How to write a cron that will run a script every day at midnight?

Solution 1:

Here's a good tutorial on what crontab is and how to use it on Ubuntu. Your crontab line will look something like this:

00 00 * * * ruby path/to/your/script.rb

(00 00 indicates midnight--0 minutes and 0 hours--and the *s mean every day of every month.)

Syntax: 
  mm hh dd mt wd  command

  mm minute 0-59
  hh hour 0-23
  dd day of month 1-31
  mt month 1-12
  wd day of week 0-7 (Sunday = 0 or 7)
  command: what you want to run
  all numeric values can be replaced by * which means all

Solution 2:

from the man page

linux$ man -S 5 crontab

   cron(8) examines cron entries once every minute.

   The time and date fields are:

          field          allowed values
          -----          --------------
          minute         0-59
          hour           0-23
          day of month   1-31
          month          1-12 (or names, see below)
          day of week    0-7 (0 or 7 is Sun, or use names)
   ...
   # run five minutes after midnight, every day
   5 0 * * *       $HOME/bin/daily.job >> $HOME/tmp/out 2>&1
   ...

It is good to note the special "nicknames" that can be used (documented in the man page), particularly "@reboot" which has no time and date alternative.

   # Run once after reboot.
   @reboot         /usr/local/sbin/run_only_once_after_reboot.sh

You can also use this trick to run your cron job multiple times per minute.

   # Run every minute at 0, 20, and 40 second intervals
   * * * * *       sleep 00; /usr/local/sbin/run_3times_per_minute.sh
   * * * * *       sleep 20; /usr/local/sbin/run_3times_per_minute.sh
   * * * * *       sleep 40; /usr/local/sbin/run_3times_per_minute.sh

To add a cron job, you can do one of three things:

1. add a command to a user's crontab, as shown above (and from the crontab, section 5, man page).

   • edit a user's crontab as root with crontab -e -u <username>
   • or edit the current user's crontab with just crontab -e
   • You can set the editor with the EDITOR environment variable
     • env EDITOR=nano crontab -e -u <username>
     • or set the value of EDITOR for your entire shell session
       1. export EDITOR=vim
       2. crontab -e
   • Make scripts executable with chmod a+x <file>

2. create a script/program as a cron job, and add it to the system's anacron /etc/cron.*ly directories

   • anacron /etc/cron.*ly directories:
     • /etc/cron.daily
     • /etc/cron.hourly
     • /etc/cron.monthly
     • /etc/cron.weekly
   • as in:
     • /etc/cron.daily/script_runs_daily.sh
     • chmod a+x /etc/cron.daily/script_runs_daily.sh -- make it executable
   • See also the anacron man page: man anacron
   • Make scripts executable with chmod a+x <file>
   • When do these cron.*ly script run?
     • For RHEL/CentOS 5.x, they are configured in /etc/crontab or /etc/anacrontab to run at a set time
     • RHEL/CentOS 6.x+ and Fedora 17+ Linux systems only define this in /etc/anacrontab, and define cron.hourly in /etc/cron.d/0hourly

3. Or, One can create system crontables in /etc/cron.d.

   • The previously described crontab syntax (with additionally providing a user to execute each job as) is put into a file, and the file is dropped into the /etc/cron.d directory.
   • These are easy to manage in system packaging (e.g. RPM packages), so may usually be application specific.
   • The syntax difference is that a user must be specified for the cron job after the time/date fields and before the command to execute.
   • The files added to /etc/cron.d do not need to be executable.
   • Here is an example job that is executed as the user someuser, and the use of /bin/bash as the shell is forced.

   File: /etc/cron.d/myapp-cron
   # use /bin/bash to run commands, no matter what /etc/passwd says
   SHELL=/bin/bash
   # Execute a nightly (11:00pm) cron job to scrub application records
   00 23 * * * someuser /opt/myapp/bin/scrubrecords.php