux distros put startup scripts in the rc subdirectories (rc1.d, rc2.d, etc.), whereas BSD systems house the system scripts in /etc/rc.d. Slackware's init setup is similar to BSD systems, though Slackware does have runlevels and has had System V compatibility since Slackware 7.

If you look in the file /etc/inittab you will see something like:

id:4:initdefault:l

0:0:wait:/etc/rc.d/rc.0l

6:6:wait:/etc/rc.d/rc.6x

1:4:wait:/etc/rc.d/rc.4

Theboot process uses these parameters to identify the default runlevel andthe files that will be used by that runlevel. In this example, runlevel4 is the default and the scripts that define runlevel 4 can be found in/etc/rc.d/rc.4.

And what is a runlevel? You might assume thatthis refers to different levels that the system goes through during aboot up. Instead, think of the runlevel as the point at which thesystem is entered. Runlevel 1 is the most basic configuration (simplesingle user access using an text interface), while runlevel 5 is themost advanced (multi-user, networking, and a GUI front end). Runlevels0 and 6 are used for halting and rebooting the system.

There are,however, differences between Linux distributions. For instance, Fedorauses runlevel 5 for X-based logins, whereas Slackware uses runlevel 4to do the same job. Therefore, you should check your documentationbefore making any changes. This table shows a generic list ofconfigurations (and some examples of different distros) taken from Linux - The Complete Reference (R.Peterson, Osbourne/McGraw-Hill).

Run Level

Generic

Fedora Core

Slackware

Debian

Asyou can see there are slight (but important) differences between Linuxdistributions. One thing is common between them -- if you want tochange the default level, you must edit /etc/initab. You will need tobe root or use sudo to edit this file, naturally.

Why would youwant to change the runlevel? Normally you will only use full GUI ortext multi-user mode -- runlevels 4 or 5. You'd only want runlevels 1or 2 if you have some system problems and you want the most basicaccess. Runlevels 0 and 6 should never be used as a default (forobvious reasons -- you don't want the system to shutdown or reboot assoon as you turn it on). You can, of course, change mode whilst thesystem is running. Type init followed by the required runlevel e.g.:

This will reboot the system.

Theboot process, or to be more accurate the init command, will decide therunlevel to select (in the example above it's 4) and from that willdecide the rc.d script files to be run. In this case either the file/etc/rc.d/rc.4 or any files in the directory /etc/rc.d/rc4.d. Let'slook at an example rc.d script file. Here's the default rc.4 file forSlackware 10.2:

# Try to use GNOME's gdm session manager:

if [ -x /usr/bin/gdm ]; 

then  exec /usr/bin/gdm -nodaemonfi

# Not there?  OK, try to use KDE's KDM session manager:

if [ -x /opt/kde/bin/kdm ];

 then  exec /opt/kde/bin/kdm -nodaemonfi

# If all you have is XDM, I guess it will have to do:

if [ -x /usr/X11R6/bin/xdm ];

 then  exec /usr/X11R6/bin/xdm -nodaemonfi

When you boot your computer, the first thing that it will do is load the bootloader -- either GRUB or LILO in most cases. The bootloader will then load the Linux kernel -- the core operating system. Next, a process called init starts. This process reads the file /etc/inittab to establish the runlevel to use. The runlevel is the start mode for the computer.

Once init knows the runlevel it will look for the appropriate files or directories as defined in /etc/initab.

Init will then either run the script files defined by /etc/initab, or run the script files in the directories defined by /etc/initab (depending on the set up of your system).

Finally, init will present you with the logon mode that you've selected.

In the other distros (such as Fedora and Debian) you'llfind that the scripts to be run are actually symbolic links to files inthe directory /etc/init.d -- the central repository for all startupscripts. So all you have to do is to write your startup script, placeit in /etc/init.d, and then create a symbolic link to it from theappropriate runlevel directory (or runlevel file, if that's what yoursystem uses).

For example, runlevel 2 is the default runlevel forDebian in non-GUI mode. If you're running Apache 2 on Debian, you'dfind an init script for Apache 2 under /etc/init.d called apache2. Asymlink, S91apache2, points to /etc/init.d/apache2 from /etc/rc2.d --this tells init to start Apache 2 in runlevel 2, but only after otherservices with lower S numbers.

When the system is shut down,there is another symlink in the /etc/rc0.d and /etc/rc6.d directories(halt and reboot, respectively) that starts with a K instead of an S,which tells init to shut down the process.

If this all stillsounds a bit too complicated, you can instead simply make use of the/etc/rc.d/rc.local file. This script file is run once, before all otherscripts have run but before the logon prompt appears. By default itlooks something like:

#!/bin/bash## /etc/rc.local - run once at boot time

# Put any local setup commands in here:

You can append your instructions onto the end of the file by defining another script to be run:

/root/bin/start_bb

Or you can modify rc.local by adding the commands themselves:

modprobe -r uhcimodprobe usb-uhcieciadsl-startiptable -Fiptables -A 

INPUT -i ppp0 -p tcp --syb -j DROPnetdate time.nist.gov

Herea USB modem is initialized, a connection set up to a broadband network,some basic security is set up, and then the local time is synchronizedwith a time server. You can also start Apache or MySQL:

apachectl startecho "/usr/bin/mysqld_safe &" | su mysql

Note that some distros, such as Debian, do not use rc.local for startup scripts. See the Debian FAQ if you'd like to add startup scripts for Debian or Debian-derived distros.

Onefinal thought -- in addition to startup scripts (for rc.local), try toremember to write close-down scripts to be added to rc.0 and rc.6. Thisensures that your services are shut down neatly and not left in strangestates when the system halts.

About shutting down -- how do youstop a service from starting when you reboot? It's just the reverse ofwhat we've already looked at. Either edit the relevant runlevel file(comment the lines out rather than removing them) or remove the linkfrom the runlevel directory. Note that it may not be necessary to dothis manually, as many distros include tools to manage services. Forexample, Red Hat and Fedora use chkconfig, while Debian uses update-rc.d.

Fromthis brief discussion, I hope you can see how useful rc.d scripts canbe when it comes to controlling the services to be run on your PC. Youcan now add your own as required, as well as look at existing ones thatyou may not require and which are slowing down your logon oroverloading your PC.