Create a Category Page for one Specific Category and Exclude this Category from the Main Page in WordPress

This blog serves as my digital notebook for more than eight years and I use to to collect all sorts of things, that I think are worth storing and sharing. Mainly, I blog about tiny technical bits, but recently I also started to write about my life here in Innsbruck, where I try to discover what this small city and its surroundings has to offer. The technical articles are written in English, as naturally the majority of visitors understands this language. The local posts are in German for the same reason. My intention was to separate this two topics in the blog and lot let the posts create any clutter between languages.

Child Themes

When tinkering with the code of your WordPress blog, it is strongly recommended to deploy and use a child theme. This allows to reverse changes easily and more importantly, allows to update the theme without having to re-implement your adaptions after each update. Creating a child theme is very easy and described here. In addition I would recommend using some sort of code versioning tool, such as Git.

Excluding a Category from the Main Page

WordPress offers user defined categories out of the box and category pages for each category. This model does not fit well for my blog, where I have static pages and a time series of blog posts on the main page. In order to prevent that the posts about Innsbruck show up at the main page, the category ‘Innsbruck’ needs to be excluded. We can create or modify the file functions.php in the child theme folder and add the following code.

<?php
add_action( 'wp_enqueue_scripts', 'theme_enqueue_styles' );
function theme_enqueue_styles() {
        wp_enqueue_style( 'parent-style', get_template_directory_uri() . '/style.css' );

}

// Exclude Innsbruck Category from Main Page
function exclude_category($query) {
     if ( $query->is_home() ) {
         // Get the category ID of the category Insbruck
         $innsbruckCategory = get_cat_ID( 'Innsbruck' );
         // Add a minus in front of the string
         $query->set('cat','-' . $innsbruckCategory);

      }
     return $query;
}
add_filter('pre_get_posts', 'exclude_category');

?>

This adds a filter which gets executed before the posts are collected. We omit all posts of the category Innsbruck, by adding a minus as prefix of the category ID. Of course you could also lookup the category ID in the administration dashboard, by hovering with your mouse over the category name and save one database query.

A Custom Page Specific for one Category

In the second step, create a new page in the dashboard. This page will contain all posts of the Innsbruck category that we will publish. Create the file page.php in your child theme folder and use the following code:

<?php
/**
 * The template for displaying all pages.
 *
 * This is the template that displays all pages by default.
 * Please note that this is the WordPress construct of pages
 * and that other 'pages' on your WordPress site will use a
 * different template.
 *
 * @package dazzling
 */

    get_header();
?>
    <div id="primary" class="content-area col-sm-12 col-md-8">
        <main id="main" class="site-main" role="main">

<?php
     // Specify the arguments for the post query
     $args = array(
        'cat' => '91', // Innsbruck category id
        'post_type' => 'post',
        'posts_per_page' => 5,
        'paged' => ( get_query_var('paged') ? get_query_var('paged') : 1),
    );

    if( is_page( 'innsbruck' )) {
        query_posts($args);
    }
?>

<?php while ( have_posts() ) : the_post(); ?>
    <?php get_template_part( 'content', 'post' ); ?>
    <?php
        // If comments are open or we have at least one comment, load up the comment template
        if ( comments_open() || '0' != get_comments_number() ) :
            comments_template();
        endif;
    ?>
<?php endwhile; // end of the loop. ?>


<div class="navigation">
    <div class="alignleft"><?php next_posts_link('&laquo; Ältere Beiträge') ?></div>
    <div class="alignright"><?php previous_posts_link('Neuere Beiträge &raquo;') ?></div>
</div>


    </main><!-- #main -->
</div><!-- #primary -->
<?php get_sidebar(); ?>
<?php get_footer(); ?>

In this code snippet, we define a set of arguments, which are used for filtering the posts of the desired category. In this example, I used the id of the Innsbruck category (91) directly. We define that we want to display posts only, 5 per page. An important aspect is the pagination. When we only display posts of one category, we need to make sure that Worpress counts the pages correctly. Otherwise the page would always display the same posts, regardless how often the user clicks on the next page button. The reason is that this button uses the global paged variable, which is set correctly in the example above.

