=== Login prompt when connecting USB null modem cable ===
File: ///etc/udev/rules.d/99-ttyusb.rules//
<code>
 KERNEL=="ttyUSB0", ACTION=="add",\
   PROGRAM="/bin/systemctl start usb-serial-ttyUSB0.service"
</code>

File: ///etc/systemd/system/usb-serial-ttyUSB0.service//
<code>
 [Unit]
 BindTo=/dev/ttyUSB0
 After=/dev/ttyUSB0.device
 
 [Service]
 Type=oneshot
 TimeoutStartSec=300
 ExecStart=/sbin/getty -L ttyUSB0 115200 vt100
</code>

Note: journalctl reports an error when unplugging the USB device. But it doesn't seem to be a problem.

=== Disable WIFI and Blutooth ===
to disable onboard WiFi and BT on Pi3 add the following to file: ///boot/config.txt//
<code>
 dtoverlay=pi3-disable-wifi
 dtoverlay=pi3-disable-bt
</code>

For pi4 you can use
<code>
 dtoverlay=disable-wifi
 dtoverlay=disable-bt
</code>

=== Disable IPv6 ===
to disable IPv6 add the following to file: ///boot/config.txt//

<code>
 ipv6.disable=1
</code>

=== Reduce write IO to SD card ===
Albeit there are different approaches to this I typically start by disabling logging to disk.

First disable rsyslogd
<code>
 systemctl disable rsyslog
</code>

Then instruct journalctl to store it's log in volatile memory (ram)

Edit file: ///etc/systemd/journald.conf// and change the line 
<code>
 #Storage=auto
</code>
to
<code>
 Storage=volatile
</code>

Reboot for changes to take effect.

=== Check CPU temperature ===
<code>
 vcgencmd measure_temp
</code>

=== Static IP address ===
Edit the file: ///etc/dhcpcd/dhcpcd.conf//

<code>
 # Example static IP configuration:
 interface eth0
 static ip_address=192.168.0.10/24
 static ip6_address=fd51:42f8:caae:d92e::ff/64
 static routers=192.168.0.1
 static domain_name_servers=192.168.0.1 8.8.8.8 fd51:42f8:caae:d92e::1
</code>

ipv6 stuff is not necessary and can be omitted.

Static IP can also be setup in ///etc/network/inerfaces.d/eth0// as described in [[Network:Network and IPtables]] Ubuntu section.
Then ///etc/dhcpcd.conf// needs the line
<code>
denyinterfaces eth0
</code>

=== Enable Power Button ===
to enable a power button connected between GND and GPIO3 add the following to file: ///boot/config.txt//

<code>
dtoverlay=gpio.shutdown
</code>