Cron jobs son el método estándar para ejecutar tareas programadas en su servidor. Cron es un servicio que corre en segundo plano el cual ejecuta tareas (jobs) en un tiempo especifico, o en intervalos regulares. Las tareas y su tiempo de ejecución se definen en un archivo llamado crontab.

Configuración

Las tareas Cron se almacenan en un archivo crontab por usuario. Este archivo se alamacena en  /var/spool/cron/crontabs o /var/spool/cron/. Este archivos NO se debe editar directamente. El comando crontab siempre se debe utilizar para realizar cambios.

Puede editar su crontab con el siguiente comando:

$ crontab -e

Este comando abre el crontab en un editor de texto donde realizara los cambios necesarios. Guarde los cambios y cierre el archivo esto instalara el nuevo crontab.

El siguiente comando le permite visualizar los cambios realizados a su crontab:

$ crontab -l

El siguiente comando remueve totalmente su crontab:

$ crontab -r

Todos los comandos anteriores afectan solo al crontab del usuario con el que inició sesión. Es decir, si ha iniciado sesión como myuser, solo afectaría al crontab de myuser. Si ha iniciado sesión como usuario privilegiado (como root), puede afectar a otros usuarios utilizando la opción -u . Por ejemplo, el siguiente comando editaría el crontab para myuser:

$ crontab -u myuser -e

La opción -u se puede proporcionar con cualquiera de las otras opciones utilizadas anteriormente.

Syntax

Un archivo crontab le dice al Cron daemon qué comandos ejecutar y cuándo ejecutarlos. Tiene tres tipos diferentes de líneas: comando cron (cron command), configuracion del entorno (environment setting), y comentario (comment). Se ignoran las líneas en blanco, los espacios iniciales y las tabulaciones.

Comentarios

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.

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).

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.

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:

0,10,20,30,40,50 * * * * /path/to/command

*/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

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".