Introduction to Cron crontab

The term cron refers to a time-based job scheduler that triggers commands at set time intervals. It is configured through a simple text file called the crontab. Within this file are a list of cron jobs, each having a command and a time interval in which they are to be run.

Cron jobs can be used in any sort of automation task. Here is a short list of tasks you could use for automation.

1. Clear out items in a temporary folder at the end of every week.
2. Perform a database dump to backup files every night.
3. Send out daily notification emails every other morning.
4. Emails that are sent every second week of the month with monthly progress reports regarding website statistics.
5. A Python script that makes a reddit bot post to a certain subreddit every night at 7:00pm.

The cron daemon

The cron daemon is a dormant process that gets run whenever encountering a time which it is to execute some command. This daemon is responsible for executing each cron job listed in our cron tabs.

If this is the first time hearing the termin daemon (pronounced DEE-mon), it's simply a background process that answers requests to services.

Crontab and examples

Each job to be performed is stored in a file called the crontab. Here is an example of one might look like.

43 2 * * * echo "It's 2:43am!"
0 8 1,15 * * echo "Run every morning at 8:00am on the 1st and 15th of every month."
*/30 9-12 * * * echo "Run every 30 minutes from 9-12 am."

We'll learn how to decipher each line in the next page. For now, let's learn how to access this crontab with the crontab command.

Crontab command and actions

The main command used to edit your crontab is the crontab command.

1) Editing your crontab

We can edit our crontab by passing in the -e option. This will open the file in our default editor. If you've never opened up the crontab before, you'll get the following boilerplate, with comments guiding you on how to set you a cron.

$ crontab -e
# Edit this file to introduce tasks to be run by cron.
# 
# Each task to run has to be defined through a single line
# indicating with different fields when the task will be run
# and what command to run for the task
# 
# To define the time you can provide concrete values for
# minute (m), hour (h), day of month (dom), month (mon),
# and day of week (dow) or use '*' in these fields (for 'any').# 
# Notice that tasks will be started based on the cron's system
# daemon's notion of time and timezones.
# 
# Output of the crontab jobs (including errors) is sent through
# email to the user the crontab file belongs to (unless redirected).
# 
# For example, you can run a backup of all your user accounts
# at 5 a.m every week with:
# 0 5 * * 1 tar -zcf /var/backups/home.tgz /home/
# 
# For more information see the manual pages of crontab(5) and cron(8)
# 
# m h  dom mon dow   command

2) Listing out current crontab file

To list out all current crontab to standard out, use the -l option.

You may also check if the cron daemon is currently running. The top output shows cron is up and running, while the second line shows the current grep command that was just used.

$ crontab -l
# Current crontab file outputted to standard out
$ ps aux | grep cron
root       787  0.0  0.3  23660  1788 ?        Ss   18:45   0:00 cron
JohnDoe   2685  0.0  0.4  15948  2236 pts/1    S+   19:38   0:00 grep --color=auto cron

3) Remove all crontabs

To discontinue the current crontab, use the -r option. We can pair this with the -i option to prompt the user 'y/Y' before actually removing the crontab.

$ crontab -r

Configuring and Scheduling Crontabs

A crontab file consists of optional variables at the top, followed by a list of scheduled tasks. The first five values of the task are the minute (m), hour (h), day-of-month (dom), day-of-week (dow), and the last value contains the command to be executed.

Here are some variables that you may choose to declare.

SHELL

The shell that the cron uses to execute the command.

PATH

Directories to be used as search path for cron.

MAILTO

Mailing out the output of each command. If no one, then output will be mailed to owner of the process.

HOME

Home directory that is used for cron.

Default is set to /etc/passwd.

Scheduling

As mentioned, the first five fields determine when the cron job will be run. The last field is the command, and you may optional add a user before this parameter. Let's take a look at the scheduling parameters available.

minute

0-59

hour

0-23

day-of-month

1-31

month

1-12

day-of-week

0-7

Both 0 and 7 values are Sunday.

Can also just use the first three letters (case-insensitive: sun, mon, tue, wed, thu, fri, sat).

There are also symbols that help you fine-tune a schedule.

Wildcards (*)

Specifies every possible time interval.

Step values (,)

Instead of specifying just one value, we can list multiple, separated by a comma (,).

Range (-)

For whole number ranges, use the hyphen (-).

Alternating (/)

To specify every two hours, we may use the slash (/). Thus, the following command would run every three hours from 6am to midnight.

MAILTO=username@example.com
SHELL=/bin/bash
0 6-21,0/3 * * * echo "Run every three hours throughout the day."

Special keywords

Cron also has available special keywords per your convenience.

@yearly

0 0 1 1 *

@daily

0 0 * * *

@hourly

0 * * * *

@reboot

Run at startup.

Setting cron jobs in the /etc directory (Linux)

Within the /etc directory, you can find subdirectories called cron.hourly, cron.daily, cron.weekly and cron.monthly. You may play a crontab in any of these directories to have it run hourly, daily, weekly or monthly.

Here is another example you can see setup by default on Ubuntu 14.You can see that each specific /etc directory is set to trigger each corresponding /etc/cron.* tab.

$ cat /etc/crontab
# /etc/crontab: system-wide crontab
# Unlike any other crontab you don't have to run the `crontab'
# command to install the new version when you edit this file
# and files in /etc/cron.d. These files also have username fields,
# that none of the other crontabs do.
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 )

Examples of schedules

Here are some examples. Try to guess the schedule per cron job before reading the output.

30 4 echo "It is now 4:30 am."
0 22 echo "It is now 10 pm."
30 15 25 12 echo "It is 3:30pm on Christmas Day."
30 3 * * * echo "Remind me that it's 3:30am every day."
0 * * * * echo "Ding. Dong. Ding. It is the start of a new hour."
0 6 1,15 * * echo "Set alarm for 6am on the 1st and 15th of every month."
0 6 * * 2,3,5 echo "Perform command at 6am on Tuesday, Wednesday and Thursdays."
59 23 * * 1-5 echo "Run a backup command before midnight on weekdays."
0 */2 * * * echo "Run something every two hours, on the dot."
0 20 * * 4 echo "8pm on a Thursday means take out the trash!"
0 20 * * Thu echo "8pm on a Thursday means take out the trash!"
*/15 9-17 * * 2-5 echo "Runs every 15 minutes from 9am-5pm on weekdays. GET TO WORK!"
@yearly echo "Happy New Year!"
@reboot echo "Let's get some work done today."

Handling Crontab Outputs

When running cron jobs, you'll often have some output. If you have a mail client installed, it will send to the email address. If not, it will be sent to /dev/null (aka be discarded). either to standard out or error. You can redirect this to your e-mail.

Mailing output

By default, the output to a cron job will be mailed to whoever owns the process, or whatever the MAILTO variable is set as.

To mail the output to someone else, pipe and use the mail command.

00 * * * * echo "Hello! How are you this hour?" | mail -s "subject of mail" username@example.com

Trashing the output

If you don't want to email altogether, redirect to /dev/null, which is a folder that essentially acts as a trash bin.

0 * * * * echo "No need for this message." >> /dev/null

Appending to a log file

We may also append to a log file, or to both standard out and error by redirecting it.

0 * * * * echo "redirect" >> log.file
0 * * * * echo "redirect" >> log.file 2>&1