Cron jobs are a method to shcedule tasks to run your server automatically. Cron runs as a service in the background that executes commands (jobs) at a specific time, or at a regular interval. Jobs are pre-defined in a configuration file called 'Crontab;.
If you have cPanel as a control panel, cron jobs can managed directly without modifying a crontab. See cPanel's Cron Jobs feature for more information.
Configuration
Cron jobs are stored in a crontab file by username. These files are stored in 'var/spool/cron/contabs or 'var/spool/cron'. These files should not be edited directly. The crontab command should always be used to make any changes.
A crontabe can be modified by using the following command.
$ crontab -e
That comman will open to crontab in a text edtor where you can make the necessary change. Saving the changes, and closing the file will install in the new crontab.
The following command allows you to view your crontab without making changes.
$ crontab -l
The following command with remove your crontab entirely:
$ crontab -r
All of the above commands affects only the crontab for the user you are logged in as. If you are logged in as the 'root' user you can affect other users using the -u option. The following command would edit the crontab for myuser.
$ crontab -u myuser -e
The -u can be supplied with any of the other options used above.
Syntax
A crontab file tells the Cron daemon what commands to run, and when to run them. It has three different kinds of lines: cron command, environment setting, and comment. Blank lines, leading spaces, and tabs are ignored.
Comments
Any line whose first non-whitespace character is a pound-sign (#) will be considered a comment. Inline comments are not allowed. Comments are not processed by the Cron daemon.
Environment Setting
Environment setting is in the form of:
name = value
Whitespace around the equal sign (=) are optional, and everything to the right of the equal sign is considered part of the value. The value can be quoted (with single or double quotes) to preserve leading or trailing spaces. Many environment variables are assigned a default value:
SHELL is set to /bin/sh.
HOME and LOGNAME are set by /etc/passwd.
SHELL and HOME can be overridden by environment settings, but LOGNAME can not.
The MAILTO variable is checked to see if mail needs to be sent. By default, mail is sent to the owner of the crontab. Mail can be sent to another email address by assigning it to MAILTO. If you don't want Cron to send mail, assign MAILTO to the empty string (MAILTO=""). The MAILTO variable will only affect where emails are sent for commands that come after the line it is assigned. If you want to change all the email address of all commands, you would need to assign MAILTO at the top of the file. If you want to change the email address for a single command, you would set MAILTO directly above the command, and then set MAILTO back to its previous value before the next command.
Cron Commands
A cron command has six fields: five date and time fields, and a command field. The Cron daemon checks the schedule of each job every minute. A job is run whenever the time, and date matches the current time and date.
A full cron command will look something like this:
0 0 * * * /path/to/command
Scheduling
The five date and time fields are minute, hour, day of the month, month, and day of the week respectively:
┌───────────── minute
│ ┌───────────── hour
│ │ ┌───────────── day of the month
│ │ │ ┌───────────── month
│ │ │ │ ┌───────────── day of the week
│ │ │ │ │
│ │ │ │ │
│ │ │ │ │
* * * * *
Above diagram courtesy of Wikipedia.
minute 0-59
hour 0-23
day of the month 1-31
month 1-12 or names (see below)
day of the week 0-7 (Sunday is 0 or 7) or names
Cron jobs are run whenever the minute , hour , and month fields match the current time, and when at least one of the day of the month , or day of the year fields matches.Names for days of the week, or months are the first three letters of a day or month (e.g Mon for Monday, Oct for October).
Crons scheduled to run at times that can be "skipped" may result in jobs not running when you expect them to. A cron scheduled to run on the 31st of each month would only run during January, March, May, July, August, October, and November. If a daylight savings time change occurs, a cron job scheduled to run at a time that is skipped, or occurs twice will be skipped or be executed twice respectively.
All fields may contain an asterisk (*) which stands for any value.
Ranges are allowed, and are written as two numbers separated by a hyphen (-). Ranges are inclusive.
Lists are allowed, and are numbers or ranges separated by commas e.g. 1,3,5,7, or 0-7,16-23.
Step values may be used with ranges. A step value is written as a range followed by a forward slash (/), and a number. The number will tell the Cron daemon how many times it should skip that job before running it again. The value 0-23/2 in the hour field tells Cron to execute the command every other hour. Step values may be used with asterisks as well so */2 will also tell Cron to run a command every other hour.
Commands
The last field of a cron command is the command to be run. Specifically, everything after the day of the week field until a newline or '%' character is considered to be part of the command, and will be executed by the shell set in the SHELL environment variable (/bin/sh by default).
All '%' characters in a command are converted to newline characters. A literal '%' must be escaped with a preceding backslash (\) e.g. \%. No other characters need to be escaped in a command.
Extensions
There are a handful of special time shorthand that may be used instead of the five date and time fields.
@reboot Runs once after a reboot.
@yearly or @annually Run once a year (0 0 1 1 *).
@monthly Run once a month (0 0 1 * *).
@weekly Run once a week (0 0 * * 0).
@daily Run once a day (0 0 * * *).
@hourly Run once an hour (0 * * * *).
Examples
A command that runs every minute:
* * * * * /path/to/command
A command that runs every 5th minute on the hour (e.g. 1:05, 2:05, 3:05 etc.):
5 * * * * /path/to/command
A command that runs every 10 minutes:
Range
0,10,20,30,40,50 * * * * /path/to/command
Step
*/6 * * * * /path/to/command
A command that runs every Monday at 8:00 AM:
0 8 * * 1 /path/to/command
A command that runs the first Tuesday of every month at 5:30 PM:
30 17 1-7 * 2 /path/to/command
Output
As previously mentioned, the Cron daemon will email the output of the command to user's email address, or the address set by MAILTO whenever a cron job is executed. This can become a problem however if a command is run frequently. Stopping the emails is an option by setting MAILTO="", but you will then lose the output of the commands. Shell redirection can be used to have the output written to a file instead of being sent as an email.
The following command will write to a log file every hour:
0 * * * * echo "Hourly message." > message.log
The command that gets executed is echo "Hourly message.". The remaining part of the line, > message.log, redirects the output to a file named message.log. It's important to note that this will replace whatever is in message.log with Hourly message. The >> operator will append to the end of a file instead of overwriting it:
0 * * * * echo "Hourly message." >> message.log
If this cron job ran 3 times, the contents of message.log would look like this:
Hourly message.
Hourly message.
Hourly message.
If you don't care about the normal output of a cron job, and only want to be notified when it encountered an error, you can redirect the output to /dev/null.
0 * * * * /path/to/my/script > /dev/null
/dev/null is a special device file in Linux (and other Unix-like operating systems) that is used when discard output. Anything that is written to this file just gets thrown away by the system.
Redirection can be used to prevent emails from being sent as well by sending both normal output and error messages to /dev/null instead of setting MAILTO="".
0 * * * * /path/to/my/script 2>&1 > /dev/null
2>&1 in short says to treat error messages like normal output. So 2>&1 > /dev/null can be read as "treat error messages like normal output, and throw away normal output".
