Do not use transparent PNGs for iOS favicons

Safari on iOS accepts an apple-touch-icon favicon that is used for stuff like desktop bookmarks. Always define a solid background color for them.

If you use PNGs with a transparent background, Safari will use just set a black background on your pretty icon. This is almost never what you want.
You can fix that by applying a white background via ImageMagick like this:

convert a...

Migrate data between Redis servers

There is a reasonable simple way to move data between Redis servers: Simply temporarily configure the new server as a replica of the old server.

To do this:

  • Make sure the new Redis server can access the old server. If they are on different networks, a simple SSH tunnel will do.
  • Connect to the new server using redis-cli.
  • Tail the log of the redis server (found in /var/logs/redis) in another terminal.
  • Make sure the server is currently master and not already a replica (check INFO replication)
  • Enable replication with `REPLICAOF...
Linked contentAuto-destruct in 46 days

Updated: Inspecting a live Ruby process

Updated the gdb-inspect card with instructions for Ruby 2.4+.

Ruby 2.4+

TL;DR live inspection:

# First, find out the PID of your Ruby process (e.g. passenger-status)
$ gdb PID
(gdb) generate-core-file                  # generate core.PID file for later inspection
(gdb) t a a bt                            # thread apply all backtrace
(gdb) call (void) close(1)                # close the existing file descriptors for std...

How Rails and MySQL are handling time zones

When working with times and dates in Rails applications, you need to deal with the following problem:

  • In Rails, Time objects have a time zone. You can get the zone name by doing
  • This zone is considered when doing time calculations, e.g. 10 AM CEST minus 8 AM UTC is zero.
  • A datetime in MySQL does not have a zone. It just stores the literal string "2010-05-01 12:00:00".
  • That means that Rails must make assumptions about timestamps loaded from and written to MySQL.

Rails has two completely different modes ...


Linux: Open a file with the default application

If you are on a Linux shell and want to open a file with whatever default application is configured for that type, this often works:

xdg-open Quote.odt
xdg-open invoice.pdf
xdg-open index.html

Make an alias so you have a simpler API (like Mac OS): alias open=xdg-open.

whenever: Installing cron jobs only for a given Rails environment or Capistrano stage

We use the whenever gem to automatically update the crontab of the servers we deploy to. By default, whenever will update all servers with a matching role (we use the :cron role ).

This card describes how to install some tasks only for a given Rails environment or for a given Capistrano stage.

Installing jobs only for a given Rails environment

In yo...

Rspec: Expecting a Rake task to be called

This seems to be obvious, but you can expect Rake tasks to be called in RSpec.

it 'deletes all Users' do
   expect(Rake::Task['notify:critical_operation']).to receive(:invoke)
   expect { described_class.clean }.to change(User, :count).from(1).to(0) 

Note: Try to avoid logic in rake tasks and prefer to just call classes in them.


desc 'Some task'
task :some_task do

Installing multiple MySQL versions on the same Linux with mysql-sandbox

Ubuntu has a package mysql-sandbox that lets you install multiple MySQL versions into your user home:

  1. Install mysql-sandbox
sudo apt install mysql-sandbox
  1. Download the version of MySQL you want to use from
    Make sure to choose "Generic Linux" instead of "Ubuntu" so you get a .tar.gz instead of .deb

  2. cd into the directory the mysql binaries will be extracted to

mkdir -p ~/bin/sandbox_dist
cd ~/bin/sandbox_dist
  1. Build the sandbox

HTML: Making browsers wrap long words

By default, browsers will not wrap text at syllable boundaries. Text is wrapped at word boundaries only.

This card explains some options to make browsers wrap inside a long word like "Donaudampfschifffahrt".

Option 1: Soft hyphens

Unicode has a soft hyphen character you can use to mark optional word division opportunities. The soft hyphen is an invisible character with zero width. Only when the browser decides to wrap at a soft hyphen, it is turned in...


How to find out what is running on a port on a remote machine

By convention, common protocols use a defined port, like 80 for HTTP or 443 for HTTPS.

