Cron & Crontab

Written by Jahangir on October 27, 2008 – 6:39 am -

CRON
Cron is a scheduling daemon process on Linux and other Unix variants, it enables unix users to execute commands or scripts (groups of commands) automatically at a specified time/date. Cron is very useful to run backup scripts.

Cron Files/Directory
/etc/crontab
/etc/cron.d
/var/spool/cron/(username)

/etc/cron.hourly
/etc/cron.daily
/etc/cron.weekly
/etc/cron.monthly

Crontab Restrictions
Cron has a built in feature of allowing you to specify who may and who may not use it. It can done by using “/etc/cron.allow” and “/etc/cron.deny” files.
If the username exist in the file “/etc/cron.allow“, that user is authorize to execute cron. If “/etc/cron.allow” file does not exist, you can use crontab if username does not appear in the file “/etc/cron.deny“. If only “cron.deny” exists and its empty, than all users can use crontab. If neither file exists, than only the root user can use crontab. The “allow/deny” files consist of one user name per line.

Crontab Options and Explanation
A crontab file has five fields for specifying time, day and date followed by the command to be run at that interval. Below is Crontab syntax,

*  *  *  *  * command to be executed
-  -  -  -  -
| | | | |
| | | | +—– day of week
| | | +——– month
| | +———– day of month
| +————— hour
+—————— minute

Minute (0-59),
Hour (0-23),
Day of the month (1-31),
Month of the year (1-12),
Day of the week (0-6 with 0=Sunday).

* Is treated as a wild card. Meaning all possible value.
*/5 is treated as ever 5 minutes, hours, days, or months. Replacing the 5 with another numerical value will change this option.
2,8,12 “,” Treated as an “OR“, so if placed in the hours, this could mean at 2, 8 or 12 o-clock. Or if put in days it would be on 2,8 and12. “,” is used to define more options [date, time, day]
6-10 Treats for any value between 6 and 10. So, if placed in day of month this would be days 6 through 16. Or if put in hours it would be between 6 and 10. “-” is used to define the range [date, time]

Crontab Commands
crontab -e Edit your crontab file, or create one if it doesn’t already exist.
crontab -l Display your crontab entries.
crontab -r Remove your current crontab entry..
crontab -u username -e This option allows you to modify or view the crontab file of a user, only super user can use this option.


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Migrating Cpanel Servers

Written by Swapnil on September 17, 2008 – 1:52 pm -

Moving all the accounts between two Servers that host the same control panel is usually not a very difficult process.

You need to move all domains from the present server to the new server. At the same time, you need the whole move done perfectly and with zero downtime for all domains. In addition, you need to move even your nameservers between two different datacenters. Its pretty complicated.
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Posted in Dedicated Server Hosting, Linux VPS Hosting | No Comments »
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