crontab -e , then 0 * * * * = every hour; then xclock

Question

After sudo crontab -e (it wrote "/tmp/crontab.XXXX70pN9h" 1L, 19C 1,1 All

Then I saved the file. And to check..? (how? i don't know)

And to check I 
ps -e | grep cron
29818 ?        00:00:00 cron

So.. but I decided to check again but

sudo cat /etc/cron.hourly
cat: /etc/cron.hourly: Is a directory
xi@localhost ~ $ sudo ls /etc/cron.hourly

Nothing! i.e. in directory

As I understand first 0 in the hour-position means every hour?

Answer 1

Your crontab and cron.hourly are separate entities. Putting something in your crontab does not put it in cron.hourly, even if it runs every hour. Putting a script in cron.hourly is merely another way of having it run every hour, that is, your cron daemon checks both your crontab and cron.hourly. Having it in both would run it twice.

Answer 2

You can check if the crontab is installed using:

sudo crontab -l

Every hour a log message should appear in /var/log/syslog that it was executed.

Further reading:

http://www.linuxweblog.com/crotab-tutorial

I tink that the first zero (0) is the minutes. But if you say you want something done when the minutes is 0 it would be performed every hour. As only once an hour the minutes are zero.

Answer 3

The fields in cron are:

m      h       dom             mon     dow          command
Minute Hour    Day of Month    Month   Day of week  <command>

And your 0 * * * * <command> then translates to "Execute command on every hour, every day, every month, and every weekday, if the minute is 0"

You can list your crontab by using crontab -l, as delh already stated.

Take this Link here, there are some Practical Cron Examples