The standard format of both the global /etc/crontab file and user-specific crontab files consists of six fields separated by spaces or tabs. The first five fields indicate when to execute the command that is specified in the sixth field. The fields are:
Minute: The minute of the hour (0-59) when the command should run.
Hour: The hour of the day (0-23) when the command should run.
Day of month: The day of the month (1-31) when the command should run.
Month: The month of the year (1-12 or Jan-Dec) when the command should run.
Day of week: The day of the week (0-7 or Sun-Sat, with 0 or 7 representing Sunday) when the command should run.
Command: The command or script to execute.
For example, the following entry in a crontab file will run the command /usr/bin/backup.sh every day at 2:30 AM:
30 2 * * * /usr/bin/backup.sh
The global /etc/crontab file has an additional field between the fifth and sixth fields, which is:
For example, the following entry in the /etc/crontab file will run the command /usr/bin/apt update as the root user every hour:
0 * * * * root /usr/bin/apt update
The other fields in the options are not part of the standard format of crontab files:
Year: This field is not supported by the standard cron daemon, but it may be available in some implementations, such as the Vixie cron. It would specify the year (1970-2099) when the command should run, and it would be placed after the month field.
Effective group ID: This field is not supported by any cron implementation, and it would not make sense to specify the group ID of the user who will execute the command, since it can be derived from the user ID.
References:
LPIC-1 Exam 102 Objectives, Topic 107: Administrative Tasks, Subtopic 107.2: Automate system administration tasks by scheduling jobs, Weight: 4, Key Knowledge Areas: Use cron and systemd timers to run jobs at regular intervals and to use anacron to manage system cron jobs. Objective: Use cron to run jobs at regular intervals.
LPIC-1 Exam 102 Learning Materials, Topic 107: Administrative Tasks, Subtopic 107.2: Automate system administration tasks by scheduling jobs, Section 107.2.1: cron, Page 18-20.
