Linux/Crontab

Crontab is how scheduled processes are defined in linux.
For example, want to run something every month at a certain time? Every week? Every day? Every third Tuesday at 5:23 am?
For all of these, crontab is the way to do it.

Accessing Crontab

To list all current cronjobs in crontab, use:

• crontab -l

To display the last time contab was edited, use:

• crontab -v

To edit contab, use:

• crontab -e

Note: Accessing crontab this way creates "user cronjobs". As in, the user account that creates the cronjob with will be the user account that runs it.
Make sure to create cronjobs using the correct user!

Conjobs in Crontab

Each line is a cronjob, and should take the format of:

• * * * * * <task_to_execute>

Each * character above represents a timeframe, in order from left to right:

• Minute - 0 to 59.
• Hour - 0 to 23.
• Day of Month - 1 to 31.
• Month - 1 to 12 OR jan, feb, mar, apr, ...
• Day of Week - 0 - 6 (with sunday=0) OR sun, mon, tue, wed, thu, fri, sat

Crontab Examples

All of these assume we're executing some script at /etc/my_project/my_script.sh.

Run script every minute:

• * * * * * /etc/my_project/my_script.sh

Run script every hour at the start of the hour:

• 00 * * * * /etc/my_project/my_script.sh

Run script every hour at 35 minutes in:

• 35 * * * * /etc/my_project/my_script.sh

Run script every day at 6:30 pm:

• 30 18 * * * /etc/my_project/my_script.sh

Run script on the 5th day of each month, at 4:23 am:

• 23 04 5 * * /etc/my_project/my_script.sh

Run script every monday at 8 am:

• 00 08 * * MON /etc/my_project/my_script.sh

Enabling Cronjob Logging

At least for Ubuntu systems, additional steps are required to enable logging of cronjobs.

First, you need to edit the following system settings to allow logging at all:

/etc/rsyslog.d/50-default.conf

In this file, there will be a line that starts with #cron.*. Uncomment this line out to enable. Optionally, also change the log path to a place of your choosing.

Then either restart your system, or run:

sudo service rsyslog restart

A new logging file should exist at the location specified. Default location is /var/log/cron.log.