The if conditional makes sure that only the pages from the Innsbruck category are displayed. The while loop then iterates over all posts and displays them. At the bottom we can see the navigational buttons for older and newer posts of the Innsbruck category.

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Vom Patscherkofel via Zirbenweg auf die Viggarspitze, zur Glungezerhütte und via Tulfeinjöchl zur Tulfeinalm

Mit dem Zirbenwegticket (21€ inkl. Bustransfer) kommt man (wie bereits beschrieben) sehr bequem auf den Patscherkofel. Von dort geht es entlang dem sehr angenehmen Zirbenweg Richtung Osten. Am Wochenende ist dieser Weg sehr überlaufen, unter der Woche war es jedoch erwartungsgemäß sehr ruhig. Nach etwa drei Kilometern des leichten Auf-und- abs erreicht man eine Abzweigung, auf der die Glungezerhütte ausgeschrieben ist. Der Weg wird schmäler und auch etwas steiler, dafür aber auch schöner, denn man wandert über große Steinplatten, die vom Latschengewächs sehr schön eingerahmt werden.

glungetzer-karte

Nach einem weiteren Kilometer erreicht man die Viggarspitze, deren Gipfelkreuz weithin sichtbar ist. Ein serpentinenartiger Steig führt einen recht Steil zum Gipfel, der sich in 2305m Seehöhe befindet. Der Ausblick ist phänomenal, man überblickt das gesamte Inntal, die Nordkette, Innsbruck natürlich aber auch den Patscherkofel und den Stubaier Gletscher. Nach kurzer Rast kann man den Gipfel auch in Richtung Süden wieder verlassen, was jedoch ein wenig Trittsicherheit abverlangt. Entlang eines sehr schönen Weges über Wiesen erreicht man wieder den Ursprünglicben Pfad zum Glungezer. Diesem folgt man weitere 3 Kilometer bis man nach einem doch recht anstrengenden Anstieg die Glungezerhütte erreicht. Dabei handelt es sich um die höchste ÖAV Hütte Nordtirols. Die Hütte schmiegt sich spektakulär an den Berg und auch hier hat man einen fantastischen Ausblick, befindet sich die Hütte doch auf 2610m Höhe. Zur Stärkung gabs Glungezer-Spaghetti à la Kathmandu, die man empfehlen kann. Auch zum Glungezer Gipfel ist es nicht mehr weit.

Glungezer wird als “Glúnggezer” ausgesprochen, mit kurzem “u” und mit Doppel-gg. Der Name Glungezer bedeutet “unter Felsblöcken dahinfließendes ‘glucksendes’ Wasser” –  Schmelzwasser, das “gluggezen tut” (Lautmalerei)

Der Glungezer trägt also einen “Wassernamen, der zum Gipfel hinaufgewandert ist”, wie es die Namensforscher nennen.

(Quelle)

Nach einem prüfenden Blick auf unsere Wadeln, hätte uns der Hüttenwirt zwar am liebsten über den Steig herunter geschickt, wir haben uns aber dennoch für die gemütlichere Variante über den Panoramasteig gewählt und uns wieder nach unten gearbeitet, Hier wären Wanderstöcke doch empfehlenswert gewesen, es geht doch etwas in die Knie. Parallel zum Zirbenweg gelangt man dann letzendlich zur Tulfeinalm, hinter der sich der Schlepplift (Glungezer Sektion 2) der sich im Sommer zu einem etwas abenteuerlichen Einersessellift wandelt, befindet.

tulfein

Mit diesem Gelangt man nach einer beschaulichen Fahrt zum Doppelsessellift, der einen schlussendlich ins Tal nach Tulfes befördert. Von dort gilt es nur noch den Wanderbus Richtung Igls zu erwischen, der einen wieder an den Ausgangspunkt zurückbringt.

