Allgemein

Relay lifetime online calculator

TechOverflow calculators:
You can enter values with SI suffixes like 12.2m (equivalent to 0.012) or 14k (14000) or 32u (0.000032) as well as values with units (like 12V).
The results are calculated while you type and shown directly below the calculator, so there is no need to press return or click on a Calculate button. Just make sure that all inputs are green by entering valid values.

Note: A cycle is typically defined as switching the relay on and switching it off again, i.e. one cycle consists of two switch actions (on + off).

In case you know how many seconds between cycles:

 

s

cycles

N_{\text{predicted}} = N_{\text{nominal}} \cdot \text{Seconds between cycles}

In case you know how many relay cycles per second:

Hz

cycles

N_{\text{predicted}} = \frac{N_{\text{nominal}}}{\text{Cycles per second}}
Posted by Uli Köhler in Allgemein

Go HTTP Server minimal example

package main

import (
    "fmt"
    "log"
    "net/http"
)

func main() {
    http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
        fmt.Fprintf(w, "Hello world!")
    })

    // log.Fatal shows you if there is an error like the
    //  port already being used
    log.Fatal(http.ListenAndServe(":8080", nil))
}

Save this file as Main.go in a directory called GoHTTPServer, run go build && ./GoHTTPServer. Then, open http://localhost:8080 in your browser and see the result for yourself.

Posted by Uli Köhler in Allgemein

Go HTTP client minimal example

This example gets the current IPv4 address from TechOverflow’s IP address HTTP API How to get your current IPv4 address using wget.

package main

import (
    "io/ioutil"
    "net/http"
)

func main() {
    // Perform request
    resp, err := http.Get("https://ipv4.techoverflow.net/api/get-my-ip")
    if err != nil {
        print(err)
        return
    }
    // Cleanup when this function ends
    defer resp.Body.Close()
    // Read all the response data into a []byte
    body, err := ioutil.ReadAll(resp.Body)
    // Decode & print
    println(string(body))
}

Save as Main.go in a new project directory (e.g. GoHttpTest) and run go build in that directory.
After that, you can run ./GoHttpTest which should print your IPv4 adress. Example:

uli@server ~/GoHttpTest % go build
uli@server ~/GoHttpTest % ./GoHttpTest 
91.59.80.56

 

Posted by Uli Köhler in Allgemein, Go

How to fix Angular Cannot find module ‘@angular/http’ after ‘ng update’

When updating to Angular 8.x+, developers commonly encouter this type of error:

app/app.module.ts:9:42 - error TS2307: Cannot find module '@angular/http'.

9 import {HttpModule, RequestOptions} from '@angular/http';

The solution

The @angular/http module has been removed in recent versions of Angular and replaced by @angular/common/http.  Furthermore, there are some name changes that might need manual fixing, including:

  • HttpModuleHttpClientModule
  • HttpHttpClient
  • URLSearchParams ⇒ HttpParams

Note that the replacements are not always exact equivalent and hence might require some work to get working

See the official @angular/http deprecation guide for more details on how to migrate.

Posted by Uli Köhler in Allgemein

How to fix vpnc-disconnect ‘no vpnc found running’

Problem:

You want to disconnect from your vpnc VPN using vpnc-disconnect, but you see this error message:

Solution:

vpnc-disconnect doesn’t know the difference between no vpnc is running and permission denied when trying to stop the vpnc daemon.

In most cases, using sudo:

sudo vpnc-disconnect

will fix the issue.

If this does not resolve the issue for you, check if you used the --pid-file argument to vpnc. vpnc-disconnect always assumes that the PID file is /var/run/vpnc.pid.

Alternatively, check out our alternate methods of stopping vpnc.

Posted by Uli Köhler in Allgemein

How to dump SQL from SQLite3 database file

If you have a SQLite3 database file (usually the filename extension is .db) and you want to dump it as SQL code, you can use

sqlite3 [database file] .dump

For example:

$ sqlite3 /usr/share/command-not-found/commands.db .dump
[...]
DELETE FROM sqlite_sequence;
INSERT INTO sqlite_sequence VALUES('packages',14669);
INSERT INTO sqlite_sequence VALUES('commands',47706);
CREATE INDEX idx_commands_command ON commands (command);
CREATE INDEX idx_packages_name ON packages (name);
COMMIT;

Missing the sqlite3 executable?

