Why my cron not working?

root@ubuntu-512mb-lon1-01:~/Documents# crontab -l

Edit this file to introduce tasks to be run by cron.

Each task to run has to be defined through a single line

indicating with different fields when the task will be run

and what command to run for the task

To define the time you can provide concrete values for

minute (m), hour (h), day of month (dom), month (mon),

and day of week (dow) or use ‘*’ in these fields (for ‘any’).#

Notice that tasks will be started based on the cron’s system

daemon’s notion of time and timezones.

Output of the crontab jobs (including errors) is sent through

email to the user the crontab file belongs to (unless redirected).

For example, you can run a backup of all your user accounts

at 5 a.m every week with:

0 5 * * 1 tar -zcf /var/backups/home.tgz /home/

For more information see the manual pages of crontab(5) and cron(8)

m h dom mon dow comman

0 * * * * @root /usr/bin/screen -dmS sh /root/Documents/

if I place /usr/bin/screen -dmS sh /root/Documents/ in command area is work but cron didnt work. Please advice

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Cron executes its scripts in an environment that is significantly different from the normal shell environment. In particular, the PATH variable has different values (and slightly different meaning) in cron context. Moreover, the vixie cron is sensitive to newlines; the crontab file must end with a blank line or the last cron in the file will be ignored.

Try replacing sh with /bin/sh