RasPi-GetWifiAddrOnStartup

This repo demonstates how to save WIFI IP address automatically to a USB memory stick for a Raspberry Pi at startup.

Introduction

Many people prefer to connect a headless Raspberry Pi over WIFI, because wiring a Ethernet cable is inconvenivent or is just impossible (at least expensive, for Raspberry Pi Zero 2W, and laptops without Ethernet port). However, if you are moving the Raspberry Pi between office and home, the WIFI address may change and you may not have access to the router's dash board to get the IP address. This repo demonstates how to use python to save the WIFI IP address to a USB memory stick when the Raspberry Pi is booting up.

Getting started

• Prepare a USB memory stick and create an empty file ipaddr.txt in its root directory.

• Create the mounting directory /mnt/myusb for the USB stick on your Raspberry Pi.

• Download save_ip.py to your Raspberry Pi, e.g. /home/pi/scripts/.

• Insert the USB memory stick and test the function with command:

  sudo python save_ip.py

Run program on startup

There are many ways to run the program on startup (see here). Using rc.local file may be the easiest one.

Open the rc.local file:

sudo nano /etc/re.loacl

and add the following command just before exit 0:

sudo python /home/pi/scripts/save_ip.py >>/home/pi/logs/save_ip.txt 2>&1 &

The above command will run the python script at background, so it will not block the normal booting process. The stdout and stderr will be redirected to /home/pi/logs/save_ip.txt for diagnostic purpose.

Pitfalls

Initially, I used &>> to redirect stdout and stderr, which did not work, because /etc/rc.local still uses dash shell (thanks to this ansower). A comparison of redirection command between bash shell and POSIX form is shown in below table:

Bash	POSIX
foo &> bar	foo >bar 2>&1
foo &>> bar	foo >>bar 2>&1
foo |& bar	foo 2>&1 | bar