Note that crontabs in this directory should not be accessed directly - the crontab command should be used to access and update them.Cron also reads /etc/crontab, which is in a slightly different format. Cron then wakes up every minute, examining all stored crontabs, checking each command to see if it should be run in the current minute.
In Debian and Redhat cron treats the files in /etc/cron.d as extensions to the /etc/crontab file (they follow the special format of that file, i.e. The intended purpose of this feature is to allow packages that require finer control of their scheduling than the /etc/cron.
Each user has their own crontab, and commands in any given crontab will be executed as the user who owns the crontab.
Uucp and News will usually have their own crontabs, eliminating the need for explicitly running su as part of a cron command.
An active line in a crontab will be either an environment setting or a cron command.
An environ‐ ment setting is of the form: name = value where the spaces around the equal-sign (=) are optional, and any subsequent non-leading spaces in value will be part of the value assigned to name.
Search for updating crontab:
Files must conform to the same naming convention as used by run-parts: they must consist solely of upper- and lower-case letters, digits, underscores, and hyphens.