Forem: Kevin John Mulligan

Resolving a common mistake with `jq`

Kevin John Mulligan — Sun, 23 Oct 2022 05:22:26 +0000

jq is a lightweight and flexible command-line JSON processor.

Scenario:

You may want to use jq to display the package.json file you are working on in a JS project but you're likely to encounter this error.

jq package.json

jq: error: package/0 is not defined at <top-level>, line 1:
package.json
jq: 1 compile error

Resolution:

The correct syntax includes an additional . which specifies which filter to use.
jq . package.json

From the website

The absolute simplest filter is .. This is a filter that takes its input and produces it unchanged as output. That is, this is the identity operator.

General Troubleshooting 🎯

Kevin John Mulligan — Wed, 25 May 2022 05:36:34 +0000

General Trouble shooting

https://elinux.org/R-Pi_Troubleshooting

Two massive problems with Pis: Power supplies and SD cards

Power supplies

The all Pis released before the Pi4 require 5 volts to run correctly. It's easy to find a 5V USB charger, however the USB cable and the Pis ciruitry cause a slight drop in voltage resulting in under voltage which can cause SD card corruption amongst other things.

Solutions

Get a high quality short USB cable to reduce the voltage drop. Low quality materials have a higher resistance, as does a longer cable.
Get a high quality or purpose built power supply for the Pi, it can accept up to 5.25V (10% tolerance) and will reduce the voltage to the required 5V on board. The official website says the Pi requires 5.1V and 2.5A for all Pis before Pi4 which is a Type-C connector and requires 3A

SD cards

Using an old SD card (especially one previously used in a phone) can waste a lot of time.

The card will be slower and is more likely to have errors than a new card.

Common issues

https://circuitdigest.com/article/top-10-common-issues-while-using-raspberry-pi

Default setup - Display IP on startup 🌐

Kevin John Mulligan — Wed, 25 May 2022 05:34:10 +0000

Display IP on startup

Show IP address on startup (for connection via SSH)

Create "display_ip.py"
Run this command to install the modules

$pip install urlopen socket json

Run this command to open the cron jobs file

$ sudo crontab -e

Add this to the cron jobs file. (Sleep for 1 minute to allow the internet to connect)

@reboot /bin/sleep 60; python3 /home/pi/display_ip.py

Addendum

File: display_ip.py

#!/usr/bin/python3

from urllib.request import urlopen
import json
import socket

from tkinter import *
import sys
import os

if os.environ.get('DISPLAY','') == '':
    print('no display found. Using :0.0')
    os.environ.__setitem__('DISPLAY', ':0.0')


rootWindow = Tk()
rootWindow.title('IP Addresses')

hostName = Label(rootWindow, font = ('TkFixedFont', 20),)
hostName.grid(sticky = W, row = 1, column = 0, padx = 10, pady = (20,10))
localIP = Label(rootWindow, font = ('TkFixedFont', 20))
localIP.grid(sticky = W, row = 2, column = 0, padx = 10, pady = (20,10))
publicIP = Label(rootWindow, font = ('TkFixedFont', 20),)
publicIP.grid(sticky = W, row = 3, column = 0, padx = 10, pady = (20,10))

ip = urlopen('http://httpbin.org/ip').read()
ip = ip.decode('utf-8')
ip = json.loads(ip)
testIP = "8.8.8.8"
s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
s.connect((testIP, 0))
ipaddr = s.getsockname()[0]
host = socket.gethostname()

localIP.config(text= "LAN:  " + ipaddr)
publicIP.config(text="WAN:  " + ip['origin'])
hostName.config(text="Host: " + host)

rootWindow.mainloop()

Unix commands ⌨️

Kevin John Mulligan — Wed, 25 May 2022 05:25:32 +0000

Navigation

cd - change directory
ls - list contents of directory

Make a file executable for users, group, owner:

sudo chmod ugo=x file.txt

Display contents of a file

cat file.txt

Raspberry Pi tips 🥧

Kevin John Mulligan — Wed, 25 May 2022 05:23:01 +0000

Using a Raspberry Pi

Background

I have used various Raspberry Pis since 2015, and during that time I've learnt a lot. Yet, I somehow manage to forget vast swaths of important information each time which results in hours of lost time due to making the same mistakes and having to relearn.

Table of contents:

Unix commands
Default setup (display the IP on startup)
General troubleshooting

Cover image from Raspberry Pi Ltd