In case you don’t have the sqlite3 executable, you can install it using

sudo apt -y install sqlite3

on Ubuntu/Debian-based system

Posted by Uli Köhler in Allgemein, SQLite

How to fix C error ‘stderr undeclared’

Problem:

You have C code like

fprintf(stderr, "test");

but when you try to compile it you see an error message like

/home/uli/myproject/main.c:9:25: error: ‘stderr’ undeclared (first use in this function)
                 fprintf(stderr, "test");

Solution:

Add

#include <stdio.h>

at the top of the source file where the error occured. Including stdio.h will include symbols like printf, fprintf, stdin, stdout and stderr.

Posted by Uli Köhler in Allgemein

How to install MicroK8S (MicroKubernetes) on Ubuntu in 30 seconds

This set of commands will install & start MikroK8S (MikroKubernetes) on Ubuntu and similar Linux distributions.

sudo snap install microk8s --classic
sudo snap install kubectl --classic
sudo microk8s.enable # Autostart on boot
sudo microk8s.start # Start right now
# Wait until microk8s has started
until microk8s.status ; do sleep 1 ; done
# Enable some standard modules
microk8s.enable dashboard registry istio

For reference see the official quickstart manual.

Posted by Uli Köhler in Allgemein, Cloud, Container, Kubernetes

How to fix kubectl ‘The connection to the server localhost:8080 was refused – did you specify the right host or port?’

Problem:

You want to configure a Kubernetes service using kubectl using a command like

kubectl patch service/"my-elasticsearch-svc" --namespace "default"   --patch '{"spec": {"type": "LoadBalancer"}}'

but you only see this error message:

The connection to the server localhost:8080 was refused - did you specify the right host or port?

Solution:

Kubernetes does not have the correct credentials to access the cluster.

Add the correct credentials to the kubectl config using

gcloud container clusters get-credentials [cluster name] --zone [cluster zone]

e.g.

gcloud container clusters get-credentials cluster-1 --zone europe-west3-c

After that, retry your original command.

In case you don’t know your cluster name or zone, use

gcloud container clusters list

to display the cluster metadata.

Credits to this StackOverflow answer for the original solution.

Posted by Uli Köhler in Allgemein, Cloud, Container, Kubernetes

Fixing NodeJS Intl.DateTimeFormat not formatting properly for locales

Symptom:

NodeJS starting from version v9.x supports the ES6 Intl.DateTimeFormat

When you use it with the ‘en-US’ locale, it works properly and prints "August 13, 2018":

const df = new Intl.DateTimeFormat('en-US', {day: 'numeric', month: 'long', year: 'numeric', timeZone: 'UTC'});
console.log(df.format(new Date("2018-08-13T04:00:00.000Z")));

However, using it with a different locale fails:

const df = new Intl.DateTimeFormat('de', {day: 'numeric', month: 'long', year: 'numeric', timeZone: 'UTC'});
console.log(df.format(new Date("2018-08-13T04:00:00.000Z")));

While you would expect this to print "13. August 2018" , it will print "2018 M08 13"

Reason:

By default, NodeJS is only built with small-icu support, thereby only installing the en-US locale in order to reduce the installation filesize.

Solution 1 (preferred):

You can use the intl polyfill module to completely replace NodeJS’s implementation of intl:

Installation:
npm i --save intl
Usage:
// Replace Intl by polyfill
Intl = require("intl")

const df = new Intl.DateTimeFormat('de', {day: 'numeric', month: 'long', year: 'numeric', timeZone: 'UTC'});
console.log(df.format(new Date("2018-08-13T04:00:00.000Z")));

This will print 13. August 2018 as expected.

Solution 2 (alternate):

You can use the full-icu package to continue using the NodeJS ICU implementation (i.e. no polyfill), but just install the ICU data.

