The Whenever Gem: Making Cron Easy

I recently had to learn how to use Cron to set up a scheduled task on our rails server. In spite of the ubiquity of cron, I found the documentation and even the “how to” tutorials to be horrible at best. Fortunately, several people pointed me to a nice little ruby gem that takes the pain out of configuring cron jobs: the whenever gem.

The Whenever File

Whenever was built to work with rails apps that need to schedule something, so it uses a few rails folder conventions. By default, it looks for a file called `config/schedule.rb` when you run the `whenever` command line tool. However, you can specify any arbitrary file you want by passing the `-f filename` parameter to the command line.

In my use of whenever, i started out with a custom file name. In the end, though, I switched to the default `config/schedule.rb` because it made the tool easier to work with and allowed me to split up my cron job configuration into different environments, similar to a rails app.

The Whenever DSL

Whenever uses a simple DSL to create cron jobs for you, and can update your crontab file as well. The DSL is fairly simple to work with and understand. For example, if I need to run a job every hour, I can write this:

every 1.hour do
  command "./my_executable --with-some-param"
end

When you run the `whenever` executable against this, it will show you the correct cron job syntax. Even better, though, if you run `whenever -w` against this file, it will write the correct cron job syntax to your crontab file. If you want to remove the job, just run `whenever -c` and it will clear out all of the generated tasks for that file.

Output To Log Files

Another thing I learned about cron is that by default, it will send an email for everything that is logged (re: output to STDOUT and STDERROR), which typically ends up in /var/mail/account_name. I don’t like this – I want real log files. So, I set up a log file in `/var/logs/` with the right permissions and then added the output configuration i needed to the top of my whenever file.

set :output, { :standard => "/var/logs/my_app.log", :error => "/var/logs/my_app.errors.log" }

When you run whenever against the file, now, it will append > syntax to pipe the output to the log files I specified.

A Lot More Than Just This

There is a lot more to whenever than just this. There are several built in command types, for example, including a rake command and a ruby script runner. I’ve only shown the ‘command’ command in this example. For more info on what it can do, check out the documentation in the readme and wiki at the github project page.

I’m also using whenever to deploy cron jobs to my linux server, to run some ruby (thor) scripts. I’ll talk about this another post, though, as it’s a fairly involved subject of it’s own.