Crontab
You can configure the behavior of cron by editing files called crontabs.
Unix keeps different copies of crontabs for each user in the /var/spool/cron folder.
You can edit your own user's crontab by running:
crontab -e
And you can list the current cronjobs for your user by running:
crontab -l
Crontab Syntax
All crontabs follow the same syntax. Each line specifies a command to be run and the time at which it should run.
(1)(2)(3)(4)(5) <command to be executed>
(1) Minute (0 - 59)
(2) Hour (0 - 23)
(3) Day (1 - 31)
(4) Month (1 - 12)
(5) Weekday (0 - 7) (Sunday is 0 or 7, Monday is 1...)
For example, the following crontab entry tells the shell to cd into the directory where security scripts are stored and run the scan.sh shell script every day at 9:30 pm. (The wildcard character * means “all”.)
30 21 * * * cd /Users/vickie/scripts/security; ./scan.sh
And in system-wide crontabs, you can also specify the user to run the command as:
* * * * * <username> <command to be executed>
For example, this entry will tell cron to run the same commands, but as the root user:
30 21 * * * root cd /Users/vickie/scripts/security; ./scan.sh
Another useful thing to know is that if you wish to run a script every 5 minutes, then you should put /5 in the minutes field, like so:
`
/5 * * * * root cd /Users/vickie/scripts/security; ./scan.sh`
Usually a use this command to read crontab in ctf:
cat /etc/crontab