You can use nmap to find out what service is running behind a given port, and most often see some details about it. This can be helpful if servers don't offer the services you expect for some ports. If you'd like to see what ports are listing on your local machine, you might want to use netstat instead of nmap.

Note that nmap's service discovery may trigger several requests.


When using nmap, adding the -A switch ...

Git: How to add changes matching a regular expression

When you have many changes, and you want to spread them across different commits, here is a way to stage all changes matching a given regular expression for a single commit.


Consider the following git diff output.

diff --git a/file1.rb b/file1.rb
index 806ca88..36d536b 100644
--- a/file1.rb
+++ b/file1.rb
@@ -1,7 +1,5 @@
-# Here is a useless comment.
-# It will be removed.
 class File1
-  def foo
+  def bar
     # ...
diff --git a/file2.rb b/file2.rb
index 550e1c6..600f4e3 100644
--- a/file2.rb
+++ b/file2...

PSA: "index: true" in Rails migrations does not work as you'd expect

Several Rails migration methods accept index: true as an option to create an index. In some cases (like #add_column), this option is silently discarded. Know what you are doing, or use #add_index instead.


Consider the following migration.

class CreateExamples < ActiveRecord::Migration
  def change
    create_table :examples do |t|
      t.references :category, index: true
      t.boolean :positive, index: true
      t.integer :number_of_participants, index: true

Linked content

Five years of "Today I Learned" from Josh Branchaud

The linked GitHub repository is a bit like our "dev" cards deck, but groomed from a single person (Josh Branchaud). It includes an extensive list of over 900 TILs on many topics that might be interesting for most of us. (e.g. Ruby, Rails, Git, Unix..)


Here is an excerpt of all the Ruby TILs that were new to me. I encourage you to take your time to skim over the original list as well!


How to use a local gem in your Gemfile

You can use local copies of gems in your Gemfile like this:

gem 'spreewald', :path => '~/gems/spreewald'

As soon as you have bundled your project with the local copy of the gem, all code changes in the copy will be available on your project. So you can for example set a debugger or add console output in the gem and use it from your project.
If you checked out the gem with your versioning tool, you can easily reset your changes afterwards or make a pull request for the gem if you improved it.

Don't commit a Gemfile with local pat...


How to simulate limited bandwidth in Google Chrome and Firefox

Your development machine is usually on a very good network connection.
To test how your application behaves on a slow network (e.g. mobile), you can simulate limited bandwidth.


  • Open the dev tools (Ctrl+Shift+I or F12) and switch to the "Network" tab
  • In the row below the dev tool tabs, there's a throttling dropdown which reads "Online" by default.
  • Inside the dropdown, you will find a few presets and an option to add your own download/upload/latency settings.


  • Open the dev tools (Ctrl+Shift+I or F12) and switc...

Video transcoding: Web and native playback overview (April 2020)


Embedding videos on a website is very easy, add a <video> tag to your source code and it just works. Most of the time.

The thing is: Both the operating system and Browser of your client must support the container and codecs of your video. To ensure playback on every device, you have to transcode your videos to one or more versions of which they are supported by every device out there.

In this card, I'll explore the available audio and video standards we have right now. The goal is to built a pipeline that transcodes unknown ...

Linked contentRepeats

Colorful output for several linux command line tools: grc

Because colors improve readability so much.
On Ubuntu 18.04 you can install it with sudo apt install grc

From github:

For the impatient - try following commands:

grc netstat
grc ping hostname
grc tail /var/log/syslog
grc ps aux

Checklist: Rails Authentication

Authentication is a special part of web applications. On the one hand, it usually is a crucial security mechanism restrict access to certain people and roles. On the other hand, most users authenticate only once, so it is very unlikely to spot issues by accident.

So, here comes a quick checklist to help you verifying your authentication solution is all set.

  • This should be default: use HTTPS with HSTS. The HSTS part is important.
  • Use a reliable authentication solution, e.g. Clearance or [Devise...
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