1. Chapter 10 Periodic Processes Unix System Administration

2. C-Ron, C-Ron Run  Cron is a daemon that periodically executes commanded defined in the crontab.  Some Unix systems provide security to restrict who can run cron jobs.  Cron jobs are executed using the Bourne shell (/bin/sh)

3. Put It on Cron’s Tab  Format of crontab  6 space separated commands  field 1 = minute of hour 0-59  field 2 = hour of day 0-23  field 3 = day of month 1-31  field 4 = month of year 1-12  field 5 = day of the week 1-7 (1 = Sunday)  field 6 (to end of line) = command to execute

4. /var/spool/cron/crontabs/* #ident "@(#)root 1.19 98/07/06 SMI" /* SVr4.0 1.1.3.1 */ # # The root crontab should be used to perform accounting data collection. # # The rtc command is run to adjust the real time clock if and when # daylight savings time changes. # 10 3 * * 0,4 /etc/cron.d/logchecker 10 3 * * 0 /usr/lib/newsyslog 15 3 * * 0 /usr/lib/fs/nfs/nfsfind 1 2 * * * [ -x /usr/sbin/rtc ] && /usr/sbin/rtc -c > /dev/null 2>&1 30 3 * * * [ -x /usr/lib/gss/gsscred_clean ] && /usr/lib/gss/gsscred_clean

5. Managing Crontab  Edit crontabs  Method #1  setenv EDITOR vi  crontab -e  Method #2  crontab -l >mycrontab  vi mycrontab  crontab mycrontab  Removing a crontab  crontab -r

6. Locking Down Cron  /etc/cron.d/cron.allow & /etc/cron.d/cron.deny  One username per line  See man crontab for details on how the rules are enforced.

7. Questions To Ask Yourself Periodically  What types of services is cron well suited for?  What happens if the program creates output and you don’t redirect it?  If /bin/sh is used to execute crontab commands, how do you execute stuff in other shells?