2 - Raspberry PI Community Projects

SECURITY
Restricting cron or atd
You can restrict access to cron by creating an explicit authorization file
(whitelist) in /etc/cron.allow, in which you indicate the only users authorized
to schedule commands. All others will automatically be deprived of
this feature. Conversely, to only block one or two troublemakers, you could
write their username in the explicit prohibition file (blacklist), /etc/cron.
deny. This same feature is available for atd, with the /etc/at.allow and
/etc/at.deny files.
The root user has their own crontab, but can also use the /etc/crontab file, or write additional
crontab files in the /etc/cron.d directory. These last two solutions have the advantage of being
able to specify the user identity to use when executing the command.
The cron package includes by default some scheduled commands that execute:
• programs in the /etc/cron.hourly/ directory once per hour;
• programs in /etc/cron.daily/ once per day;
• programs in /etc/cron.weekly/ once per week;
• programs in /etc/cron.monthly/ once per month.
Many Debian packages rely on this service, they put maintenance scripts in these directories,
which ensure optimal operation of their services.
9.7.1. Format of a crontab File
TIP
Text shortcuts for cron
cron recognizes some abbreviations which replace the first five fields in a
crontab entry. They correspond to the most classic scheduling options:
• @yearly: once per year (January 1, at 00:00);
• @monthly: once per month (the 1st of the month, at 00:00);
• @weekly: once per week (Sunday at 00:00);
• @daily: once per day (at 00:00);
• @hourly: once per hour (at the beginning of each hour).
SPECIAL CASE
cron and daylight savings
time.
In Debian, cron takes the time change (for Daylight Savings Time, or in fact for
any significant change in the local time) into account as best as it can. Thus,
the commands that should have been executed during an hour that never
existed (for example, tasks scheduled at 2:30 am during the Spring time change
in France, since at 2:00 am the clock jumps directly to 3:00 am) are executed
shortly aer the time change (thus around 3:00 am DST). On the other hand,
in autumn, when commands would be executed several times (2:30 am DST,
then an hour later at 2:30 am standard time, since at 3:00 am DST the clock
turns back to 2:00 am) are only executed once.
Chapter 9 — Unix Services
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