Cron Jobs is used for scheduling the particular tasks to run on the server.it is a Linux utility command or script which will schedule a task automatically run on the server by giving specific time and date.
We can schedule the particular task(back up files , delete temporary files, etc.)
it is the one of the most useful tool in a Linux or UNIX like operating systems.
They are mostly used for automatic system maintenance and administration. The cronjobs once the command is executed it will run in the background while another task or process is going on.
Define Cron :-
It is a time based job scheduler in UNIX like os Linux and so on.these jobs or task are called as “Cron Jobs”
There is a separate cron named “daemon” that runs on Linux systems.it is a program that runs in the background at all time. This cron daemon is responsible for launching these Con Jobs Schedule.
Need or Use of Cron Jobs in Linux:
- If there is a membership site, where accounts have expiration dates, there we can schedule cron jobs to regularly deactivate or delete accounts that are past their expiration dates.
- Next if we are having summary tables in our database, it can be regularly updated with a cron job. For example you may store every web page hit in a table, but another summary table may contain daily traffic summaries.
- We can erase the cached data file and expire files at certain interval by using this Cron Jobs.
- We can auto-check our website content for broken links and have a report e-mailed to ourself regularly.
- Than we schedule a long-running tasks to run from a command line script, or rather than running it from a web script.
Syntax:
Here is a simple cron job:
10 * * * * /usr /bin /php /www/ virtual /user name/ cron.php > /dev /null 2>1
There are two main parts:
- The first part is "10 * * * *". This is where we schedule the timer.
- The rest of the line is the command as it would run from the command line.
The command itself in this example has three parts:
- "/usr/bin/php". PHP scripts usually are not executable by themselves. Therefore we need to run it through the PHP parser.
- "/www/virtual/username/cron.php". This is just the path to the script.
- "> /dev/null 2>&1". This part is handling the output of the script. More on this later.
Timing Syntax
This is the first part of the cron job string, as mentioned above. It determines how often and when the cron job is going to run.
It consists of five parts:
- minute
- hour
- day of month
- month
- day of week
Here is an illustration:
* * * * * command to be executed - - - - - | | | | | | | | | ----- Day of week (0 - 7) (Sunday=0 or 7) | | | ------- Month (1 - 12) | | --------- Day of month (1 - 31) | ----------- Hour (0 - 23) ------------- Minute (0 - 59)
Asterisk
Quite often, you will see an asterisk (*) instead of a number. This represents all possible numbers for that position. For example, asterisk in the minute position would make it run every minute.
| Field | Range of values |
|---|---|
| minute | 0-59 |
| hour | 0-23 |
| day | 1-31 |
| month | 1-12 |
| day-of-week | 0-7 (where both 0 and 7 mean Sun, 1 = Mon, 2 = Tue, etc) |
| command-line-to-execute | the command to run along with the parameters to that command if any |
The fields have to be in that exact order, with no empty or missing fields, and everything must be placed on a single line.
"Minute" is a number from 0 to 59. "Hour" is a number from 0 to 23. They represent the time of the day in a 24-hour day format, so for example, if you want a certain command to run at 5.30 am, you will have to code it as
30 5
if we want something run at 8 pm,it has to be coded as
0 20
since 2000 hours is 8 pm in the 24-hour time format.
"Day" and "month" refer to dates. "Day" takes a value between 1 and 31, and "month", as you may have already guessed, can take any value between 1 and 12.
We need to look at a few examples to fully understand this Syntax.
Examples:
This cron job will run every minute, all the time:
* * * * * [Enter this command]
This cron job will run at minute zero, every hour (i.e. an hourly cron job):
0 * * * * [Enter this command]
This is also an hourly cron job but run at minute 15 instead (i.e. 00:15, 01:15, 02:15 etc.):
15 * * * * [Enter this command]
This will run once a day, at 2:30am:
30 2 * * * [Enter this command]
This will run once a month, on the second day of the month at midnight (i.e. January 2nd 12:00am, February 2nd 12:00am etc.):
0 0 2 * * [Enter this command]
This will run on Mondays, every hour (i.e. 24 times in one day, but only on Mondays):
0 * * * 1 [Enter this command]
You can use multiple numbers separated by commas. This will run three times every hour, at minutes 0, 10 and 20:
0,10,20 * * * * [Enter this command]
Division operator is also used. This will run 12 times per hour, i.e. every 5 minutes:
*/5 * * * * [Enter this command]
Dash can be used to specify a range. This will run once every hour between 5:00am and 10:00am:
0 5 -10 * * * [Enter this command]
Also there is a special keyword that will let you run a cron job every time the server is rebooted:
@reboot [Enter this command]
Each every web hosting companies will provide the control panel to their customers / clients.by using this control panel we can easily manage the above cron jobs.
1.open the crontab manager for user window or open the SSH putty root and login with root as username and enter the password.
2.editing the Crontab--the below command will lanch the vi text editor there we can edit the contents of the Crontab:
crontab -e
3.if we want to see the existing crontab without editing it,just enter the below command:
crontab -l
4.To delete the contents of the crontab:
crontab -r
5.To loading the file :-here we can write all our cronjobs and push them to crontab.
crontab cron.txt
From all the above steps we can easily use the Cronjobs to schedule the task in Linux server automatically without affecting any other task.
