Cron is a linux utility that allows users to execute scripts or programs at specified time or date interval. Cron can be typically used to take backup of server, automate downloading of emails, running virus scan, etc.

Cron stores its entries in a file called crontab. In most of the linux flavors, this file is found in /etc folder. A typical crontab file looks like as below:

                SHELL=/bin/bash
                PATH=/sbin:/bin:/usr/sbin:/usr/bin
                MAILTO=root
                HOME=/

                # run-parts
                01 * * * * root run-parts /etc/cron.hourly
                02 4 * * * root run-parts /etc/cron.daily
                22 4 * * 0 root run-parts /etc/cron.weekly
                42 4 1 * * root run-parts /etc/cron.monthly
            

The first four lines defines the environment in which cron executes the tasks. The definition of the variables are as following:

1. SHELL: This variable specify the system that which shell environment should be used which in this case is bash shell.
2. PATH: This variable defines the path used to execute commands.
3. MAILTO: This variable specify the username to which the output of cronjob should be emailed. If it is defined as empty string(MAILTO=""), emails will not be sent.
4. HOME: This variable set the home directory which will be used during the execution of cronjobs.
5. LOGNAME: This variable set username for the execution of cronjobs.

The schedule to run the cronjobs can be defined in the below format i.e.

                minute   hour   day   month   dayofweek   command
            

The first five fields in the format defines the date and time and the last field defines the command which should be run at the scheduled interval. The permitted values for these fields are:

1. minute: any integer from 0-59
2. hour: any integer from 0-23
3. day: any integer from 1-31 (should be valid if month is specified)
4. month: any integer from 1-12 (or can be of shorthand notation for month like jan, feb, etc)
5. dayofweek: any integer from 1-7 (or can be of shorthand notation for week day like sun, mon, etc)

Apart from the exact values, we can also use either wildcard(*), range of values(-), list of values(,) or step values(/) in any of the above fields to define the intervals.

1. Wildcard(*) : Asterisk (*) can be used as a default value when you dont want to specify values for the fields. For e.g. if * is specified in hour field will run the cron every hour within the other fields value constraints.
2. Range of values(-) : Hyphen (-) can be used between two integers for any field value to specify a range of time. For e.g. 0-5 specified for hour field will run the cron at 0, 1, 2, 3, 4, 5 hours.
3. List of values(,) : Comma(,) seperated integers for any field value will define a list of values. For e.g. 3,4,5 specified for hour field will run the cron at 3, 4, 5 hours
4. Step values(/) : Forward slash(/) can be used as step values for any field. The integer defined after / will be skipped within the range. For e.g. */2 defined in hour field will run the cronjobs every other hour. It can also be used with the range of values. For e.g. 0-59/2 will provide the same effect as */2 in hour field. Step values are also called as repeat pattern for cron.

Instead of defining these first five fields in crontab, special strings called as shorthand notation can also be used to define common time intervals. They are:

1. @hourly: Run once an hour. Same as 0 * * * *
2. @daily: Run once in a day. Same as 0 0 * * *
3. @midnight: same as @daily
4. @weekly: Run once in a week. Same as 0 0 * * 0
5. @monthly: Run once in a month. Same as 0 0 1 * *
6. @yearly: Run once in a year. Same as 0 0 1 1 *
7. @annually: same as @yearly
8. @reboot: Run once at startup

The users other than root can also set the cronjobs using crontab utility. All the cronjobs set by users are stored in /var/spool/cron directory. The cron file name for users are stored username.

The cron daemon(crond) first checks the /etc/crontab file, then /etc/cron.d directory and finally /var/spool/cron directory for changes every minute. The modified changes are loaded into memory, therefore it does not require restart of cron daemon.

There are two important files which controls the access to cron. These files are /etc/cron.allow and /etc/cron.deny. The format of these files are, having a username on each new line. These access files are read evertime cron is added or deleted by a user. If a username is listed in the cron.deny file, he would not be able to add, modify or delete cronjobs. It is not always essential that these files are present on the server. If you dont found it and want to control the accessibility of cron to particular users, create this file in /etc location. Please note that this file can only be created or deleted by root user.

