For past week, I have been building a side project that sends me a summary of the day's weather via text message in the morning before I wake up. I have been looking at ways to automate running ruby files in schedule and came across some interesting options. It took be couple of days to figure out how these things work but in the end, the findings were very useful. The two schedulers that I liked working with are called Cron and Rufus-Scheduler.

What is cron?

Cron is a time-based job scheduler in Unix-like operating systems(Mac, Linux, and etc..).

There is a cron "daemon" that runs on these systems. A daemon is a program that runs in the background all the time, usually initiated by the system. This cron daemon is responsible for launching these cron jobs on schedule.

The jobs or tasks that scheduler execute are called cron jobs. The cron jobs are written in a file called cron-tab and gets executed in designated schedules.

Cron syntax

Here is a basic template for cron syntax

* * * * * [command for the task goes here]

• First asterisk represents minutes from (0-59)
• Second asterisk represents hours from (0-23)
• Third asterisk represents days (0-31)
• Fourth asterisk represents months (1-12)
• Last asterisk represents day of week (0-6) *Starts with sunday as 0

In general, asterisk (*) means run on every possible number of the unit)
Example above runs command every minute.

Examples:

run command at every hour

0 * * * * [command]

run command at every 30 minute

30 * * * * [command]

run command every 15 minutes

0,15,30,45 * * * * [command]

run command at every day 1PM

0 13 * * * [command]

run command once a month at 5th day

0 0 5 * * [command]

run command every Tuesday at 8:30AM

30 8 * * 2 [command]

After the cron tab file is done, I decided to save as a file called ~/.crontab
You can specify the cron tab file for cron to run by

$ crontab ~/.crontab

Also crontab provides editing functionality via termial

$ crontab -e

and viewing list of cron jobs

$ crontab -l

My thoughts about using cron

After days of attempt at trying to use cron to schedule jobs,
I found out that cron jobs themselves can not run ruby files.
Here is what I found out after some google search:

Cron runs as sh shell. it doesn't read bashrc or profile.d files like
rvm.sh. so it will not know where to find ruby that's installed in rvm.

Great! -_-;;;; After another couple days of attempt at trying to get cron up and running without any gem dependency, I gave up and found a perfect gem to do the job.

Setting up cron jobs using Whenever gem

There is a cool gem that makes setting up cron jobs using ruby called Whenever.

Install the gem by

$ gem install whenever

and initialize by typing the command in the project folder

$ wheneverize .

It will create a file in config/schedule.rb in the project

Whenever examples:

Here are examples of whenever syntax provided by the author of the gem

every 3.hours do
  runner "MyModel.some_process"
  rake "my:rake:task"
  command "/usr/bin/my_great_command"
end

every 1.day, :at => '4:30 am' do
  runner "MyModel.task_to_run_at_four_thirty_in_the_morning"
end

every :hour do # Many shortcuts available: :hour, :day, :month, :year, :reboot
  runner "SomeModel.ladeeda"
end

every :sunday, :at => '12pm' do # Use any day of the week or :weekend, :weekday
  runner "Task.do_something_great"
end

every '0 0 27-31 * *' do
  command "echo 'you can use raw cron syntax too'"
end

# run this task only on servers with the :app role in Capistrano
# see Capistrano roles section below
every :day, :at => '12:20am', :roles => [:app] do
  rake "app_server:task"
end

Setting up crontab using whenever

The Whenever gem's github readme fine states that if you use RVM, it might cause your cron job to hang. It can be resolved by adding following line in the ~/.rvmrc file

rvm_trust_rvmrcs_flag=1

After your schedule is setup and .rvmrc file is set, go to the project's folder in the terminal and type

$ whenever -w [identifier]

The gem will parse your schedule.rb and write the crontab for you. Your cron jobs should be up and running right after the crontab file is generated.

You can also look at more usage of whenever gem by typing

$ whenever --help

in the command line.

For more information about using Whenever gem, visit the gitup repo here
https://github.com/javan/whenever