Tuesday, May 24, 2011

How-To: Managing services with update-rc.d

SkyHi @ Tuesday, May 24, 2011
Linux services can be started, stopped and reloaded with the use of scripts stocked in /etc/init.d/.
However, during start up or when changing runlevel, those scripts are searched in /etc/rcX.d/ where X is the runlevel number.
This tutorial will explain how one can activate, deactivate or modify a service start up.
When installing a new service under debian, the default is to enable it. So for instance, if you just installed apache2 package, after you installed it, apache service will be started and so will it be upon the next reboots.
If you do not use apache all the time, you might want to disable this service from starting up upon boot up and simply start it manually when you actually need it by running this command:
# /etc/init.d/apache2 start
You could either disable this service on boot up by removing any symbolic links in /etc/rcX.d/SYYapache2 or by using update-rc.d.
The advantage of using update-rc.d is that it will take care of removing/adding any required links to /etc/init.d automatically.
Taking apache2 as an example, let's examine how /etc/rcX.d is looking like:
# ls -l /etc/rc?.d/*apache2
lrwxrwxrwx 1 root root 17 2007-07-05 22:51 /etc/rc0.d/K91apache2 -> ../init.d/apache2
lrwxrwxrwx 1 root root 17 2007-07-05 22:51 /etc/rc1.d/K91apache2 -> ../init.d/apache2
lrwxrwxrwx 1 root root 17 2007-07-05 22:51 /etc/rc2.d/S91apache2 -> ../init.d/apache2
lrwxrwxrwx 1 root root 17 2007-07-05 22:51 /etc/rc3.d/S91apache2 -> ../init.d/apache2
lrwxrwxrwx 1 root root 17 2007-07-05 22:51 /etc/rc4.d/S91apache2 -> ../init.d/apache2
lrwxrwxrwx 1 root root 17 2007-07-05 22:51 /etc/rc5.d/S91apache2 -> ../init.d/apache2
lrwxrwxrwx 1 root root 17 2007-07-05 22:51 /etc/rc6.d/K91apache2 -> ../init.d/apache2
As you can see, for runlevels 0, 1 and 6 there is a K at the beginning of the link, for runlevels 2, 3, 4 and 5, there is a S. Those two letters stands for Kill and Start.
On Debian and Ubuntu, runlevels 2, 3, 4 and 5 are multi-users runlevels.
Runlevel 0 is Halt.
Runlevel 1 is single user mode
Runlevel 6 is reboot

1. Removing a Service

If you want to totally disable apache2 service by hand, you would need to delete every single link in /etc/rcX.d/. Using update-rc.d it is as simple as:
# update-rc.d -f apache2 remove
The use of -f is to force the removal of the symlinks even if there is still /etc/init.d/apache2.
Note: This command will only disable the service until next time the service is upgraded. If you want to make sure the service won't be re-enabled upon upgrade, you should also type the following:
# update-rc.d apache2 stop 80 0 1 2 3 4 5 6 .

2. Adding a service

2.1. Default priorities

Now, if you want to re-add this service to be started on boot up, you can simply use:
# update-rc.d apache2 defaults
Adding system startup for /etc/init.d/apache2 ...
/etc/rc0.d/K20apache2 -> ../init.d/apache2
/etc/rc1.d/K20apache2 -> ../init.d/apache2
/etc/rc6.d/K20apache2 -> ../init.d/apache2
/etc/rc2.d/S20apache2 -> ../init.d/apache2
/etc/rc3.d/S20apache2 -> ../init.d/apache2
/etc/rc4.d/S20apache2 -> ../init.d/apache2
/etc/rc5.d/S20apache2 -> ../init.d/apache2

2.2. Custom priorities

But as you can see, the default value is 20 which is pretty different than 91 ... a S20 link is started before a S91 and and K91 is kill before K20.
To force apache2 to be started with priorities 91 for both Start and Kill, we need to use the following command:
# update-rc.d apache2 defaults 91
Adding system startup for /etc/init.d/apache2 ...
/etc/rc0.d/K91apache2 -> ../init.d/apache2
/etc/rc1.d/K91apache2 -> ../init.d/apache2
/etc/rc6.d/K91apache2 -> ../init.d/apache2
/etc/rc2.d/S91apache2 -> ../init.d/apache2
/etc/rc3.d/S91apache2 -> ../init.d/apache2
/etc/rc4.d/S91apache2 -> ../init.d/apache2
/etc/rc5.d/S91apache2 -> ../init.d/apache2

