If you use Linux you most likely have heard of the init.d directory. But what exactly does this directory do? It ultimately does one thing but it does that one thing for your entire system, so init.d is very important. The init.d directory contains a number of start/stop scripts for various services on your system. Everything from acpid to x11-common is controlled from this directory. Of course it’s not exactly that simple.If you look at the /etc directory you will find directories that are in the form rc#.d (Where # is a number reflects a specific initialization level – from 0 to 6). Within each of these directories is a number of other scripts that control processes. These scripts will either begin with a “K” or an “S”. All “K” scripts are run before “S” scripts. And depending upon where the scripts are located will determine when the scripts initiate. Between the directories the system services work together like a well-oiled machine. But there are times when you need to start or stop a process cleanly and without using the kill or killall commands. That is where the /etc/init.d directory comes in handy.Now if you are using a distribution like Fedora you might find this directory in /etc/rc.d/init.d. Regardless of location, it serves the same purpose.In order to control any of the scripts in init.d manually you have to have root (or sudo) access. Each script will be run as a command and the structure of the command will look like:/etc/initi.d/command OPTIONWhere command is the actual command to run and OPTION can be one of the following:startstopreloadrestartforce-reloadMost often you will use either start, stop, or restart. So if you want to stop your network you can issue the command:/etc/init.d/networking stopOr if you make a change to your network and need to restart it, you could do so with the following command:/etc/init.d/networking restartSome of the more common init scripts in this directory are:networkingsambaapache2ftpdsshddovecotmysqlOf course there may be more often-used scripts in your directory – it depends upon what you have installed. The above list was taken from a Ubuntu Server 8.10 installation so a standard desktop installation would have a few less networking-type scripts.But what about /etc/rc.localThere is a third option that I used to use quite a bit. This option is the /etc/rc.local script. This file runs after all other init level scripts have run, so it’s safe to put various commands that you want to have issued upon startup. Many times I will place mounting instructions for things like nfs in this script. This is also a good place to place “troubleshooting” scripts in. For instance, once I had a machine that, for some reason, samba seemed to not want to start. Even after checking to make sure the Samba daemon was setup to initialize at boot up. So instead of spending all of my time up front with this I simply placed the line:/etc/init.d/samba startin the /etc/rc.local script and Samba worked like a charm. Eventually I would come back and trouble shoot this issue.Final ThoughtsLinux is flexible. Linux is so flexible there is almost, inevitably, numerous ways to solve a single problem. Starting a system service is one such issue. With the help of the /etc/init.d system (as well as /etc/rc.local) you can pretty much rest assured your service will start.