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Persistent Data in a MySQL Docker Container

Running MySQL in Docker

In a recent article on Docker in this blog, we presented some basics for dealing with data in containers. This article will present another popular application for Docker: MySQL containers. Running MySQL instances in Docker allows isolating database infrastructure with ease.

Connecting to the Standard MySQL Container

The description of the MySQL docker image provides a lot of useful information how to launch and connect to a MySQL container. The first step is to create standard MySQL container from the latest available image.

sudo docker run \
   --name=mysql-instance 
   -e MYSQL_ROOT_PASSWORD=secret 
   -p 3307:3306 
   -d 
   mysql:latest

This creates a MySQL container where the root password is set to secret. As the host is already running its own MySQL instance (which has nothing to do with this docker example), the standard port 3306 is already taken. Thus we publish utilise the port 3307 on the host system and forward it to the 3306 standard port from the container.

Connect from the Host

We can then connect from the command line like this:

mysql -uroot -psecret -h 127.0.0.1 -P3307

We could also provide the hostname localhost for connecting to the container, but as the MySQL client per default assumes that a localhost connection is via a socket, this would not work. Thus when using the hostname localhost, we needed to specify the protocol TCP, wo that the client connects via the network interface.

mysql -uroot -psecret -h localhost --protocol TCP -P3307

Connect from other Containers

Connecting from a different container to the MySQL container is pretty straight forward. Docker allows to link two containers and then use the exposed ports between them. The following command creates a new ubuntu container and links to the MySQL container.

sudo docker run -it --name ubuntu-container --link mysql-instance:mysql-link ubuntu:16.10 bash

After this command, you are in the terminal of the Ubuntu container. We then need to install the MySQL client for testing:

# Fetch the package list
root@7a44b3e7b088:/# apt-get update
# Install the client
root@7a44b3e7b088:/# apt-get install mysql-client
# Show environment variables
root@7a44b3e7b088:/# env

The last command gives you a list of environment variables, among which is the IP address and port of the MySQL container.

MYSQL_LINK_NAME=/ubuntu-container/mysql-link
HOSTNAME=7a44b3e7b088
TERM=xterm
MYSQL_LINK_ENV_MYSQL_VERSION=5.7.14-1debian8
MYSQL_LINK_PORT=tcp://172.17.0.2:3306
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
MYSQL_LINK_PORT_3306_TCP_ADDR=172.17.0.2
MYSQL_LINK_PORT_3306_TCP=tcp://172.17.0.2:3306
PWD=/
MYSQL_LINK_PORT_3306_TCP_PORT=3306
SHLVL=1
HOME=/root
MYSQL_LINK_ENV_MYSQL_MAJOR=5.7
MYSQL_LINK_PORT_3306_TCP_PROTO=tcp
MYSQL_LINK_ENV_GOSU_VERSION=1.7
MYSQL_LINK_ENV_MYSQL_ROOT_PASSWORD=secret
_=/usr/bin/env

You can then connect either manually of by providing the variables

mysql -uroot -psecret -h 172.17.0.2
mysql -uroot -p$MYSQL_LINK_ENV_MYSQL_ROOT_PASSWORD -h $MYSQL_LINK_PORT_3306_TCP_ADDR -P $MYSQL_LINK_PORT_3306_TCP_PORT

If you only require a MySQL client inside a container, simply use the MySQL image from docker. Batteries included!

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Persistent Docker Containers

Docker Fundamentals

Docker has become a very popular tool for orchestrating services. Docker it much more lightweight than virtual machines. For instance do containers not require a boot process. Docker follows the philosophy that one container serves only one process. So in contrast to virtual machines which often bundle several services together, Docker is built for running single services per container. If you come from the world of virtualised machines, Docker can be a bit confusing in the beginning, because it uses its own terminology. A good point to start is as always the documentation and there are plenty of great tutorials out there.