2.3. Different priorities for Start and Kill

Alternatively, if you want to set different priorities for Start than for Kill, let say Start with 20 and Kill with 80, you will need to run:
# update-rc.d apache2 defaults 20 80
Adding system startup for /etc/init.d/apache2 ...
/etc/rc0.d/K80apache2 -> ../init.d/apache2
/etc/rc1.d/K80apache2 -> ../init.d/apache2
/etc/rc6.d/K80apache2 -> ../init.d/apache2
/etc/rc2.d/S20apache2 -> ../init.d/apache2
/etc/rc3.d/S20apache2 -> ../init.d/apache2
/etc/rc4.d/S20apache2 -> ../init.d/apache2
/etc/rc5.d/S20apache2 -> ../init.d/apache2

3. Specifying custom runlevels

Finally, if you only want to Start and Kill on specific runlevels, like for instance starting apache with priority 20 on runlevels 2, 3, 4 and 5 and Kill with priority 80 on runlevels 0, 1 and 6:
# update-rc.d apache2 start 20 2 3 4 5 . stop 80 0 1 6 .
Adding system startup for /etc/init.d/apache2 ...
/etc/rc0.d/K80apache2 -> ../init.d/apache2
/etc/rc1.d/K80apache2 -> ../init.d/apache2
/etc/rc6.d/K80apache2 -> ../init.d/apache2
/etc/rc2.d/S20apache2 -> ../init.d/apache2
/etc/rc3.d/S20apache2 -> ../init.d/apache2
/etc/rc4.d/S20apache2 -> ../init.d/apache2
/etc/rc5.d/S20apache2 -> ../init.d/apache2
Or, to start with priority 20 for runlevel 2, 3 and 4 and priority 30 for runlevel 5 and kill with priority 80 for runlevel 0, 1 and 6:
# update-rc.d apache2 start 20 2 3 4 . start 30 5 . stop 80 0 1 6 .
Adding system startup for /etc/init.d/apache2 ...
/etc/rc0.d/K80apache2 -> ../init.d/apache2
/etc/rc1.d/K80apache2 -> ../init.d/apache2
/etc/rc6.d/K80apache2 -> ../init.d/apache2
/etc/rc2.d/S20apache2 -> ../init.d/apache2
/etc/rc3.d/S20apache2 -> ../init.d/apache2
/etc/rc4.d/S20apache2 -> ../init.d/apache2
/etc/rc5.d/S30apache2 -> ../init.d/apache2 

 You could install sysvconfig package and then type:
$ sudo service samba status

Unfortunately, samba does not support the status option.
However, you can find out when a service is running by checking in /var/run or in /var/run/service/ for files with the extension .pid. For instance, samba stores its pid in /var/run/samba/nmbd.pid and /var/run/samba/smbd.pid .
The service command is pretty tight to red hat based distros. Debian does not have such a command out of the box and sysvconfig seems to be a port of te Red Hat tool.

Managing services in Debian


I’m used to System V style commands used in Red Hat / Fedora and derivatives as well as SUSE, so I found Debian service management methods unintuitive at first.
Below are a couple of personal notes I made on the subject to help myself out, as I was getting used to the Debian way of things…
List existing services:

ls /etc/rc2.d/S*

Manage services [start,stop,reload,etc]:

/etc/init.d/service command

Make service start on boot:

update-rc.d name defaults

The ‘defaults’ value makes the service start in runlevels 0123 and stop in 456. Alternatively, you can specify which runlevels the service will start and stop in with:

update-rc.d name start runlevel stop runlevel

To remove service from starting on boot:

update-rc.d name remove

BTW, for a really excellent breakdown of update-rc.d options, check out this well written post.
If you install sysvconfig (apt-get install sysvconfig) you can also use System V style commands to start/stop services:

service name command

Unfortunately, even having installed sysvconfig in Debian, the command to list all services with status (e.g. running, unused, etc) is still missing. In SUSE, you’d run it with:

service -s

And would get a nice little list like this:
In the absence of the above, you could just list running processes with ‘ps’.. Another option is to list files in /var/run whose filename ends with ‘pid’, as each of those corresponds to a service:

find . /var/run | grep pid

Yet another option, with sysvconfig installed on Debian, is to just run:


…which produces a clean and simple ncurses GUI which allows to manage service startup and edit runlevels: