Joining PDFs with Linux command line
There are several ways to merge two (or more) PDF files to a single file using the Linux command line.
If you're looking for graphical tools to edit or annotate a PDF, we have a separate card for that.
PDFtk (recommended)
PDFtk is a great toolkit for manipulating PDF documents. You may need to install it first (sudo apt install pdftk
).
Merging multiple files works like this:
pdftk one.pdf two.pdf cat output out.pdf
Unlike pdfjam, PDFtk should not mess with page sizes but simply joins pages as they are.
...
Known MariaDB version incompatibilities
MariaDB (and MySQL) is released in different versions with different behaviors. For backwards compatibility this can be managed with the sql_mode
option.
Following you can find behavior differences which cannot be changed by this option. This can cause a regression in an existing application. There is no workaround available. The application needs to be adapted.
Queries with aggregated date types
For a database instance of MariaDB-10...
Ruby: How to use global variables for a conditional debugger
You can share a state in Ruby with global variables. Even if you should avoid them whenever possible, for debugging an application this could be temporary quite handy.
Example:
class User
after_save { byebug if $debug; nil }
def lock
self.locked = true
save
end
end
Rspec.describe User do
let(:user) { create(:user) }
before do
# Many users are created and saved in this hook, but we don't want the debugger to stop for them...
Regular Expressions: Excessive backtracking can get yourself in trouble
Two weeks ago, Cloudflare was struck by a global outage that lasted ~30 minutes. The incident was rooted on a CPU exhaustion caused by a single regular expression containing some catastrophic backtracking:
.*(?:.*=.*)
This is a small reminder do keep using the lazy operator ?
whenever possible and furthermore be aware that regular expressions should not only be unit-tested but also evaluated in terms of performance. See <https://makandracards.com/makandra/494822-regul...
Webpack(er): A primer
webpack is a very powerful asset bundler written in node.js to bundle (ES6) JavaScript modules, stylesheets, images, and other assets for consumption in browsers.
Webpacker is a wrapper around webpack that handles integration with Rails.
This is a short introduction.
Installation
If you haven't already, you need to install node.js and Yarn.
Then, put
gem 'webpacker', '~> 4.x' # check if 4.x is still cu...
RubyMine: Efficiently filtering results in the "Finder" overlay
RubyMine comes with a nice way to grep through your project's files: The finder (ctrl + shift + f
). Don't be discouraged about the notice 100+ matches in n+ files
if your searched keyword is too general or widely used in your project.
RubyMine comes with a few ways to narrow down the resulting list, don't hesitate to apply those filters to speed up your search. Your keybinding might vary based on your personal settings.
File mask (alt + k
)
If you already know the file extension of your ...
Jasmine: using async/await to write nice asynchronous specs
Jasmine has long standing support for writing asynchronous specs. In days gone by we used the done
callback to achieve this, but these days it is possible to write much more readable specs.
Async specs
As a first example, say we want to check that some form disables the submit button while working.
// bad (how we used to do it)
beforeEach(() => {
this.form = setupMyForm()
this.submitButton = findTheSubmitButton()
})
it('disables the submit button while working', (done) => {
expect(this.submitButton.disabled).toBe(false)
...
ActiveType 1.2 supports "change_association"
With ActiveType 1.2 you can modify associations (has_many
etc.) after they have been defined.
One common use case for this is to change an association inside a form model, like this:
class Credential < ActiveRecord::Base
end
class User < ActiveRecord::Base
has_many :credentials
end
class SignUpCredential < ActiveType::Record[Credential]
end
class SignUp < ActiveType::Record[User]
change_association :credentials, class_name: 'SignUpCredential'
end
Now, if you load `credentials...
Katapult EOL
Katapult was an endeavor to dramatically speed up starting a new Rails application. However, it turned out to save less time than expected, while requiring more time for maintenance than anticipated. Since its benefits fell too low, we have decided to not continue developing Katapult.
You can still use Katapult for generating ready-to-run applications with model CRUD, authentication and all of Katapult's features, but the rapid development of the web will quickly render the generated code antiquated. Nevertheless, its architecture may remai...
Deal with certain travis CI failures
Travis changed their default distribution from Ubuntu 14.04 (trusty) to 16.04 (precise). This might break your test setup for new builds.
You can solve this issue by freezing your test distribution in the .travis.yml
to Ubuntu 14.04 until you have the time to solve all the issues you will have in 16.04:
dist: trusty
Error details
Here are few indicators that you ran into this issue:
Connection to the PostgreSQL database does not work anymore
Your travis-ci builds might have started failing on the usual
psql -c...
Documenting your Rails project's Node.js version in .nvmrc
Not all versions of Node.js are compatible with each other. Also npm packages may require a minimum or maximum version of Node.js. We use nvm on our development PCs so we can operate multiple versions of Node.js in parallel.
To make sure that all developers use a compatible version of Node.js, your Rails project should declare the required Node.js in a file called .nvmrc
.
When a .nvmrc
exists, developers can cd
in your project directory and activate...
How to recognize CVE-2019-5418
If you get requests with values for formats
like this:
{:locale=>[:de], :formats=>["../../../../../../../../../../etc/services{{"], :variants=>[], :handlers=>[:erb, :builder, :raw, :ruby, :coffee, :haml]}
or fails like this:
Invalid query parameters: invalid %-encoding (../../../../../../../../../etc/passwd%%0000.html)
Someone tries to exploit CVE-2019-5418.
If you use the latest Rails (or latest Rails LTS) you're...
