Crontab

crontab -a filename
Install filename as your crontab file. On many systems, this command is executed simply as crontab filename (i.e., without the -a  option).
crontab -e
Edit your crontab file, or create one if it doesn't already exist.
crontab -l
Display your crontab file.
crontab -r
Remove your crontab file.
crontab -v
Display the last time you edited your crontab file. (This option is available on only a few systems.)
crontab -u user
Used in conjunction with other options, this option allows you to modify or view the crontab file of user. When available, only administrators can use this option.

Field
Value
Description
minute
0-59
The exact minute that the command sequence executes
hour
0-23
The hour of the day that the command sequence executes
day
1-31
The day of the month that the command sequence executes
month
1-12
The month of the year that the command sequence executes
weekday
0-6
The day of the week that the command sequence executes (Sunday = 0, Monday = 1, Tuesday = 2, and so forth)
command
Special
The complete sequence of commands to execute. The command string must conform to Bourne shell syntax. Commands, executables (such as scripts), or combinations are acceptable.

# Example 1:  0,30 8-17 * * 1-5 cmd


Show/Hide Answer (+/-)

Run cmd on the half-hour from 8:00 AM to 5:30 PM, Monday thru Friday



# Example 2:  0 12 1,15 * 5 cmd


Show/Hide Answer (+/-)

Run cmd at noon each Friday AND the first and fifteenth of every month



# Example 3:  17 3 * * 1 cmd


Show/Hide Answer (+/-)

Run cmd at 3:17 AM Monday (a backup program perhaps)



# Example 4:
# This command sends a "popup" message to the user's screen,
# at 3:00 PM Fridays, if they are logged in:
0 15 * * 5 echo "Time for staff meeting" | write $LOGNAME >/dev/null 2">"&1



# Example 5:
# This command sends a multi-line "popup" message to the user's screen,
# at 3:00 PM Fridays, if they are logged in:
0 15 * * 5 write $LOGNAME '>'/dev/null 2'>'&1 %Time for the%staff meeting.