CRON – A daemon to execute scheduled commands

cron is a time-based job scheduler in Linux operating systems. cron enables users to schedule jobs (commands or shell scripts) to run periodically at certain times or dates. It is commonly used to automate system maintenance or administration, though its general-purpose nature means that it can be used for other purposes, such as connecting to the Internet and downloading email.

To start the cron service all you need to do is:
# /etc/init.d/cron start
OR
#start service cron

cron searches its spool area (/var/spool/cron/crontabs) for crontab files. A crontab file contains instructions to the cron daemon of the general form: "run this command at this time on this date". Each user has their own crontab, and commands in any given crontab will be executed as the user who owns the crontab. crontabs found are loaded into memory.
cron then wakes up every minute, examining all stored crontabs, checking each command to see if it should be run in the current minute. When executing commands, any output is mailed to the owner of the crontab (or to the user named in the MAILTO environment variable in the crontab, if such exists).
Along with the crontab files, there is also a command of the same name – crontab. crontab is the program used to install, remove or list the tables used to drive the cron. Check out the crontab(1) man page to refer the list of options that the crontab command provides (click here).

Crontab Format

Commands are executed by cron when the minute, hour, and month of year fields match the current time, and when at least one of the two day fields (day of month, or day of week) match the current time.

A field may be an asterisk (*), which always stands for "first-last".

Ranges of numbers are allowed. Ranges are two numbers separated with a hyphen. The specified range is inclusive. For example, 8-11 for an "hours" entry specifies execution at hours 8, 9, 10 and 11.

Lists are allowed. A list is a set of numbers (or ranges) separated by commas. Examples: "1,2,5,9", "0-4,8-12".

Step values can be used in conjunction with ranges. Following a range with "/" specifies skips of the number's value through the range. For example, "0-23/2" can be used in the hours field to specify command execution every other hour. Steps are also permitted after an asterisk, so if you want to say "every two hours", just use "*/2".

Names can also be used for the "month" and "day of week" fields. Use the first three letters of the particular day or month (case doesn't matter). Ranges or lists of names are not allowed.

The "sixth" field (the rest of the line) specifies the command to be run. The entire command portion of the line, up to a newline or % character, will be executed by /bin/sh or by the shell specified in the SHELL variable of the crontab file. Percent-signs (%) in the command, unless escaped with backslash (\), will be changed into newline characters, and all data after the first % will be sent to the command as standard input.

Here are some examples of crontab lines. Use the command "crontab -e" to edit your crontab file.

           This line executes the "ping" command every minute of every hour of every day of every month. The standard output is redirected to dev null so we will get no e-mail but will allow the standard error to be sent as an e-mail

 *    *    *    *    *       /sbin/ping -c 1 192.168.0.1 > /dev/null
   

     This line executes the "ping" and the "ls" command every 12am and 12pm on the 1st day of every 2nd month. It also puts the output of the commands into the log file /var/log/cronrun.

   0,12 1 */2 * /sbin/ping -c 192.168.0.1; ls -la >>/var/log/cronrun 

                  

           This line executes the disk usage command to get the directory sizes every 2am on the 1st through the 10th of each month. E-mail is sent to the email addresses specified with the MAILTO line. The PATH is also set to something different.

PATH=/usr/local/sbin:/usr/local/bin:/home/user1/bin

MAILTO=user1@nowhere.org,user2@somewhere.org

0 2 1-10 * * du -h --max-depth=1 /

Joohi Sinha
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