While this reduces the total installation filesize, installation is slow and the exact method depends on the NodeJS version and requires more work than just using the intl polyfill.

In order to install, use

npm i --save full-icu

This will take some time to compile the data and then will print instructions like this:

 √ icudt62l.dat (link)
Node will use this ICU datafile if the environment variable NODE_ICU_DATA is set to “node_modules/full-icu”
or with node --icu-data-dir=node_modules/full-icu YOURAPP.js
 For package.json:
{"scripts":{"start":"node --icu-data-dir=node_modules/full-icu YOURAPP.js"}}

By the way, if you have full data, running this in node:
> new Intl.DateTimeFormat('es',{month:'long'}).format(new Date(9E8));
... will show “enero”. If it shows “January” you don't have full data.
News: Please see https://github.com/icu-project/full-icu-npm/issues/6

In order to actually use full-icu, you need to use the --icu-data-dir=node_modules/full-icu argument every time you run node. In order to run node interactively, use

node --icu-data-dir=node_modules/full-icu

If you use scripts in your application (e.g. the start script, i.e. what gets executed if you run npm start), you need to adjust the configuration in package.json:

Instead of

// [...]
"scripts": {
    "start": "node index.js"
}
// [...]

use

// [...]
"scripts": {
    "start": "node --icu-data-dir=node_modules/full-icu index.js"
}
// [...]

Depending on your application, you might need to use a different script name than index.js – common names include server.js and start.js

Posted by Uli Köhler in Allgemein, Javascript, NodeJS

What do ko, Mo & Go mean?

ko , Mo and Go are french for kb, Mb and Gb:

ko (kilooctet) = kb (kilobytes)
Mo (Mégaoctet) = Mb (Megabytes)
Go (Gigaoctet) = Gb (Gigabytes)
To (Téraoctet) = Tb (Terabytes)
Posted by Uli Köhler in Allgemein

How to use BIOS instead of UEFI firmware for a VMWare virtual machine

Recent versions of VMWare’s VMPlayer use UEFI firmwares by default if you select a UEFI-compatible operating system during VM creation.
There are some cases, however, in which you have to change the VM to a classical BIOS firmware afterwards, for example if you have your operating system installed without UEFI support or if you selected the wrong operating system.

Currently, it seems to be impossible do change the firmware setting in the GUI, therefore you first have to find the .vmx file corresponding to the virtual machine. Usually it is located at ~/vmware/<Name of VM>/<Name of VM>.vmx.

After stopping the VM , open that file in your favourite text editor and look for this line:

firmware = "efi"

Change it to

firmware = "bios"

After that, save the file, restart VMPlayer and start your VM.

Posted by Uli Köhler in Allgemein

Downloading & reading a ZIP file in memory using Python

Problem:

You want to retrieve a ZIP file by downloading it from an URL in Python, but you don’t want to store it in a temporary file and extract it later but instead directly extract its contents in memory.

Solution:

In Python3 can use io.BytesIO together with zipfile (both are present in the standard library) to read it in memory.
The following example function provides a ready-to-use generator based approach on iterating over the files in the ZIP:

import requests
import io
import zipfile

def download_extract_zip(url):
    """
    Download a ZIP file and extract its contents in memory
    yields (filename, file-like object) pairs
    """
    response = requests.get(url)
    with zipfile.ZipFile(io.BytesIO(response.content)) as thezip:
        for zipinfo in thezip.infolist():
            with thezip.open(zipinfo) as thefile:
                yield zipinfo.filename, thefile

 

Posted by Uli Köhler in Allgemein

Fixing VTiger “Illegal request” for links from other domains

Problem:

You’ve got a link to your VTiger installation from another domain, but any time you open it, you get an Illegal request error message, even though you are logged in correctly.

Solution:

The reason for this error message is that vtiger validates the Referer (i.e. source URL of the request) as a protection layer against certain security issues, for example CSRF (cross-site request forgery). We will disable the referer check. Be sure to understand the implications before you do as suggested.

