Add systemd service to zabbix post.

content/System Administration/zabbix-sound-level.md

 Title: Measuring sound level with Zabbix
 Description: Test consumer electronics with enterprise tools!
 Date: March 22, 2019
-Cover: /media/zabbix/graph-real.png
-Featured_Project: true
 
-Here's a way to observe sound level from a microphone using an IT monitoring system.
+Here's how I watched the sound level from a microphone with a Raspberry Pi and an IT monitoring system.
 
 # Why?
 
-- I wanted to measure how long an old MP3 player could play until the battery died
-- ...without having to listen to the music
-- ...and I wanted to find out as soon as it stopped playing.
+I wanted to...
+
+- measure how long an old MP3 player could play until the battery died
+- ... without having to listen to the music
+- ... and I wanted to find out as soon as it stopped playing.
 
 ![Photo](/media/zabbix/photo.jpg)
 
 # Zabbix
 
-Zabbix is an open-source monitoring tool. I already use it to keep an eye on my servers. Maybe it can help here?
+[Zabbix](https://zabbix.com) is an open-source enterprise IT monitoring tool.
+I already use it to keep an eye on my servers. Maybe it can help here?
 
 # Measuring volume
 
@@ -55,17 +56,19 @@ First, some housekeeping on the Zabbix server:
 2. Create a new **Item** for the `soundmonitor` host. Let's call it `soundlevel`. ![Item](/media/zabbix/item.png)
 3. Put the Item on a **Graph**. ![Graph](/media/zabbix/graph.png)
 
-Now we can test the `zabbix_sender` command.
+Let's test the `zabbix_sender` command...
 
 ```bash
 zabbix_sender -z zabbix.example.net -s soundmonitor -k soundlevel -o "0.5"
 ```
 
-Take a look at the graph. It should now have our fake datapoint!
+...and check the graph. It should have our fake datapoint.
 
 ![Graph 0.5](/media/zabbix/graph-0.5.png)
 
-# All together
+There it is!
+
+# Assembling the pieces
 
 We can use [command substitution](https://www.gnu.org/software/bash/manual/html_node/Command-Substitution.html) to string together a one-liner.
 
@@ -76,9 +79,9 @@ zabbix_sender -z zabbix.example.net \
   -o "$(rec -n stat trim 0 .5 2>&1 | awk '/^Maximum amplitude/ { print $3 }')"
 ```
 
-This takes the numeric sound level from the `rec|awk` combination and passes it as the `-o` argument of `zabbix_sender`.
+This takes the numeric sound level from `rec | awk` and passes it as an argument to `zabbix_sender`.
 
-Now, we just need to put this in a loop...
+Now just put that in a loop...
 
 ```bash
 while true; do
@@ -89,8 +92,42 @@ while true; do
 done
 ```
 
+...and we have a working pipeline.
+
+
 # Ta-da!
 
 ![Graph (for real)](/media/zabbix/graph-real.png)
 
 Pausing the iPod produced that valley in the middle. Based on this test, I set a **Trigger** for `soundlevel` falling below `0.5` for 60 seconds or longer.
+
+# Extra credit
+
+The Raspberry Pi might lose power and restart. We can address that with a `systemd` service running the `zabbix_sender` loop.
+
+### `/etc/systemd/system/zs.service`
+
+```ini
+[Unit]
+Description=ZabbixSoundMonitor
+Wants=network-online.target
+After=network.target network-online.target
+
+[Service]
+Environment="AUDIODEV=hw:1,0"
+Environment="AUDIODRIVER=alsa"
+ExecStart=/usr/local/bin/zs.sh
+Restart=always
+RestartSec=5
+
+[Install]
+WantedBy=multi-user.target
+```
+
+This takes care of a few things:
+
+1. Waits until a network is available.
+2. Sets the `AUDIODEV` and `AUDIODRIVER` environment variables required by `rec`.
+3. Runs the script at `/usr/local/bin/zs.sh` (which contains the loop we wrote).
+
+Enable the new service with `systemctl enable zs.service`, and start it with `service zs start`. It will automatically start on reboot.