How to rotate log files explicitly

Usually, the logrotate service takes care of renaming log files each night or so to avoid logs becoming huge. That will rename your.log to your.log.1, the next time to your.log.2.gz, etc. Here is how to make that happen out of band (you should rarely need to do that).

Logrotate won't touch all your logs automagically. There is a config file for each service which you can tell logrotate to use.

So if you need logs to be rotated right now, do this (as root):

logrotate --force PATH_TO_CONFIG_FILE

For example, to rotate all y…

How to horizontally center absolute positioned container with CSS

Note: We have card with all CSS centering options. You probably want to head over there and get an overview over what techniques are available for your use case and browser requirements.

Horizontally centering a static element in CSS is normally handled by setting the left and right margins to auto, for example:

$container_width: 100px
  width: $container_width
  margin: 0 auto

However, this w…

Center a float horizontally

Note: We have card with all CSS centering options. You probably want to head over there and get an overview over what techniques are available for your use case and browser requirements.

If you cannot use display: inline-block, centering a float can be useful if you want to center something and at the same time make its width automatically fit some content.

Use this SASS:


How to set the user agent in tests

The User-Agent HTTP header identifies the client and is sent by "regular" browsers, search engine crawlers, or other web client software.


In Rack::Test, you can set your user agent like this on Capybara:

Given /^my user agent is "(.+)"$/ do |agent|
  page.driver.browser.header('User-Agent', agent)
  # Or, for older Capybaras:
  # page.driver.header('User-Agent', agent)

For Selenium tests with Firefox, it seems you can set the general.useragent.override profile setting to your preferred value. [See StackOverflo…

How to make Rational#to_s return strings without denominator 1 again

The way Rational#to_s works on Ruby has changed from Ruby 1.9 on. Here is how to get the old behavior back.

You may want this for things where Rationals are being used, like when subtracting Date objects from one another.

What's happening?

Converting a Rational to a String usually does something like this:

1.8.7 > Rational(2, 3).to_s
=> "2/3"
1.9.3 > Rational(2, 3).to_s
=> "2/3"
2.0.0 > Rational(2, 3).to_s
=> "2/3"

However, when you have a Rational that simplifies to an integer, you will only get a St…

Cucumber step to set cookies in your Capybara session

To set a cookie in your test browser for cucumber tests, you can use this step:

Given /^I have a "([^\"]+)" cookie set to "([^\"]+)"$/ do |key, value|
  headers = {}
  Rack::Utils.set_cookie_header!(headers, key, value)
  cookie_string = headers['Set-Cookie']


Note that Rack::Utils is only used to find out the correct cookie header string (you don't want to generate it yourself) – it is then passed to Capybara's browser. The browser will …

What `var` actually does in Javascript

TL;DR: Variables not declared using var are stored outside the current scope, most likely in the global scope (which is window in web-browsers).

Declaring a variable in Javascript is done like var x = 5. This creates a new variable in the current scope (e.g. the function you're in). What happens when don't use var?

Javascript needs to be clever when you do an assignment without declaring a variable, e.g. x = 7. To find that variable,

  • it first looks up x in the current scope
  • next, it goes up the scope chain, lookin…

YAML syntax compared with Ruby syntax

yaml4r is a juxtaposition of yaml documents and their Ruby couterpart. Thus, it does a great job as YAML-doc, e.g. when writing Rails locale files. Did you know that …

  • << is a merge key (similar to & in SASS)
  • there are variables, called aliases. Definition: &alias Some content, usage: *alias.


Specifying a key twice does not merge the sub keys, but override the first definition, e.g.

  car: # overridden
    door: Tür

Rails: How to use a n:m association as 1:n association

Sometimes you might want to limit the number of associated records in a has_many association, but cannot add a foreign key to the other model (using belongs_to).

There are many takes on limiting the number of associated records in has_many associations, but none feels smooth.

However, when your…

Spreewald: When using `patiently do`, don't reuse existing variable names

Spreewald's patiently repeats the given block again and again until it either passes or times out.

Be careful to give patiently a block that can actually be repeated. E.g. the following block can not be repeated:

Given /^the field "(.*?)" is empty$/ do |field|
  patiently do
    field = find_field(field)
    field.text.should be_blank

The reason the above code will fa…

Test redirects to an external URL with Cucumber/Capybara

When a controller action redirects to an external URL (like you will find that this is hard to test with Cucumber and Capybara:

  • A non-Javascript Rack::Test scenario will just ignore the host and try to open /some/path in your local application
  • A Selenium test will actually follow the redirect, which you probably don't want either

There are two workarounds for this. You can use either, or a combination of both.

  1. Write a controller spec

Controller specs can test if a resp…

Capybara 0.3.9 Bug: Chaining .find to scope doesn't work

The following code doesn't work like expected: page.find(css_selector).find(other_css_selector)
The second .find will search the whole dom instead of a scope.

Rumor has it this is fixed in Capybara 0.4.1.

ApacheBench may return "Failed requests" for successful requests

When you use ab to do some performance benchmarking, you might run into output like this:

Complete requests:      200
Failed requests:        5
   (Connect: 0, Receive: 0, Length: 5, Exceptions: 0)

Note that in our example these "Failed requests" actually never failed.\
For some requests, the application just returned a response with a different content length than the first response. This is indicated by the "Length: 5" bit in the example above.

If you see requests that failed with other kinds of errors, they probably fail…

External content

Article "Pricing Experiments You Might Not Know, But Can Learn From"

We all struggle with pricing. Here is some interesting information that helps you with your pricing decisions.

How to use helper methods in a controller

Rails 3+


Rails 2


Also see How to use helper methods inside a model.

def vs. define_method

Ever wondered about the difference between def and define_method? Turns out there are three implicit contexts in Ruby. def and define_method differ in which one they use.


  • Ruby keyword, starts a method definition
  • Opens a new, isolated scope. Variables defined outside are not accessible inside and vice versa.
  • Defines an instance method on the receiver (specified before the method name, e.g. def; implicit receiver is the default definee

The default definee is not self and…

How to disable cookies in cucumber tests

Unfortunately, Capybara does not offer a switch to disable cookies in your test browser. However, you can work around that by using a tiny Rack middleware – it works for both Selenium and non-Selenium tests.

Wouldn't it be nice to say something like this?

Given cookies are disabled
When I try to sign in
Then I should see "Can't sign you in. Please enable cookies."

You can! Put the code below into some place like lib/rack/cookie_stripper.rb.

module Rack
  class CookieStripper
    ENABLED = false

External content

The Plight of Pinocchio: JavaScript's quest to become a real language -

Great presentation about writing Javascript like you write everything else: Well-structured and tested.

JavaScript is no longer a toy language. Many of our applications can’t function without it. If we are going to use JavaScript to do real things, we need to treat it like a real language, adopting the same practices we use with real languages.

This framework agnostic talk takes a serious look at how we develop JavaScript applications. Despite its prototypical nature, good object-oriented programming principles are still relevant. The…

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