1. Commands
   crontab -e   # Edit (or create) the crontab file
crontab -l   # View the crontab file
crontab -r   # Remove the crontab file
2. Restrictions
   Crontab access is controlled with a cron.allow and a cron.deny file, listing the associated usernames.  If cron.deny exists, and cron.allow is missing, all users can use crontab.  If both missing then only root is allowed.
   whereis cron
3. Crontab file syntax
   The syntax contains 5 fields specifying the scheduling datetime values, and the last portion contains the Unix command to run.
   # +---------------- minute (0 - 59)
# | +------------- hour (0 - 23)
# | | +---------- day of month (1 - 31)
# | | | +------- month (1 - 12)
# | | | | +---- day of week (0 - 7) (Sunday=0 or 7)
# | | | | |
  * * * * * command to be executed
   On each of the 5 fields, 3 operators can be used to specify the datetime selection.
   # Comma (',') specifies a list of values, eg: "1,3,4,7"
# Dash ('-') specifies a range, eg: "1-3" or "1,2,3"
# Asterisk ('*') specifies all possible values for a field.
4. Crontab file example
   The following job lists the tmp directory structure and appends the output to a log file every minute.
   0-59 * * * *  ls -la /tmp >> /tmp/log.txt

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