Friedrich Ewald My Personal Website

Posts

  • Syntax for Crontab

    Sep 12, 2016 · 1 min read

    Just a quick reminder for the syntax of the crontab for cronjobs: 

    # 1. Entry: Minute when the process will be started [0-60]
# 2. Entry: Hour when the process will be started [0-23]
# 3. Entry: Day of the month when the process will be started [1-28/29/30/31]
# 4. Entry: Month of the year when the process will be started [1-12]
# 5. Entry: Weekday when the process will be started [0-6] [0 is Sunday]
#
# all x min = */x
    Found on Stackoverflow

  • Angular2 - Loading indicator for Http service

    Aug 30, 2016 · 3 min read

    I wanted to notifiy the user, whenever I am doing an AJAX request in Angular2. To achieve this in an existing project, the goal was, to edit as little files as possible. The solution to this problem is very straightforward: The class Http needs to be extended which allows own code to be used. First, I needed to create the EventHttpService with the following code: 

    import { Http } from '@angular/http';
export class EventHttpService extends Http{}
    So far this class does nothing, it just extends the Http class, providing the same functionality. Therefore I needed to override all the methods which should perform a different action. Below is an example for the get method, with all the required imports. Of course, the whole class needs to be decorated with Injectable() so that we can inject it via Dependency Injection. 
    import { Injectable } from '@angular/core';
import { Http, RequestOptions, RequestOptionsArgs, Response, ConnectionBackend } from '@angular/http';
import { Observable } from 'rxjs/Observable';

@Injectable()
export class EventHttpService extends Http {
	public get(url: string, options?: RequestOptionsArgs) : Observable<Response> {
			// Own code before the request is performed
			var response = super.get(url, options);
			response.subscribe(null, error => {
				// Own code on error
			}, () => {
				// Own code on success
			});
			return response;
	}
}
    The comments in the source code above mark the points where the developer can implement own code. I used it, to increment and decrement a counter. When the counter starts from 0, an injected loading bar is shown and when the counter finally reaches 0, the loading bar is hidden. So far, the class does nothing, since we are not using it. The right place to use it, is app.module.ts: 
    @NgModule({
  imports:      [],
  declarations: [],
  providers: [
    HTTP_PROVIDERS,
    provide(Http, {
      useFactory: (xhrBackend: XHRBackend, requestOptions: Request) => new EventHttpService(xhrBackend, requestOptions),
      deps: [XHRBackend, RequestOptions]})
    ],
  bootstrap:    [ AppComponent ]
})
export class AppModule { }
    With this example, whenever Http.get is used, the EventHttpService is used automatically, executing my custom code. If you want to inject other services into Http, this is also straightforward, just add these to the constructor of the EventHttpService so that it looks as follows. In my case, I am using the awesome ng2-slim-loading-bar to show a loading bar, when the first AJAX request is performed. 
    public constructor(_backend: ConnectionBackend, _defaultOptions: RequestOptions, private loadingBar: SlimLoadingBarService) {
	super(_backend, _defaultOptions);
}
    Below is the complete code of the event-http.service.ts: 
    import { Injectable } from '@angular/core';
import { Http, RequestOptions, RequestOptionsArgs, Response, ConnectionBackend } from '@angular/http';
import { Observable } from 'rxjs/Observable';
import { SlimLoadingBarService } from 'ng2-slim-loading-bar';

@Injectable()
export class EventHttpService extends Http {
	private currentRequests: number = 0;

	public constructor(_backend: ConnectionBackend, _defaultOptions: RequestOptions, private loadingBar: SlimLoadingBarService) {
		super(_backend, _defaultOptions);
	}

	public get(url: string, options?: RequestOptionsArgs) : Observable<Response> {
		this.incrementRequestCount();
		var response = super.get(url, options);
		response.subscribe(null, error => {
			this.decrementRequestCount();
		}, () => {
			this.decrementRequestCount();
		});
		return response;
	}

	private decrementRequestCount() {
		if (--this.currentRequests == 0) {
			this.loadingBar.complete();
		}
	}

	private incrementRequestCount() {
		if (this.currentRequests++ == 0) {
			this.loadingBar.start();
		}
	}
}

  • VIM cheatsheet

    Jul 15, 2016 · 1 min read

    A cheatsheet for vom can be found here. I find this quite useful to begin with this powerful editor. The most important keys, if you just want to edit a file are:

    • i insert
    • esc exit insert mode
    • j move cursor down
    • k move cursor up
    • h move cursor left
    • l move cursor right
    • :w write (append the filename)
    • :q quit
    • :q! quit and do not save changes

  • Big O notation for many datatypes

    May 20, 2016 · 1 min read

    An interesting site which shows the Big O notation for all many commonly used datatypes is the Big O Cheatsheet.

  • Show open ports on the linux shell

    Apr 11, 2016 · 1 min read

    If you want to show all the currently open ports on linux, just use the following command: 

    netstat -lntu
    This gives you the following output: 
    Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN
tcp        0      0 0.0.0.0:3128            0.0.0.0:*               LISTEN
tcp6       0      0 :::22                   :::*                    LISTEN
udp        0      0 0.0.0.0:39790           0.0.0.0:*
udp        0      0 0.0.0.0:68              0.0.0.0:*
udp        0      0 0.0.0.0:41121           0.0.0.0:*
udp6       0      0 :::57063                :::*
udp6       0      0 :::42147                :::*
    Found here

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