as part of a cron command.
non-space  character is a pound-sign (#) are comments, and are ignored.
ridden by settings in the crontab; LOGNAME may not.
will  be	 sent.	 Otherwise  mail  is sent to the owner of the crontab.
address, otherwise, ``root'' will be used.
locale specified by the LC_*  environment  variables  (see  locale(7)).
constant for the whole run time of the daemon.
cron(8) examines cron entries once every minute.
A field may be an asterisk (*), which always stands for "first-last".
"hours" entry specifies execution at hours 8, 9, 10 and 11.
an asterisk, so if you want to say "every two hours", just use "*/2".
matter).	 Ranges or lists of names are not allowed.
on the first line is optional.
- set correct role for directory, which is used for input/output.
monthly.	  /var/spool/cron/  usual  place  for  storing	users crontab.
Sunday.	BSD and ATT seem to disagree about this.
@reboot	  :    Run once after reboot.
@yearly	  :    Run once a year, ie.  "0 0 1 1 *".
@monthly	  :    Run once a month, ie. "0 0 1 * *".
@weekly	  :    Run once a week, ie.  "0 0 * * 0".
@daily	  :    Run once a day, ie.   "0 0 * * *".
@hourly	  :    Run once an hour, ie. "0 * * * *".
symlinked crontabs are not automatically noticed by  the	 cron  daemon.
a limitation of inotify API.