As basic as the task of creating a startup script may sound, even experienced sysadmins sometimes run into problems with having services start at the right time during the boot process or stop during the shutdown. Two major reasons for this: the procedure is a bit convoluted due to linking. Also, writing startup scripts is not something you have to do very often these days. So a quick overview of the bootup process is in order. We will use RHEL 5.5 in this example, but the structure and process are nearly identical for most Linux flavors and versions.

Runlevels

Linux uses System V-style initialization with seven runlevels – modes of the OS operation. These are:

0 – runlevel 0 is used for shutdown. Entering “init 0” at the command prompt as root will cause the system to shut down and, in most cases, to power off.

1 – this runlevel is also known as the single-user mode. It is used primarily for troubleshooting. Only the most fundamental services are started. You will have access to all locally-attached storage. Network services are disabled, so this mode can only be used if you have direct access to the system console.

2 – similar to level 1, this runlevel is used mainly for troubleshooting. Networking is enabled and multiple users can log in. However, network file-sharing services like NFS or SMB are disabled. This mode can be useful for troubleshooting systems that hang during bootup on trying to mount remote filesystems.

3 – this runlevel is most commonly used by Unix servers. It starts all services with the exception of X-Windows. Multiple users can connect to the system, but only from command line. There is no support for GUIs.

4 – runlevel 4 is rarely used. It usually includes everything from runlevel 3, plus some custom services than may be needed for development or testing purposes.

5 – this runlevel is used by personal computers and some servers. It offers the X-Windows service and GUI desktop environment.

6 – runlevel 6 is used to reboot the system. Typing “init 6” at the command prompt as root is usually equivalent to typing “reboot”.

The default runlevel is specified in the /etc/inittab file. In the case of our test system, the default runlevel is 3, as shown below:

[root@deathstar]# grep initdefault /etc/inittab | grep -v "#"
id:3:initdefault:

A common mistake: when sysadmins add service startup and shutdown scripts, they usually concentrate on runlevels 3 and 5, forgetting to include runlevels 0 and 6 (shutdown and reboot). As a result, services are not properly shut down when the system is being powered down.

Init Directories and Files

Startup/shutdown scripts are located in /etc/rc.d/init.d

There are seven other sub-directories under /etc/rc.d

[root@deathstar]# ls /etc/rc.d | grep rc..d
rc0.d
rc1.d
rc2.d
rc3.d
rc4.d
rc5.d
rc6.d

Each of these directories contain soft links to the startup/shutdown scripts located in /etc/rc.d/init.d

[root@deathstar]# ls -als /etc/rc.d/rc3.d

8 drwxr-xr-x  2 root root 4096 Dec 15 16:03 .
8 drwxr-xr-x 10 root root 4096 Sep 26 00:29 ..
4 lrwxrwxrwx  1 root root   17 Sep 25 23:48 K00ipmievd -> ../init.d/ipmievd
4 lrwxrwxrwx  1 root root   17 Sep 25 23:51 K05wdaemon -> ../init.d/wdaemon
4 lrwxrwxrwx  1 root root   14 Sep 26 00:28 K10cups -> ../init.d/cups
...
4 lrwxrwxrwx  1 root root   22 Sep 25 23:54 S97yum-updatesd -> ../init.d/yum-updatesd
4 lrwxrwxrwx  1 root root   19 Sep 25 23:53 S99firstboot -> ../init.d/firstboot
4 lrwxrwxrwx  1 root root   11 Sep 25 23:48 S99local -> ../rc.local
4 lrwxrwxrwx  1 root root   23 Sep 26 00:48 S99OVCtrl -> /etc/rc.d/init.d/OVCtrl
4 lrwxrwxrwx  1 root root   16 Sep 25 23:51 S99smartd -> ../init.d/smartd

The links starting with “K” pass the “stop” argument to the startup/shutdown scripts. The links starting with “S” pass the “start” argument. The two-digit number following “K” or “S” determines the order in which the scripts are executed.

A common mistake: when adding a startup/shutdown script for a service that requires network access or access to a network-mounted filesystem, sysadmins often forget to make sure the service doesn’t try to start before network or network file-sharing services or that it shuts down before network.

Writing a Startup Script

Below is a simple example of a startup/shutdown script. These scripts can get very complex, but the basic structure is always the same. At the very minimum, your script needs to have the “start” and “stop” directives. If your startup script is for service that runs under a graphical desktop, such as Gnome or KDE, a “status” directive may also be needed.

#! /bin/sh
# Sample Unix System V-style initialization script

case "$1" in
  start)
        echo -n "Starting your_service"
        # To launch a process as root"
        /path/to/command/to/start/your_service
        # To launch a process as another user:
        /bin/su - username -c "/path/to/command/to/start/your_service argument1 argument2"
        echo "."
        ;;
  stop)
        echo -n "Stopping your_service"
        # To stop a process as root:
        /path/to/command/to/stop/your_service
        # To stop a process as another user:
        /bin/su - username -c "/path/to/command/to/stop/your_service argument1 argument2"
        echo "."
        ;;
  *)
        echo "Usage: /sbin/service your_service {start|stop}"
        exit 1
esac
exit 0

Modify this script as needed and place it in /etc/rc.d/init.d
In most cases, permissions for the startup/shutdown scripts should be 755.

A common mistake: some processes need to be started under a username other than root. The syntax for doing this is shown above. Sysadmins often forget to use double-quotes around the command containing space-separated argument. For example, the command below will work:

/bin/su - username -c "/path/to/command/to/start/your_service argument1 argument2"

And this command will not work:

/bin/su - username -c /path/to/command/to/start/your_service argument1 argument2

Creating Runlevel Links

Now that your startup/shutdown script has been written and tested, it is time to create soft links to the script from the appropriate /etc/rc.d/rc#.d directories. Let’s say you need your service to start in runlevels 3 and 5. Take a look in /etc/rc.d/rc3.d and in /etc/rc.d/rc5.d and determine when you want your service to start. In most cases, you will want your new service to start after everything else and to stop before everything else, when the system is being shut down or rebooted.

The script below will help you create appropriate soft links for starting and stopping your new service in runlevels 3 and 5, as well as for stopping them in runlevels 0 and 6.

#!/bin/sh

# Create shutdown links for runlevels 0, 3, 5 and 6
for i in 0 3 5 6
do
   ln -s /etc/rc.d/init.d/your_service /etc/rc.d/rc${i}.d/K01your_service
done

# Create startup links for runlevels 3 and 5
for i in 3 5
do
   ln -s /etc/rc.d/init.d/your_service /etc/rc.d/rc${i}.d/S99your_service
done

Always test your startup/shutdown scripts before rebooting the system. A simple typo in the script may cause your system to hang on startup or shutdown, requiring direct console access to fix the problem. Once you are convinced that your script works, do a reboot to verify. Never assume everything will work as expected just because the great you is on the job.