[ubuntu]how to automatically execute a task

How to execute a task automatically?For example,updating news or graping web pages.We can do as below.

###1.install cron

sudo apt-get install cron

###2.find out whether cron runs

pgrep cron

if you can see a list of pids,cron runs successfully.if not,run:

service cron start

###3.add a automatically task

Now,we can edit a profile which storing information for a automatically task.

Run:

crontab -e

then, we choose a edit ide to edit this profile.

m       h     dom    mon   dow   command
0-59   0-23  1-31    1-12  0-7   command

dom means day of month.

for example:

59 * * * * java -jar /home/ruben/update_news.jar

other command list below:

usage: crontab [-u user] file
       crontab [ -u user] [ -i ] { -e | -l | -r }
               (default operation is replace, per 1003.2)
       -e      (edit user's crontab)
       -l      (list user's crontab)
       -r      (delete user's crontab)
       -i      (prompt before deleting user's crontab)

if you want to see cron log,you can run:

grep CRON /var/log/syslog

/*note that CRON is upcase.*/

then,you can see a list of log info in the screen.