How to setup a cron job

Cron jobs can help you to automate certain procedures e.g. backup. This article is
a brief "how-to" for creating a cron field which defines when a specific
application or script should be executed.

Execute the command "crontab -e" to generate or edit your cron-file. Alternatively
use "crontab -l" for listing your cron jobs or "crontab -r" for deleting all cron
jobs.

One line in the cron file looks like this:
1 2 3 4 5 /etc/myScript arguments

The five numbers at the beginning of the line represent the following:
1: minutes (0-59)
2: Hours (0-23)
3: Day (0-31)
4: Month (0-12 or January, February, ... 0 and 12=December)
5: Day of the week(0-7 or Monday, Tuesday, Wednesday, ... 0 and 7=Sunday)
This enables you to exactly define when a script is supposed to be started.

Wait, there is more! You can make the file even more flexible like this:
*: applies every time
1-4: start if 1,2,3 or 4 matches
1-4/2 step-by-step, here: 1 and 3
1,5,6: enumeration
*/3: every third
1-4,10-15: enumeration and range combined

Moreover you can define what SHELL is to execute the commands and who you want to
send the results to (via e-mail):
SHELL=/bin/bash
MAILTO=me@localhost

A few more examples:
# every day at 0:05
5 0 * * * $HOME/bin/daily.job >> $HOME/tmp/out 2>&1

# every first day of the month at 14:15
15 14 1 * * $HOME/bin/monthly

# on weekdays at 22:00
0 22 * * 1-5 mail -s "It's 10pm" joe%Joe,%%Where are your kids?%

# every day 23 minutes after every even hour (0:23, 2:23, ...)
23 0-23/2 * * * $HOME/status.sh

# Sundays at 4:05
5 4 * * sun $HOME/backup.sh full

Weekdays and calender days do not exclude each other but are considered
separately. If either pattern matches the script will execute.

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