Control Startup Services

Control Startup Services

Use GUI and command-line tools to start, stop, disable, and enable services.

As Ubuntu boots, you might notice text scrolling by, detailing all of the different things Ubuntu is doing. Among these things are a number of services that Ubuntu enables at boot time, such as the cron scheduling service, the system logger, and the graphical login manager. If you have installed other services on your system, such as a web server, those services will also be enabled at boot. Sometimes, though, you may want to either stop or temporarily disable these services, and Ubuntu provides a number of ways to do this, both graphically and through the command line. This hack shows some of the more common ways to control startup services.

Services Administration Tool

Ubuntu provides a graphical tool to manage services that start up at boot time. Click SystemServices, or type:

$ sudo services-admin
            

to start the Services Administration Tool. Figure shows how simple the program is: you have a list of services with a checkbox next to each of them. To disable a service, just deselect its checkbox and click OK. Currently, this application supports changing only whether a service starts at boot, so to manually start, stop, or restart a service, you will need to refer to the command-line method.

Ubuntu Services Administration Tool

Command-Line Method

Before learning how to start, stop, and disable services from the command line, it's important to understand Ubuntu's startup process and how Ubuntu determines which programs to run when it starts. For most Linux distributions (including Ubuntu), System V init scripts govern which programs start at boot and which programs don't. All System V init scripts that could potentially be run at boot are typically located in the /etc/init.d/ directory for Ubuntu. Not every script in /etc/init.d is executed at boot, however. Linux organizes which scripts to run for different circumstances into runlevels; most Linux systems have seven runlevels, ranging from 0 to 6. Think of a runlevel as a checklist of programs for Ubuntu to start before it presents a login.

A few of these runlevels are set aside for special states in a Linux system:

Runlevel 0

Halts the system.

Runlevel 1

Sets up single-user mode.

Runlevels 2-5

Set up different multiuser modes. Although, typically, only one or two of these are used by a distribution.

Runlevel 6

Reboots the system.

Each runlevel has a directory that stores symlinks to certain init scripts in /etc/init.d, which are started when that runlevel is selected and stopped when it is exited. Ubuntu puts these symlinks under /etc/rc<runlevel>.dfor example, all runlevel 2 scripts are located in /etc/rc2.d/.

If you look in one of these runlevel directories, you'll notice that many of the symlinks to scripts in /etc/init.d have odd names that begin with an S, K, or D; then a number; and finally the name of the script. Ubuntu defaults to runlevel 2, so here is a sample /etc/rc2.d directory:

greenfly@ubuntu:~$ ls -l /etc/rc2.d/
total 0
lrwxrwxrwx 1 root root 20 2006-01-21 14:48 K77ntp-server -> ../init.d/ntp-serverlrwxrwxrwx 1 root root 17 2006-01-07 08:46 S05vbesave -> ../init.d/vbesave
lrwxrwxrwx 1 root root 15 2006-01-07 08:19 S10acpid -> ../init.d/acpid
lrwxrwxrwx 1 root root 18 2006-01-07 08:17 S10sysklogd -> ../init.d/sysklogd
lrwxrwxrwx 1 root root 15 2006-01-07 08:17 S11klogd -> ../init.d/klogd
lrwxrwxrwx 1 root root 14 2006-01-07 08:47 S12dbus -> ../init.d/dbus
lrwxrwxrwx 1 root root 13 2006-01-07 08:50 S13gdm -> ../init.d/gdm
lrwxrwxrwx 1 root root 13 2006-01-07 08:46 S14ppp -> ../init.d/ppp
lrwxrwxrwx 1 root root 16 2006-01-07 08:48 S19cupsys -> ../init.d/cupsys
lrwxrwxrwx 1 root root 15 2006-01-07 08:52 S19hplip -> ../init.d/hplip
lrwxrwxrwx 1 root root 14 2006-01-07 08:47 S20apmd -> ../init.d/apmd
lrwxrwxrwx 1 root root 22 2006-01-07 08:48 S20hotkey-setup -> ../init.d/hotkey-setup
lrwxrwxrwx 1 root root 21 2006-01-07 08:46 S20laptop-mode -> ../init.d/laptop-mode
lrwxrwxrwx 1 root root 17 2006-01-07 08:16 S20makedev -> ../init.d/makedev
lrwxrwxrwx 1 root root 23 2006-01-07 08:18 S20nvidia-kernel -> ../init.d/nvidia-kernel
lrwxrwxrwx 1 root root 19 2006-01-07 08:48 S20powernowd -> ../init.d/powernowd
lrwxrwxrwx 1 root root 15 2006-01-07 08:46 S20rsync -> ../init.d/rsync
lrwxrwxrwx 1 root root 21 2006-01-07 08:47 S25bluez-utils -> ../init.d/bluez-utils
lrwxrwxrwx 1 root root 15 2006-01-07 08:17 S25mdadm -> ../init.d/mdadm
lrwxrwxrwx 1 root root 17 2006-01-07 08:46 S89anacron -> ../init.d/anacron
lrwxrwxrwx 1 root root 13 2006-01-07 08:46 S89atd -> ../init.d/atd
lrwxrwxrwx 1 root root 14 2006-01-07 08:46 S89cron -> ../init.d/cron
lrwxrwxrwx 1 root root 17 2006-01-07 08:18 S98usplash -> ../init.d/usplash
lrwxrwxrwx 1 root root 22 2006-01-07 08:46 S99acpi-support -> ../init.d/acpi-support
lrwxrwxrwx 1 root root 19 2006-01-07 08:16 S99rmnologin -> ../init.d/rmnologin
lrwxrwxrwx 1 root root 23 2006-01-07 08:16 S99stop-bootlogd -> ../init.d/stop-bootlogd