Images and Containers

Docker images serves as templates for the containers. As images and containers both have hexadecimal ids they are very easy to confuse. The following example shows step by step how to create a new container based on the Debian image and how to open shell access.

# Create a new docker container based on the debian image
sudo docker create -t --name debian-test debian:stable 
# Start the container
sudo docker start  debian-test
# Check if the container is running
sudo docker ps -a
# Execute bash to get an interactive shell
sudo docker exec -i -t debian-test bash

A shorter variant of creating and launching a new container is listed below. The run command creates a new container and starts it automatically. Be aware that this creates a new container every time, so assigning a container name helps with not confusing the image with the container. The command run is in particular tricky, as you would expect it to run (i.e. launch) a container only. In fact, it creates a new one and starts it.

sudo docker run -it --name debian-test debian:stable bash

Important Commands

The following listing shows the most important commands:

# Show container status
sudo docker ps -a
# List available images
sudo docker images 
# Start or stop a container
sudo docker start CONTAINERNAME
sudo docker stop CONTAINERNAME
# Delete a container
sudo docker rm CONTAINERNAME

You can of course create your own images, which will not be discussed in this blog post. It is just important to know that you can’t move containers from your host so some other machine directly. You would need to commit the changes made to the image and create a new container based on that image. Please be aware that this does not include the actual data stored in that container! You need to manually export any data and files from the original container and import it in the new container again. This is another trap worth noting. You can, however,  also mount data in the image, if the data is available at the host at the time of image creation. Details on data in containers can be found here.

Persisting Data Across Containers

The way how Docker persists data needs getting used to in the beginning, especially as it is easy to confuse images with containers. Remember that Docker images serve only as the template. So when you issue the command sudo docker run …  this actually creates a container from an image first and then starts it. So whenever you issue this command again, you will end up with a new container which does share any data with the previously created container.

Docker 1.9 introduced data volume containers, which allow to create dedicated data containers which can be used from several other containers. Data volume containers can be used for persisting data. The following listing shows how to create a data volume container and mount the volume in a container.

# Create a data volume
sudo docker volume create --name data-volume-test
# List all volumes
sudo docker volume ls
# Delete the container
sudo docker rm debian-test
# Create a new container, now with the data volume 
sudo docker create -v data-volume-test:/test-data -t --name debian-test debian:stable
# Start the container
sudo docker start debian-test
# Get the shell
sudo docker exec -i -t debian-test bash

After we logged into the shell, we can see the data volume we mounted on the directory test-data:

root@d4ac8c89437f:/# ls -la
total 76
drwxr-xr-x  28 root root 4096 Aug  3 13:11 .
drwxr-xr-x  28 root root 4096 Aug  3 13:11 ..
-rwxr-xr-x   1 root root    0 Aug  3 13:10 .dockerenv
drwxr-xr-x   2 root root 4096 Jul 27 20:03 bin
drwxr-xr-x   2 root root 4096 May 30 04:18 boot
drwxr-xr-x   5 root root  380 Aug  3 13:11 dev
drwxr-xr-x  41 root root 4096 Aug  3 13:10 etc
drwxr-xr-x   2 root root 4096 May 30 04:18 home
drwxr-xr-x   9 root root 4096 Nov 27  2014 lib
drwxr-xr-x   2 root root 4096 Jul 27 20:02 lib64
drwxr-xr-x   2 root root 4096 Jul 27 20:02 media
drwxr-xr-x   2 root root 4096 Jul 27 20:02 mnt
drwxr-xr-x   2 root root 4096 Jul 27 20:02 opt
dr-xr-xr-x 267 root root    0 Aug  3 13:11 proc
drwx------   2 root root 4096 Jul 27 20:02 root
drwxr-xr-x   3 root root 4096 Jul 27 20:02 run
drwxr-xr-x   2 root root 4096 Jul 27 20:03 sbin
drwxr-xr-x   2 root root 4096 Jul 27 20:02 srv
dr-xr-xr-x  13 root root    0 Aug  3 13:11 sys
drwxr-xr-x   2 root root 4096 Aug  3 08:26 test-data
drwxrwxrwt   2 root root 4096 Jul 27 20:03 tmp
drwxr-xr-x  10 root root 4096 Jul 27 20:02 usr
drwxr-xr-x  11 root root 4096 Jul 27 20:02 var