Disabling involves only editing a single code line. I tested this with VTiger 6.5.0, but likely only minor adjustments have to be made for other versions.

Steps to fix:

  • Open <your vtiger directory>/includes/http/Request.php in a text editor
  • In the editor. search for Illegal request. You will see a code block like this:
protected function validateReferer() {
$user=  vglobal('current_user');
        // Referer check if present - to over come 
        if (isset($_SERVER['HTTP_REFERER']) && $user) {//Check for user post authentication.
                global $site_URL;
                if ((stripos($_SERVER['HTTP_REFERER'], $site_URL) !== 0) && ($this->get('module') != 'Install')) {
                        throw new Exception('Illegal request');
                }
        }
        return true;
}
 
  • Comment out throw new Exception('Illegal request'); with // (results in //throw new Exception('Illegal request');)
  • The code block should now look like this:
protected function validateReferer() {
$user=  vglobal('current_user');
        // Referer check if present - to over come 
        if (isset($_SERVER['HTTP_REFERER']) && $user) {//Check for user post authentication.
                global $site_URL;
                if ((stripos($_SERVER['HTTP_REFERER'], $site_URL) !== 0) && ($this->get('module') != 'Install')) {
                        //throw new Exception('Illegal request');
                }
        }
        return true;
}
 
  • Save the file
  • The fix should be in effect immediately, else restart your webserver.
Posted by Uli Köhler in Allgemein

Script user input

Problem

You want to remote control a program but unfortunately this program has only a “klick&gaudy”(*) interface.
(*) Okok – only has a graphical user interface (short GUI).

Solution

You may use xdotool in order to script user actions. To install this tool, use:

sudo apt-get install xdotool

Now you can get the position of the mouse pointer with:

xdotool getmouselocation

or set it via:

xdotool mousemove 400 300 (This means set the mouse to position x=400, y=300; Point of origin is the top left corner of the screen.)

In order to click use:

xdotool click 1

And in order to type a text (e.g. into a control field of the GUI)

xdotool type 'Hello World'

(P.S.: xdotool has much more options … Once this tool got installed type man xdotool in order to see them all.)

Posted by Yann Spöri in Allgemein

Play a sound in a Webbrowser

Problem:

You want to play some sounds in a webbrowser.

Solution:

Modern Browsers have a fancy integrated AudioContext that allows you to play sounds. Here is an example (JavaScript Code):

// get the AudioContext
window.AudioContext = window.AudioContext || window.webkitAudioContext;

// Initialize audio context
var context = new AudioContext();

// Create an oscillator ... via this oscillator we can then play different sounds
var oscillator = context.createOscillator();
oscillator.frequency.value = 440; // this is an "A"
oscillator.type = "square";

// attach the oscillator to the sound output
oscillator.connect(context.destination);

oscillator.start(0); // start the oscillator (0=now) ...
oscillator.stop(1);  // stop playing this sound after 1 second
Posted by Yann Spöri in Allgemein

Graph layouting via Graphviz

Problem:

You want to display a Graph.

Solution:

Create a simple text file describing your graph and save it with a .dot file extension:

graph {
node1 -- node2;
node2 -- node3;
node3 -- node4;
node4 -- node1;
}

Afterwards you can use a program of the graphviz package (sudo apt-get install graphviz) in order to visualize the graph. This package contains different layouting programs like dot, neato, fdp (all from the GraphViz project) etc. Simply call one of these programs in order to visualize the graph:

neato -Tsvg yourFile.dot -o outputFile.svg

Output from this command:

Posted by Yann Spöri in Allgemein

Yet another Atom Arduino blinker

While experimenting with Atom I produced a minimal example for making my old Arduino Uno blink its LED periodically.

Although there are plenty of examples out there (some of them even work!) I didn’t want to introduce any dependency to the Arduino libraries. Besides making it harder to build (even if arscons is quite nice) it increases the object size and Arduino only supports a limited range of processors. I need the flexibility to use the code on controlles like the ATTiny45

Continue reading →

Posted by Uli Köhler in Allgemein, Embedded