2 = 1 ?!? — not really

As many people, we came across the following photo:

False proof that 2 = 1

In this mathematical “proof” the author tries to convince us that there is an error in math and it can be demonstrated using the above example. The steps taken by this person are the following:

  1. The proof starts by defining a equal to b (a = b).
  2. Then, it multiplies both sides with a which results to a^2 = ab.
  3. Next, it subtracts from both sides b^2 and we get a^2-b^2 = ab-b^2.
  4. Following, it extracts the common multiplier of both sides (a-b) resulting to (a+b)(a-b) = b(a-b).
  5. Here’s the nice part, the author divides both sides with (a-b) which results to a+b = b.
  6. Since a = b, it replaces a on the left side with a b and produces this state b+b = b => 2b = b.
  7. Finally, it divides by b both sides which results to 2 = 1.

The reason that this proof is false is because in step 5, where it divides both sides with (a-b) it really is dividing both sides with 0 (since we know for sure that a = b from step 1) which it cannot be defined as a valid division. So everything from step 5 and forward are invalid and thus the proof is wrong.

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The update cannot be installed because we will be unable to copy some files. This is usually due to inconsistent file permissions.: wp-admin/includes/update-core.php

Recently, we were trying to update a WordPress installation that was running under apache in a CentOS 7. When we would press the lovely blue button to update WordPress we would get the following error:

The update cannot be installed because we will be unable to copy some files. This is usually due to inconsistent file permissions.: wp-admin/includes/update-core.php

Installation Failed

At first we thought it was an issue with the privileges on the folder, so we did the following steps to fix the issue:

Find which user is controlling apache server:

We executed the following command to get the name of the local user that was handling the apache web server:

ps -ef | grep apache;

From which, we got the following results:

apache 7289 8523 1 10:35 ? 00:00:03 /usr/sbin/httpd -DFOREGROUND
apache 7293 8523 0 10:35 ? 00:00:02 /usr/sbin/httpd -DFOREGROUND
apache 7316 8523 0 10:35 ? 00:00:01 /usr/sbin/httpd -DFOREGROUND
apache 7317 8523 0 10:35 ? 00:00:01 /usr/sbin/httpd -DFOREGROUND
apache 7335 8523 1 10:35 ? 00:00:03 /usr/sbin/httpd -DFOREGROUND
apache 7347 8523 0 10:35 ? 00:00:01 /usr/sbin/httpd -DFOREGROUND
apache 7348 8523 0 10:35 ? 00:00:01 /usr/sbin/httpd -DFOREGROUND
apache 7349 8523 0 10:35 ? 00:00:01 /usr/sbin/httpd -DFOREGROUND
apache 7350 8523 0 10:35 ? 00:00:00 /usr/sbin/httpd -DFOREGROUND
apache 7351 8523 0 10:35 ? 00:00:01 /usr/sbin/httpd -DFOREGROUND
root 8071 7978 0 10:40 pts/1 00:00:00 grep --color=auto apache

The first column is the name of the user, so we found out that apache local user is the user executing the apache web server.

Repair all file and folder access rights:

After getting the above information we proceeded to update the access rights in our WordPress installation folder (/var/www/example.com)

find /var/www/example.com -type d -exec chmod 755 '{}' \;
find /var/www/example.com -type f -exec chmod 644 '{}' \;
chown -R apache:apache /var/www/example.com;

To our disappointment, nothing changed and we would still get the same error. Luckily enough we had another ace in our sleeve!

Checking the security context of each file and folder:

We executed the following command (ls -Z) to get the security context of the files and folders in the installation folder of WordPress:

ls -Z /var/www/example.com;

Where we got the following results:

