Adding cron jobs cleanly

Background

Adding jobs to /etc/cron.x keeps things modular and ‘clean’. You can add
hourly, daily, weekly and monthly (regular) scripts by creating your
script in the appropriate folder: /etc/cron.{hourly,daily,weekly,monthly}

The cron daemon will run these hourly, daily, etc scripts at intervals
defined in /etc/crontab

SHELL=/bin/sh  
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin  
  
# m h dom mon dow user    command  
17 *  * * *   root    cd / && run-parts --report /etc/cron.hourly  
25 6  * * *   root    test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.daily )  
47 6  * * 7   root    test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.weekly )  
52 6  1 * *   root    test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.monthly )

Observe the PATH variable set. This is why you don’t need to provide
explicit paths (e.g. /bin/rm), but it’s a good idea to do it anyway.

If you need to run a script akin to adding it to the crontab, you’ll
need to place it in /etc/cron.d

Adding regular scripts

Let’s say you want something to run weekly.

• Create your script
• Move it to /etc/cron.weekly

Before you rejoice,

• You must add the #!/bin/sh line to the top of your scripts! Not
  doing so will result in Exec format error messages.
• You must set the permissions to 755

Adding crontab-style scripts

You’d add these to /etc/cron.d/''scriptname''. The format’s the same,
except that you must explicitly provide the user executing the script
(makes sense.) For example:

#!/bin/sh  
09,39 * * * *  root  /bin/rm -rf /tmp/junk/*

Add that to a file, move it to /etc/cron.d. Make sure that:

• The #!/bin/sh line exists and is first.
• The permissions to 755.

crontab Syntax

Stolen from here since, even though I’ve done this for years, I cannot remember this for the life of me.

# +---------------- minute (0 - 59)
# |  +------------- hour (0 - 23)
# |  |  +---------- day of month (1 - 31)
# |  |  |  +------- month (1 - 12)
# |  |  |  |  +---- day of week (0 - 6) (Sunday=0)
# |  |  |  |  |
  *  *  *  *  *  command to be executed