Creating /boot/ssh and /boot/wpa_supplicant.conf (for a headless setup) works, but how?

Have you ever wondered HOW this works? I did. And it was quite the rabbit hole to go down.

• First I Googled and found the documentation about creating the file /boot/ssh
• I never managed to find any official documentation on creating the file /boot/wpa_supplicant.conf (Though I did find lots of references that stated that it did work.)
• Then I studied https://github.com/RPi-Distro/pi-gen extensively.
• But I couldn't find anything that looked for /boot/ssh or /boot/wpa_supplicant.conf
• Then I started looking for packages that may contain the implementation.

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  ~/src/pi-gen $ cat $(find stage[012] -type f -name '*package*') | tr " " "\n" | grep 'pi' | sort libraspberrypi-bin libraspberrypi-dev libraspberrypi-doc libraspberrypi0 pi-bluetooth python-rpi.gpio raspberrypi-bootloader raspberrypi-net-mods raspberrypi-sys-mods raspi-config raspi-copies-and-fills rpi-update

• I grabbed all the matching packages I could find in https://archive.raspberrypi.org/debian/pool/main/r/
• I got lucky with raspberrypi-net-mods and raspberrypi-sys-mods

I finally discovered that it is a bit of systemd magic that makes each happen.

raspberrypi-net-mods.service

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[Unit] Description=Copy user wpa_supplicant.conf ConditionPathExists=/boot/wpa_supplicant.conf Before=dhcpcd.service [Service] Type=oneshot RemainAfterExit=yes ExecStart=/bin/mv /boot/wpa_supplicant.conf /etc/wpa_supplicant/wpa_supplicant.conf ExecStartPost=/bin/chmod 600 /etc/wpa_supplicant/wpa_supplicant.conf [Install] WantedBy=multi-user.target

Notice the ConditionPathExists and ExecStart! That's pretty clever.

Similarly, but slightly more clever.

raspberrypi-sys-mods.sshswitch.service

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[Unit] Description=Turn on SSH if /boot/ssh is present ConditionPathExistsGlob=/boot/ssh{,.txt} After=regenerate_ssh_host_keys.service [Service] Type=oneshot ExecStart=/bin/sh -c "update-rc.d ssh enable && invoke-rc.d ssh start && rm -f /boot/ssh ; rm -f /boot/ssh.txt" [Install] WantedBy=multi-user.target

Now that I've uncovered this, I can finally finish the project I started with this thread nearly 5 years ago. It's been a long journey of discovery, this $35 whim I indulged in back in May 2012

https://www.raspberrypi.org/forums/viewtopic.php?t=198660