Unix systems are built on commands, each designed to do a single simple task. Cron allows you to run commands according to a repeating schedule defined in a crontab file located in /var/spool/cron/crontabs. The command crontab is used to create and modify the user's file.
| /etc/cron.d | main cron directory |
| /etc/cron.d/FIFO | used as a lock file |
/etc/cron.d/cron.allow |
Allow/deny access to cron |
| /etc/cron.d/queuedefs | queue description file for at, batch, and cron. |
| /etc/cron.d/logchecker | moves log file to /var/cron/olog if log file exceeds system ulimit. |
| /etc/default/cron | contains cron default settings |
| /var/cron/log | cron history information |
| /var/spool/cron | spool area |
Crontab files are tab/space separated fields beginning with time indicators:
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)
Followed by a commmand
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