When scheduling a cron job each column indicates a different time period:
minute hour [Day of Month] [Month] [Day of Week]
Thus, to schedule a job to run at 5 minutes after every hour:
5 * * * * /job
every five minutes:
0-59/5 * * * * /job
some versions of cron allow the abbreviation: */5 * * * * /job
every hour on the hour:
0 * * * * /job
On the first of every month (at 1 am):
0 1 1 * * /job
—
Field Locations and Meaning:
- Minute (0-59)
- Hour (2-24)
- Day of month (1-31)
- Month (1-12, Jan, Feb, …)
- Day of week (0-6) 0 = Sunday, 1 = Monday, … or Sun, Mon, etc)
- Command to execute
Some versions of cron allow an extra field before the command to
specify the user that the command will run as.
—
35 2 * * * root tar czf /usr/local/backups/daily/abc.tar.gz /abc >/dev/null 2>&1
This will run tar czvf /usr/local/backups/daily/abc.tar.gz /abc at 2:35am every day.
The > /dev/null 2>&1 part means to send any standard output to /dev/null and to redirect standard error (2) to the same place as the standard output (1).
Basically it runs the command without any output to a terminal.
To view the cronjob currently on server use this command :
- crontab -l
To edit the cronjob on the server use this command :
- crontab -e
Once your cronjobs on the editor (by default it using “vi”) you can do the necessary amendments and save it by using the “vi” command :wq – which will overwrite current crontab settings.
Cronjob is very useful to schedule your requirements by setting up specific time to start, so no need hands on to execute the linux / unix command at time time.
eg : If you want to run a mysql database backup at the midnight from your server, can setup the cronjob like this :
0 0 * * * sh /root/backup/dbbackup
and the /root/backup/dbbackup contain the mysqldump command :
- !/bin/sh
cd /root/backup /usr/bin/mysqldump -uroot -ppassword –opt databasename > /root/backup/dbname.sql
So, every midnight, the cronjob automatically execute and do a mysql backup on /root/backup directory. But this method will always try to overwrite last night’s backup. If you want to backup everyday with a different name based on date, simply use this way :
- !/bin/sh
cd /root/backup egwdate=/root/backup/db_backup_$(date +%Y%m%d).sql /opt/mysql41/bin/mysqldump -uroot -ppassword –delayed-insert –add-drop-table databasename > $egwdate
The above command, please replace the username and password for the mysqldump, with your own user ame and password. This is just for your information only. As a result you should be the database backup name added with the data, which look something like this :
-rwxrw-rw- 1 users users :36 db_backup_ -rwxrw-rw- 1 users users :13 db_backup_
It will keep increment, until you manage it.
Tags:schedule, Unix cron job