There are few additional subdirectories supplied default by cron which are checked by the cron daemon while execution. They are:

1. /etc/cron.hourly: This folder consists of scripts or programs which should be executed every hour by cron.
2. /etc/cron.daily: This folder consists of scripts or programs which should be executed daily by cron.
3. /etc/cron.weekly: This folder consists of scripts or programs which should be executed weekly by cron.
4. /etc/cron.monthly: This folder consists of scripts or programs which should be executed monthy by cron.

To start the cron daemon, use service crond start

To stop the cron daemon, use service crond stop

To find out whether the cron daemon is running, use ps aux | grep crond

To list all the cronjobs set by root user, use crontab -l

To list all the cronjobs set by specific user, use crontab -u <username> -l

To create cronjobs for root user, use crontab -e

To create cronjobs for specific user, use crontab -u <username> -e

To remove cronjobs of root user, use crontab -r

To remove cronjobs of specific user, use crontab -u <username> -r

To import cronjobs from a file for root user, use crontab <file>

To import cronjobs from a file for specific user, use crontab -u <username> <file>

There are few environment variables which affect the execution of crontab. They are:

1. EDITOR: It specifies the editor which would be used to edit crontab entries when -e option is used. The default editor is vi.
2. LANG: It specifies a default value for the internationlisation variables that are unset or null. If LANG is unset or null, the corresponding value from the implementation-dependent default locale will be used. If any of the internationalisation variables contains an invalid setting, the utility will behave as if none of the variables had been defined.
3. LC_ALL: If set to a non-empty string value, override the values of all the other internationalisation variables.
4. LC_CTYPE: Determine the locale for the interpretation of sequences of bytes of text data as characters (for example, single- as opposed to multi-byte characters in arguments and input files).
5. LC_MESSAGES: Determine the locale that should be used to affect the format and contents of diagnostic messages written to standard error.
6. NLSPATH: Determine the location of message catalogues for the processing of LC_MESSAGES .

The exit values returned by crontab are:

1. 0: Successful completion
2. >0: In case of error

Although cron is very simple to implement but still many peoples find it difficult to set. Following are the most common problems which peoples get while setting cronjobs.

The script is executing from console but is not executing from cron.

There could be various reasons for this issue. They are:

1. The script is not having execute permissions. Use chmod a+x <scriptname> to provide permissions.
2. In case of php, perl or other scripting language, the path to interpretor or program is not correct. Always provide actual path to the interpretor while defining command to run in cron. To find out where the program is located, use whereis <program> command. For e.g. for php, whereis php. for perl, whereis php, etc.
3. The environment variables required to run the script are unavailable. The cron doesn't use the same environment of a user. It uses its own environment while running the commands. So the variables defined in .login or .profile are not visible to him. The most common issue is related to PATH variable which is not similar.

Crontab is unaccessible by user

This issue is due to the permissions provided to crontab program or the particular username is listed in /etc/cron.deny file.

Cron is unable to send emails.

Cron uses linux mail to send emails. If cron is not able to find mail program or have insufficient privileges to execute mail command then it would be unable to send emails.

Cron is not running

It could be due to the reason that cron daemon is not running. You can check whether it is running or not by executing ps aux | grep crond command. If it is not running, you can start it by executing service crond start.

The very first step to debug a cron problem is to enable cron logging. Cron logging will store all the events generated by crontab and can give you an idea of which commands does it executed and at which time.

Linux uses syslog for cron logging. To cron logging settings is defined in /etc/syslog.conf file as following.

                  # Log cron stuff
                  cron.*          /var/log/cron
          

If you don't find an entry like this in that file, you can add it to enable logging. After adding the entry, restart syslog daemon. The cron log entries can be then found in /var/log/cron file.

You can do a grep over /var/log/cron file to check whether your command has ran or not. If you don't find it there then it could be due to reasons explained in Cron Troubleshooting section.

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