### Cron - Scheduler ###

  1. Scheduler
  2. Rules (Cron entries) are based on times:
   a. minute (0-59)
   b. hour (0-23)
   c. day of the month (1-31)
   d. month (1-12)
   e. day of the week (Sun,Mon,Tue, etc. OR 0-7)
   f. command to execute (shell, perl, php, etc.)
 3. Wakes up every minute in search of programs to execute
 4. Reads cron entries from multiple files
 5. Maintains per-user and system-wide (/etc/crontab) schedules

/etc:
cron.d/      
cron.deny - denies cron execution by user
cron.monthly/ - runs jobs monthly
cron.weekly/ - runs jobs weekly 
cron.daily/ - runs jobs daily
cron.hourly/ - runs jobs hourly
crontab - contains system-wide schedules

Note: '*' wildcard in a time column means to run for all values

Per-user Crontabs:
Stored in: /var/spool/cron

Task:
  1. Create a cron entry for the user 'student1'
   a. su student1
   b. crontab -e
   c. create an entry, minus the name of the user

Note: 'crontab -l' - enumerates per-user cron entries


System-wide Crontab:
Stored in: /etc/crontab

Task:
  1. Create a cron entry in: /etc/crontab

Note: 'crontab -l -u username' - enumerates per-user cron entries