Automatically starting a Python script at boot on Raspbian Jessie

As part of the Hackspace Manchester door control system, we have a raspberry pi running a little script that checks scanned cards against a database of members and opens the door if the card is known.  This has been humming along happily for around 3 years now, until recently it stopped updating card IDs when they were changed via the webui.

This led to a bit of a bug hunt, concluding with the fact the version of openssl on raspbian wheezy was waaaaay out of date, and we'd recently updated our members system to disable insecure cyphers on the HTTPS protocol.  We fixed it by upgrading to jessie, which as a side effect completely killed the auto-start of the door opening programme. Yay?

So.  Jessie.  Systemd.  Init system is a a bit different from sysvinit, but on the whole i find it a lot more sensible.  We want to run a script as the user 'alfred' (the door entry service user).  We also want to wait until the system is booted, and the serial port is available.

My script is called alfred, so i create the following file in /lib/systemd/system/alfred.service

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[Unit]
Description=alfred
After=dev-ttyAMA0.device multi-user.target

[Service]
Type=idle
ExecStart=/home/alfred/FRED/fred/fred.py
WorkingDirectory=/home/alfred/FRED/fred/
User=alfred

[Install]
WantedBy=multi-user.target

The After= says what services need to be up before this is run. In this case, it wants ttyAMA0 to be available, along with multi-user (this is the point where you could normally log in)

ExecStart= specifies the script I will be running, WorkingDirectory= is the directory to run the thing from (as i use relative paths in my python script, i need to set this), and User= says what unprivileged user to run the script as, since you don't want to be running random things as root if at all possible!

WantedBy= says that this should be started at the same time as multi-user.target, so at the end of the boot process, at the point you could normally log in.

We can then set up our new service to start on boot, and run it for the first time:

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$ systemctl enable alfred
$ systemctl start alfred
No errors, so lets check the status:
$ sudo systemctl status alfred
● alfred.service - alfred
Loaded: loaded (/lib/systemd/system/alfred.service; enabled)
Active: active (running) since Wed 2016-07-06 16:42:59 BST; 30min ago
Main PID: 709 (fred.py)
CGroup: /system.slice/alfred.service
└─709 /usr/bin/python /home/alfred/FRED/fred/fred.py
Jul 06 16:42:59 alvin systemd[1]: Starting alfred...
Jul 06 16:42:59 alvin systemd[1]: Started alfred.
Jul 06 16:43:01 alvin fred.py[709]: 2016-07-06 16:43:01,577 FRED 0.7

Looks good, service is started, and we're getting some log output from it. Reboot to check everything comes up correctly and you're done!

... Though i wasnt. One gotcha I ran into is that because of the way raspbian's networking is set up, you can't get systemd to wait until after you have a network connection configured before starting the script (it will wait until the networking service is started, but not wait for the network to actually be up). This can be worked around pretty easily by setting the "Wait for network on boot" option in raspi-config, which will pause the whole boot process until it gets a DHCP lease.