drwxr-xr-x. apache apache unconfined_u:object_r:httpd_sys_content_t:s0 api
-rw-r--r--. apache apache unconfined_u:object_r:httpd_sys_content_t:s0 favicon.ico
-rw-r--r--. apache apache unconfined_u:object_r:httpd_sys_content_t:s0 index.php
-rw-r--r--. apache apache unconfined_u:object_r:httpd_sys_content_t:s0 license.txt
-rw-r--r--. apache apache unconfined_u:object_r:httpd_sys_content_t:s0 readme.html
-rw-r--r--. apache apache unconfined_u:object_r:httpd_sys_content_t:s0 r-file.php
-rw-r--r--. apache apache unconfined_u:object_r:httpd_sys_content_t:s0 wordfence-waf.php
-rw-r--r--. apache apache unconfined_u:object_r:httpd_sys_content_t:s0 wp-activate.php
drwxr-xr-x. apache apache unconfined_u:object_r:httpd_sys_content_t:s0 wp-admin
-rw-r--r--. apache apache unconfined_u:object_r:httpd_sys_content_t:s0 wp-blog-header.php
-rw-r--r--. apache apache unconfined_u:object_r:httpd_sys_content_t:s0 wp-comments-post.php
-rw-r--r--. apache apache unconfined_u:object_r:httpd_sys_content_t:s0 wp-config.php
-rw-r--r--. apache apache unconfined_u:object_r:httpd_sys_content_t:s0 wp-config-sample.php
drwxr-xr-x. apache apache unconfined_u:object_r:httpd_sys_rw_content_t:s0 wp-content
-rw-r--r--. apache apache unconfined_u:object_r:httpd_sys_content_t:s0 wp-cron.php
drwxr-xr-x. apache apache unconfined_u:object_r:httpd_sys_content_t:s0 wp-includes
-rw-r--r--. apache apache unconfined_u:object_r:httpd_sys_content_t:s0 wp-links-opml.php
-rw-r--r--. apache apache unconfined_u:object_r:httpd_sys_content_t:s0 wp-load.php
-rw-r--r--. apache apache unconfined_u:object_r:httpd_sys_content_t:s0 wp-login.php
-rw-r--r--. apache apache unconfined_u:object_r:httpd_sys_content_t:s0 wp-mail.php
-rw-r--r--. apache apache unconfined_u:object_r:httpd_sys_content_t:s0 wp-settings.php
-rw-r--r--. apache apache unconfined_u:object_r:httpd_sys_content_t:s0 wp-signup.php
-rw-r--r--. apache apache unconfined_u:object_r:httpd_sys_content_t:s0 wp-trackback.php
-rw-r--r--. apache apache unconfined_u:object_r:httpd_sys_content_t:s0 xmlrpc.php

What we got from this result: all files and folders marked with the httpd_sys_content_t are not directly editable by the apache web server even though the user has the rights to do so as it is a security step by SELinux.

httpd_sys_content_t
Use this type for static web content, such as .html files used by a static website. Files labeled with this type are accessible (read only) to httpd and scripts executed by httpd. By default, files and directories labeled with this type cannot be written to or modified by httpd or other processes. Note that by default, files created in or copied into /var/www/html/ are labeled with the httpd_sys_content_t type.

https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/6/html/managing_confined_services/sect-managing_confined_services-the_apache_http_server-types

The simple solution

We just changed the security context for the folders and files to httpd_sys_rw_content_t which allows apache web server to edit the files:

httpd_sys_rw_content_t
Files labeled with this type can be written to by scripts labeled with the httpd_sys_script_exec_t type, but cannot be modified by scripts labeled with any other type. You must use the httpd_sys_rw_content_t type to label files that will be read from and written to by scripts labeled with the httpd_sys_script_exec_t type.

https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/6/html/managing_confined_services/sect-managing_confined_services-the_apache_http_server-types
chcon -R --type httpd_sys_rw_content_t /var/www/example.com;

After this command finished, we updated WordPress as normal!

(In case you want to change the security context back to the original just execute chcon with httpd_sys_content_t as parameter on the folders and files you want to operate as read-only).


Ένα προληπτικό μέτρο για την προστασία της μπαταρίας του αυτοκινήτου σας κατά την διάρκεια του κατ’οίκον περιορισμού

Τα περισσότερα αυτοκίνητα (αν όχι όλα) καταναλώνουν ενέργεια ακόμη και όταν είναι σταθμευμένα, με τον κινητήρα σβηστό και χωρίς ορατά συστήματα που λειτουργούν. Τα ηλεκτρικά συστήματα, συμπεριλαμβανομένου του ρολογιού και των συστημάτων υπολογιστών επί του οχήματος, αποστραγγίζουν την μπαταρία σιγά-σιγά μέχρι να την αδειάσουν εντελώς.

Όταν το επίπεδο ενέργειας σε μια μπαταρία είναι χαμηλό, μπορεί να την αποτρέψει από το να είναι σε θέση να ξεκινήσει το αυτοκίνητό σας και θα χρειαστείτε μια ώθηση για να κάνετε το αυτοκίνητό σας να λειτουργήσει ξανά. Σε χειρότερο σενάριο, οι μπαταρίες που είναι εντελώς στραγγισμένες μπορεί να υποστούν ζημιά και πρέπει να αντικατασταθούν.

