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Crontab

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Introduction

The crontab will automatically perform recurring tasks.
Crontab is the program name to edit the configuration table of cron. The crontab command edits a file on the user who executes it, and checks it syntax. This file is often in the /var tree:


• /var/spool/cron/tabs/user (SuSE GNU/Linux)
• /var/spool/cron/crontabs/user (Debian GNU/Linux and Ubuntu)
• /var/cron/tabs/user (FreeBSD et OpenBSD)

The cron file for the root user on an Ubuntu machine will be in:

• /var/spool/cron/crontabs/root

Some systems also have a centralized crontab in /etc/crontab.

Syntax

To edit the cron table, just run the following command:

crontab -e

will launch the default editor (usually vi) with the current table (or an empty file if the first launch from crontab).
Each line of the file corresponds to a task to perform, informed as follows:

mm hh dd MMM DDD task > log

Where:

• mm represents minutes (0-59)
• hh represents the hour (0-23)
• dd represents the day number of month (1-31)
• MMM represents the month number (1 to 12) or the abbreviated month name (jan, feb, mar, apr, ...)
• DDD is the abbreviated name of day or the number corresponding to the day of the week (0 is Sunday, 1 represents Monday, ...)
• task represents the command or shell script to run
• log is the name of a file in which to store the log of operations. If the clause >log is not specified, cron will automatically send a confirmation email. To avoid this, simply specify ">/dev/null"

For each time units (minutes / hours /...) notations are possible:

• * : At each time unit
• 2-5: time units 2,3,4,5
• */3 : all three time units (0,3,6 ,...)
• 5,8 : 5 and 8 time units

Examples

Suppose we want to establish an automated diary (in the file / logs / log_df.log example) of free disk space (df) at specific time intervals::
• Every day at 23:30:

  30 23 * * * df >> /logs/log_df.log

• Every hour past 5 minutes:

  5 * * * * df >> /logs/log_df.log

• Every first of month at 23:30:

  30 23 1 * * df >> /logs/log_df.log

• Every monday, at 22h28:

  28 22 * * 1 df >> /logs/log_df.log

• From 2 to 5 of each month at 10:12

  12 10 2-5 * * df >> /logs/log_df.log

• Every peer day of the month at 23:59:

  59 23 */2 * * df >> /logs/log_df.log

• Every 5 minutes:

  */5 * * * * df >> /logs/log_df.log

  It is also possible to automatically execute more complex commands using a shell script. Simply declare it as a job in the cron table.

 

Video tutorial of Crontab usage

 

 

 

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