System V style init and systemd in practice

Runlevels and Targets

The runlevels continue to exist in the form of corresponding targets. However, they differ by distribution. The entries here originate from a Debian 8, but are more or less the same in Ubuntu. Table 6 contains a list of comparisons.

Table 6

Runlevel Targets

Init	Runlevel	Action
0	poweroff.target	Power and shut down computer
1	rescue.target	Single user mode without network
2	multi-user.target	Multi-user mode on
3	(As above - In Debian 7 corresponds to runlevel 2)	init [2-5] , exit in s or 1
4	(As above)	Multi-user, network, graphical user interface)
5	(As above)	Will be output as 5 with who -r
6	reboot.target	Restart computer
Additionally, there are more targets which can more precisely divide the system states.

Your Own Service (systemd)

After all that theory, I'll now show how to create your own systemd service. The shell script in Listing 6 collects a list of the hosts found on the network at a given interval. It is saved in executable form under /usr/sbin . You will be able to see the results using tail -f /tmp/netlist.txt .

Listing 6

netshow.sh

01 #! /bin/sh
02 while true;
03 do
04
05  echo "List of active network users" > /tmp/netlist
06  echo "------------------------------------" >> /tmp/netlist
07  date +%d.%m.%Y-%H:%M:%S >> /tmp/netlist
08  echo "------------------------------------" >> /tmp/netlist
09
10  # Execute fping and save entire output in log file
11
12  fping -r 0 -g 192.168.0.0/24 > fping.log 2>&1
13
14 # Change 192.168.0.0/24 to whatever works for your network
15
16  # Filter out the unreachable hosts
17
18  cat fping.log | grep "alive" | sort  >> /tmp/netlist
19  echo "------------------------------------" >> /tmp/netlist
20  sleep 120
21 done

You should first create the shell script shown in Listing 6 and save it in the /usr/sbin directory as netshow.sh (for it to work, you may have to install fping first using apt-get ). Remember to make it executable with

sudo chmod 700 netshow.sh

so that systemd can start the program. Then, save the unit file netshow.service (Listing 7) in /etc/systemd/system .

Listing 7

netshow.service

01 [Unit]
02 Description=Listing of active hosts
03 Documentation=man:fping(8)
04
05 [Service]
06 ExecStart=/usr/sbin/netshow.sh
07 IgnoreSIGPIPE=false
08
09 [Install]
10 WantedBy=multi-user.target

Now systemd must be instructed to process the unit file and start the application. To do so, the service must be enabled with sysctl in order to establish a permanent start: