Crontabs are stored in several locations, collectively known as crontab groups or crongroups, for short. These are:
The file /etc/crontab. It is used for site-wide cronjobs, i.e. tasks necessary for the normal functioning of the system.
A collection of crontab files in the /etc/cron.d directory. These are used for per-package cronjobs.
Crontab files located in /var/spool/cron/crontabs contain per-user cronjobs and are editable by users.
A special crontab group intended for use with pseudo-accounts, such as ‘apache’ or ‘bind’. Crontabs of this group are located in subdirectories of /var/spool/cron/crongroups named by the corresponding account. This crontab group is described in detail in User Group Crontabs.
Each active (i.e. non-empty and non-comment) line in a crontab specifies a schedule and a command line to be run according to that schedule. Active lines in master and system crontabs specify also login name of the user on behalf of whom the command must be run.
Both master and system crontabs are writable only by the super-user.
User and user group crontabs belong to particular users, and
instructions they contain are executed on behalf of their owners. To
enable users to manipulate their crontabs, the
command is provided (see crontab).