Capybara: Testing file downloads
Download buttons can be difficult to test, especially with Selenium. Depending on browser, user settings and response headers, one of three things can happen:
- The browser shows a "Save as..." dialog. Since it is a modal dialog, we can no longer communicate with the browser through Selenium.
- The browser automatically downloads the file without prompting the user. For the test it looks like nothing has happened.
- The browser shows a binary document in its own window, like a PDF. Capybara/Selenium freaks out because there is no HTML docum...
SameSite cookies
TL;DR Most web applications do not require action on this. SameSite=None
(old browser default) will continue to work, and SameSite=Lax
(new Chrome default, gradually rolled out) is an even better default for cookies. Set SameSite=Strict
only for extra security in special cases (see below). If your application is rendered in an iframe (e.g. a video player or some news stream), you need to configure its relevant cookies as SameSite=None
.
The SameSite
cookie attribute targets **c...
Self-expiring URLs with Apache
When delivering non-public uploaded files (images, documents etc), one has to decide whether and how to do authorization. The usual approaches are:
- Using
send_file
with a regular controller. This is secure, but potentially slow, especially for large collections of images. - Using unguessable URLs. This is fast (because Apache can deliver assets without going through Rails), but less secure.
When going with the "unguessable URL" approach, it is possible to somewhat increase security by using expiring URLs. The idea is to encode the expi...
Function Composition in Ruby
Along with a number of other cool new features and performance improvements, Ruby 2.6 added function composition to the Proc and Method classes. Today we’ll take a look at how this allows us to use some functional programming goodness in our Ruby code.
Vortrag: Elasticsearch Grundlagen und Rails-Integration mit searchkick
Was ist Elastic?
- Suchmaschine, basierend auf Apache Lucene
- größtenteils Open-Source
- einige kommerzielle Features ("Elastic Stack", früher "X-Pack")
- Zugriffsrechte (bis vor kurzen)
- Monitoring
- Reporting
- Graph-Unterstützung
- Machine Learning
- REST-Api (JSON über HTTP)
Grundlagen
Elastic antwortet per Default auf Port 9200
http GET :9200
{
"name": "ntK2ZrY",
"cluster_name": "elasticsearch",
"cluster_uuid": "Bbc-ix5bQZij5vfFU29-Cw",
"version": {
"number": "6.7.1",
"build_flavor": "...
Tod: A Gem for handling daytime without a date
Tod is a gem for working with daytimes. That's a tuple of (hour, minute second)
without a day, month or year.
Another additional gem?
Thus SQL has a time
datatype for storing time of day in the format hh:mm:ss
, neither Ruby nor Rails themselves offer an elegant way to deal with day times.
Time
and DateTime
both handle daytime values AND calendar date, using them to only store the time of day will end in inconsistent and thus confusing data, e. g. Time.new
will initialize with the current Time in your Timezone, `DateTime.n...
When reading model columns during class definition, you must handle a missing/empty database
When doing some meta-programming magic and you want to do something for all attributes of a class, you may need to access connection
or some of its methods (e.g. columns
) during class definition.
While everything will be fine while you are working on a project that is in active development, the application will fail to boot when the database is missing or has no tables. This means that Raketasks like db:create
or db:migrate
fail on a freshly cloned project.
The reason is your environment.rb
which is loaded for Raketasks and calls...
Migration from the Asset Pipeline to Webpacker
This is a short overview of things that are required to upgrade a project from the Asset Pipeline to Webpacker. Expect this upgrade to take a few days even the diff is quite small afterwards.
Preparations
1. Find all libraries that are bundled with the asset pipeline. You can check the application.js
and the application.css
for require
and import
statements. The source of a library is most often a gem or a vendor directory.
2. Find an working example for each library in the application and write it down.
3. Find out the ver...
ActionMailer: Previewing mails directly in your email client
In Rails, we usually have a mailer setup like this:
class MyMailer < ActionMailer::Base
def newsletter
mail to: 'receiver@host.tld',
from: 'sender@host.tld',
subject: 'My mail'
end
end
If you want to preview your mail in the browser, you can use the Action Mailer Preview. To inspect the mail directly in your email client, just create an .eml
file and open it with your client:
mail = MyMailer.newsletter
Fil...
Why you can't use timezone codes like "PST" or "BST" for Time objects
Rails' ActiveSupport::TimeWithZone
objects have both a timezone code and offset, e.g. Thu, 28 Mar 2019 16:00:00 CET +01:00
. Ruby's stdlib TZInfo
also has time zones, but with different identifiers.
Unfortunately, not all timezone codes can be used to parse strings or to move time objects into another time zone.
Some timezone codes like CET
are supported by ActiveSupport extensions like String#in_time_zone
, while many codes will actually not work:
>> '2019-03-01 12:00'.in_time_zone('PST')
ArgumentError (Invalid Timezone: PST)
...
rails_state_machine 1.1.2 released
Fixed bugs where calling valid?
would change the record's state
.
How to fix parallel_tests with Redis on powerful machines
When you have a powerful machine with many CPU cores, you might run into an error like
ERR DB index is out of range (Redis::CommandError)
This is because Redis defaults to at most 16 databases (0 to 15) and running tests in parallel might exceed that (your tests might run on databases 1..n
or 2..(n+1)
).
You can increase that limit:
-
Get number of CPUs of your machine.
nproc --all
-
Open up Redis configuration file.
sudo vim /etc/redis/redis.conf
-
Find
databases
row and increase it, e.g. set to 32:
...