If the allow file exists, then you must be listed therein in order to be allowed to use this command. If the allow file does not exist but the deny file does exist, then you must not be listed in the deny file in order to use this command. If neither of these files exists, then depending on site-dependent configuration parameters, only the super user will be allowed to use this command, or all users will be able to use this command.
If the -u option is given, it specifies the name of the user whose crontab is to be tweaked. If this option is not given, crontab examines "your" crontab, i.e., the crontab of the person executing the command. Note that su(8) can confuse crontab and that if you are running inside of su(8) you should always use the -u option for safety's sake.
The -l option causes the current crontab to be displayed on standard output.
The -d option causes the current crontab to be deleted.
The -r option is used to replace the current crontab (if any) with the contents of the named file.
/var/cron/allow /var/cron/deny
Paul Vixie, paul@vixie.sf.ca.us