Simple cron example

Simple cron example (tested on Ubuntu):

• Edit your (user) crontab file

  $ crontab -e

  This will bring up your editor (nano by default in Ubuntu)
  
  
• Enter the following inside. This will append the current date to a log file every minute. The 6 fields of the crontab file are: minute, hour, day of month, month, day of week, command.

  * * * * * /bin/date >> /tmp/cron_output
  

  Be sure to put a blank line at the end of the file.
  (NOTE 1: >> only redirects STDOUT to a file. To redirect both STDOUT and STDERR, use something like /bin/date >> /tmp/cron_output 2>&1)
  (NOTE 2: If output is not redirected, cron will try to email the output to you. To do this, a mail transfer agent such as sendmail or postfix must be installed.)
  (NOTE 3 (added 2015-06-24): When I created my cron script in /etc/cron.d with Emacs using sudo::, cron didn't pick up my script. When I created it with nano, cron picked it up. It seems the cause is the permissions of the cron script. Emacs created the script with 664 permissions while nano created the script with 644 permissions. When I changed the permissions to 644, it started working. I am running Ubuntu 15.04. This Ask Ubuntu answer confirms that a 644 permission is problematic because it is considered insecure. See /var/log/syslog for cron messages. The Ask Ubuntu page has a lot of other good tips: Reasons why crontab does not work)
  
  
• Exit the editor. It should output:

  crontab: installing new crontab

• Check that it is working:

  tail -f /tmp/cron_output

  You should see the date updated every minute on the minute (or close to it):

  Tue Sep 16 23:58:01 PDT 2008
  Tue Sep 16 23:59:01 PDT 2008
  Wed Sep 17 00:00:01 PDT 2008
  Wed Sep 17 00:01:01 PDT 2008
  ...
  

See also my post: Postgres backup with cron