We can navigate into that folder and create a 100 M data file with random data.

root@d4ac8c89437f:~# cd /test-data/
root@d4ac8c89437f:/test-data# dd if=/dev/urandom of=100M.dat bs=1M count=100
100+0 records in
100+0 records out
104857600 bytes (105 MB) copied, 6.69175 s, 15.7 MB/s
root@d4ac8c89437f:/test-data# du -h .
101M	.

When we exit the container, we can see the file in the host file system  here:

stefan@stefan-desktop:~$ sudo ls -l /var/lib/docker/volumes/data-volume-test/_data
insgesamt 102400
-rw-r--r-- 1 root root 104857600 Aug  3 15:17 100M.dat

We can use this volume transparently in the container, but it is not depending on the container itself. So whenever we have to delete to container or want to use the data with a different container, this solution works perfectly. Thw following command shows how we mount the same volume in an Ubuntu container and execute the ls command to show the content of the directory.

stefan@stefan-desktop:~$ sudo docker run -it -v data-volume-test:/test-data-from-debian --name ubuntu-test ubuntu:16.10 ls -l /test-data-from-debian
total 102400
-rw-r--r-- 1 root root 104857600 Aug  3 13:17 100M.dat

You can display a lot of usefil information about a container with the inspect command. It also shows the data container and where it is mounted.

sudo docker inspect ubuntu-test

...
        "Mounts": [
            {
                "Name": "data-volume-test",
                "Source": "/var/lib/docker/volumes/data-volume-test/_data",
                "Destination": "/test-data-from-debian",
                "Driver": "local",
                "Mode": "z",
                "RW": true,
                "Propagation": "rprivate"
            }
        ],
...

We delete the ubuntu container and create a new one. We then start the container, open a bash session and write some test data into the directory.

stefan@stefan-desktop:~$ sudo docker create -v data-volume-test:/test-data-ubuntu -t --name ubuntu-test ubuntu:16.10
f3893d368e11a32fee9b20079c64494603fc532128179f0c08d10321c8c7a166
stefan@stefan-desktop:~$ sudo docker start ubuntu-test
ubuntu-test
stefan@stefan-desktop:~$ sudo docker exec -it  ubuntu-test bash
root@f3893d368e11:/# cd /test-data-ubuntu/
root@f3893d368e11:/test-data-ubuntu# ls
100M.dat
root@f3893d368e11:/test-data-ubuntu# touch ubuntu-writes-a-file.txt

When we check the Debian container, we can immediately see the written file, as the volume is transparently mounted.

stefan@stefan-desktop:~$ sudo docker exec -i -t debian-test ls -l /test-data
total 102400
-rw-r--r-- 1 root root 104857600 Aug  3 13:17 100M.dat
-rw-r--r-- 1 root root         0 Aug  3 13:42 ubuntu-writes-a-file.txt

Please be aware that the docker volume is just a regular folder on the file system. Writing from both containers the same file can lead to data corruption. Also remember that you can read and write the volume files directly from the host system.

Backups and Migration

Backing up data is also an important aspect when you use named data volumes as shown above. Currently, there is no way of moving Docker containers or volumes natively to a different host. The intention of Docker is to make the creation and destruction  of containers very cheap and easy. So you should not get too attached to your containers, because you can re-create them very fast. This of course is not true for the data stored in volumes. So you need to take care of your data yourself, for instance by creating automated backups like this sudo tar cvfz Backup-data-volume-test.tar.gz /var/lib/docker/volumes/data-volume-test and re-store the data when needed in a new volume. How to backup volumes using a container is described here.

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