Επομένως, εάν δεν μετακινήσετε το αυτοκίνητό σας για μεγάλο χρονικό διάστημα (όπως κατά τη διάρκεια της περιόδου κλειδώματος), τότε η εκτεταμένη έλλειψη χρήσης μπορεί να καταστρέψει την μπαταρία και σας αναγκάσει να την αντικαταστήσετε.

Απλή λύση

Ελέγξτε το εγχειρίδιο λειτουργίας του αυτοκινήτου σας ή επικοινωνήστε με τον αντιπρόσωπό σας πριν αποσυνδέσετε την μπαταρία. (Δεν είμαστε σίγουροι αν υπάρχει ένα αυτοκίνητο εκεί έξω που δεν θα ήθελε να αφαιρεθεί η μπαταρία του)

Αποσυνδέστε την μπαταρία από το αυτοκίνητό σας για να σταματήσετε την αποστράγγιση ενέργειας.
Για να το κάνετε, αφαιρέστε το καλώδιο από την αρνητική θύρα (έχει το σύμβολο "-" και είναι συνήθως χρώματος μαύρου).

Μην αφήσετε τα εκτεθημένα μεταλλικά άκρα των αρνητικών και θετικών καλωδίων να αγγίζουν σε καμία περίπτωση.


Handbook of COVID-19 Prevention and Treatment

Handbook of COVID-19 Prevention and Treatment (23 downloads)

This is an unprecedented global war, and mankind is facing the same enemy, the novel corona- virus. And the first battlefield is the hospital where our soldiers are the medical workers.
To ensure that this war can be won, we must first make sure that our medical staff is guaranteed sufficient resources, including experience and technologies. Also, we need to make sure that the hospital is the battleground where we eliminate the virus, not where the virus defeats us.
Therefore, Jack Ma Foundation and Alibaba Foundation have convened a group of medical experts who have just returned from the frontlines of fighting the pandemic. With the support of The First Affiliated Hospital, Zhejiang University School of Medicine (FAHZU), they quickly published a guidebook on the clinical experience of how to treat this new coronavirus. The treatment guide offers advice and reference against the pandemic for medical staff around the world who are about to join the war.
Thanks to the medical staff from FAHZU. While taking huge risks in treating COVID-19 patients, they wrote down their treatment experience day and night in this Handbook.
Over the past 50 days, 104 confirmed patients have been admitted to FAHZU, including 78 severe and critically ill ones. Thanks to the pioneering efforts of medical staff and the application of new technologies, to date, we have witnessed a miracle. No staff is infected, and there is no missed diagnosis or patient deaths.
Today, with the spread of the pandemic, these experiences are the most valuable sources of information and the most important weapon for medical workers on the battlefield. This is a brand-new disease, and China was the first to suffer from the pandemic. Isolation, diagnosis, treatment, protective measures, and rehabilitation have all been started from scratch, but we hope that with the advent of this Handbook doctors and nurses in other affected areas can learn from our experience when entering the battlefield and they won’t have to start from zero.
This pandemic is a common challenge faced by mankind in the age of globalization. At this moment, sharing resources, experiences and lessons, regardless of who you are, is our only chance to win. Because the real remedy for epidemics is not isolation, but cooperation.
This war has just begun.

Handbook of COVID-19 Prevention and Treatment (23 downloads)

Handbook of COVID-19 Prevention and Treatment (23 downloads)


How to find differences between two directories using diff

Gotta love diff!

Finding all files and folders that are different in two locations is extremely easy. Using only two parameters we can exhaustively compare two directories, including their sub-directories and produce a list of their differences as such:

diff -qr folder-1/ folder-2/;

The -q parameter instructs diff to print only the files that are different and thus not spam us with thousands of files that are the same.

The -r parameter turns on the recursive feature which instructs diff to check all sub-folders and their files in the two directories under investigation.

Example run:

diff -qr Desktop/source/ /media/tux/My\ Disk/backup\ A/

Files Desktop/source/Camera/20191023_171328.jpg and /media/tux/My\ Disk/backup\ A/Camera/20191023_171328.jpg differ

Files Desktop/source/Camera/VID_20191011_115231.mp4 and /media/tux/My\ Disk/backup\ A/Camera/VID_20191011_115231.mp4 differ

diff: /media/xeirwn//media/tux/My\ Disk/backup\ A/Camera/IMG_20191225_165939.jpg: Input/output error