The /etc/crontab file is the system-wide crontab file that can be edited only by root. It has a different format from the normal crontab files that can be edited by individual users using the crontab command. The differences are:
* The /etc/crontab file can specify a year field as the sixth field in a cron entry. This allows for scheduling jobs that run only in specific years. The normal crontab files do not have a year field and assume the current year for all entries.
* The /etc/crontab file has a user field as the seventh field in a cron entry. This allows for running commands as different users from the crontab owner (root). The normal crontab files do not have a user
* field and run commands as the crontab owner.
* The /etc/crontab file does not need to be installed with the crontab command. It is read by the cron daemon automatically. The normal crontab files need to be installed with the crontab command to be recognized by the cron daemon.
* The /etc/crontab file and the normal crontab files both allow for environment variable substitution.
However, the /etc/crontab file sets some default environment variables such as SHELL, PATH, MAILTO, and HOME, which can be overridden by entries in the file. The normal crontab files inherit the environment variables from the cron daemon, which are usually minimal.
References:
* crontab(5) - Linux manual page
* Linux Crontab Format
* How to schedule a task using Linux crontab (/etc/crontab) file
* /etc/crontab - Linux Bash Shell Scripting Tutorial Wiki