Troubleshooting: Cron Jobs

Cron is a service for Linux servers that automatically executes scheduled commands. A cron job can be a series of shell commands, scripts, or other programs. Cron tasks or jobs can perform a variety of functions and once ran can send out an e-mail message to inform you of its completion or errors. If you receive an error, there are many ways to troubleshoot the cron task.  Use this article for troubleshooting assistance or a tutorial on the basics of cron jobs. If you would like to learn more about creating a cron job check out our Knowledge Base tutorials on the subject.

Checking Configurations with Crontab

From the command line, you can review the scheduled cron jobs by listing the crontab for the user. This command outputs the contents of the user’s crontab to the terminal.

As the user you can run:
crontab -l

As root, you can see any user’s crontab, by specifying the username.
crontab -l -u username

You can find some detailed information about how to format the cron jobs in the /etc/crontab file.  Below is the example within that file.  Each asterisk can be replaced with a number or to its corresponding field.  Or you can leave the asterisk in place to represent all possible numbers for that position.  For example, if left with all asterisk this means that the cron job will run every minute, all the time.

SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
# For details see man 4 crontabs
# Example of job definition:
# .---------------- minute (0 - 59)
# | .------------- hour (0 - 23)
# | | .---------- day of month (1 - 31)
# | | | .------- month (1 - 12) OR jan,feb,mar,apr ...
# | | | | .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
# | | | | |
# * * * * * user-name command to be executed

 

Altering the E-mail Address of a Cron

Once initiated, a cron sends a notification to an email address, set within the MAILTO line of the crontab.

MAILTO="user@domain.com"

To edit the crontab, you can run the following commands as the user:

crontab -e

Or if logged in as root, you can type in the username for any of your users to see a scheduled task that they have created.

crontab -e -u usernameThese open the crontab of the user in the default editor. Typically the vim or nano command will open the file. Be aware that this similar to opening any other text file where you’ll save before closing.

The MAILTO line indicates where the execution status of a cron should be sent. The sending address will typically be the cron task creator’s username along with the server’s hostname. So an email’s sender address would follow this syntax, unixuser@serverhostname.com. If you don’t see an email right away, it may be a good idea to check your spam box.

Silenced Crons

Sometimes cron jobs are configured to either produce no output or have its output silenced, even if set with a MAILTO address. If you see a cron job listed with any of the following on end, it is a sign that the cron has output has been silenced. These send any output to the null device (the black hole on a Linux server). In cases like this, you’ll need to remove the line from the cron job script to generate an output.

&> /dev/null

2>&1 /dev/nullSome cron jobs are disabled entirely. These will have a “#” in front of the command, resulting ignored lines when executed. Remove the “#” to re-activate the cron job.

Verifying the Crond Service

Once you’ve confirmed the correct settings, it’s time to verify that the cron system is enabled and running. The three following commands can each be used to verify if the crond (the cron service) is running.

/etc/init.d/crond status

service crond status

systemctl status crond

After running any of the above commands, if you find the crond service is not running, you can start it with one of the following.

/etc/init.d/crond start

service crond start

systemctl start crond

/var/log/cron

Once you know that the cron is enabled, not silenced, and crond is running, then it’s time to check the cron log, located in the path of /var/log/cron.

cat /var/log/cron

Example Output:

Oct 2 23:45:01 host CROND[3957]: (root) CMD (/usr/local/lp/apps/kernelupdate/lp-kernelupdate.pl > /dev/null 2>&1)
Oct 2 23:50:01 host CROND[4143]: (root) CMD (/usr/lib64/sa/sa1 1 1)
Oct 2 23:50:01 host CROND[4144]: (root) CMD (/usr/local/maldetect/maldet --mkpubpaths >> /dev/null 2>&1)
In the log, you will see if, when and what user ran the cron. If initiated, you’ll see the date and time of execution followed by brackets of the individual cron number. This timestamp does not confirm that the script ran normally or at all, it only signifies when the cron system last ran the task. Beyond that, you may need to investigate the cron script itself or application-level configurations and their respective logs to ensure the code is executing correctly.

Other Cron Services

This article is merely an overview of the main crond service as there many other cron tasks services. The anacron system is a commonly used cron service that configures daily or hourly jobs, and can even be set to run at reboot. The logs for these kinds of tasks are within /var/log/cron, and are not executed by crond.

Other scheduled tasks, although also referred to as cron jobs, are not executed from the crond system. These cron jobs are often configured within the code or configuration of a website. To determine if executed, you’ll need to investigate other configurations and logs that the cron script interacts with.

As with all cron services, automated jobs can be manipulated to execute numerous daily tasks, so you don’t have to. Cron tasks can occasionally go awry even without altering them or years, but knowing where to look is half the battle in troubleshooting a cron job.

 

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    Author Bio

    About the Author: Mark Cunningham

    Mark currently works as an Enterprise System Administrator, whose long-term goal is to actually turn his job into a series of tiny shell scripts. He also enjoys making things outside of cyberspace. You might find him woodworking, machining, or on a photography outing when not working on servers all day.

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