As you can see, this directory is full of symlinks that point to a script in the init.d directory. The letter at the beginning of each filename tells init when to execute this script. If the script begins with an S, then init starts the script when it goes through the runlevel. If the script begins with a K, then init stops (or kills) the script when it changes to a different runlevel. If the script begins with a D, then that script is disabled for the time being, and init ignores it. init runs the scripts in numerical order, so the numbers in each script let you know in which order they are to be run. This is useful to ensure that dependent services start after the service they are dependent on.

When Linux boots and starts the init process, it reads its configuration from /etc/inittab, which configures each available runlevel, the default runlevel to use, and some other settings. Next, init loads any system scripts from a special system runlevel directory at /etc/rcS.d. These scripts load daemons and services that are vital to the boot process. Lastly, init runs any startup scripts for the default runlevel in alphabetical order.

Scripts in /etc/rcS.d are run in runlevels 1 through 5, so you should generally leave them alone unless you know what you are doing. If you accidentally disable a crucial service, you may have to resort to a rescue disc to undo the mistake.

Change the Runlevel

You can change the runlevel yourself on the command line with the init command. To switch to single-user mode from the command line, type:

$ sudo init 1
            

If you're running X11 when you issue this command, beware, since it will kill X and your desktop environment!

This command runs all of the shutdown scripts for your current runlevel and then any startup scripts for single-user mode. To change back to the default multiuser runlevel for Ubuntu, type:

$ sudo init 2
            

You can also use init to halt or reboot a machine: just change to runlevel 0 and runlevel 6, respectively.

Manually Start and Stop Services

You can start and stop scripts manually by running the script with the start or stop argument. For example, to stop the CUPS service from the command line, type:

$ sudo /etc/init.d/cupsys stop
            

To start the service back up, type:

$ sudo /etc/init.d/cupsys start
            

Most scripts also support a restart argument that will run stop, then start for you. Most init scripts are also configured to output the list of arguments they support when you execute them without any options:

greenfly@ubuntu:~$ sudo /etc/init.d/cupsys
Password:
Usage: /etc/init.d/cupsys {start|stop|restart|force-reload|status}

Disable Scripts from Starting

To disable a script, you must know your default runlevel. On Ubuntu, the default runlevel is usually set to 2, but you may want to double-check your default runlevel before you start disabling services. All runlevels are defined in /etc/inittab, so to check the default runlevel, type:

greenfly@ubuntu:~$ grep initdefault /etc/inittab
id:2:initdefault:

As you can see, in this example, the default runlevel is in fact 2. Now change to the directory containing all of the scripts for that runlevel (/etc/rc2.d) and find the script you want to disable. To disable a service, just rename its script by changing the S to a D. For example, to disable the cupsys service, type:

greenfly@ubuntu:~$ cd /etc/rc2.d
greenfly@ubuntu:/etc/rc2.d$ sudo mv S19cupsys D19cupsys
            

To enable it again, rename it back by changing the D to an S:

greenfly@ubuntu:~$ cd /etc/rc2.d
greenfly@ubuntu:/etc/rc2.d$ sudo mv D19cupsys S19cupsys
            

You'll still need to stop the service as shown earlier if you want to shut it down right away, but renaming it will control whether it's started the next time you reboot (or change runlevels).