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crontab: A command in linux to automatically start/repeat a process at certian time and interval

crontab - A command to manage crontab files for each users and schedule commands ans other scripts.
Format:
crontab [-u user] file
crontab [-u user] [-l | -r | -e]

 

Schedule your jobs in Linux

schedule a job

 


DESCRIPTION
Crontab is the program used to creat, remove or show the tables used to drive the cron(8) daemon in ISC Cron.

It is meant to be used for scheduling re-occurring jobs. If you want to schedule a job for one time exection, you should use at command in Linux.

Each user can have their own crontab, and though these are files in /var, they are not intended to be edited directly.
 


Related files: /etc/cron.allow, /etc/cron.deny

Note:-
Cron.allow: If the cron.allow file exists, then you must be listed therein in order to be allowed to use this command.

Cron.deny: If the cron.allow file does not exist but the cron.deny file does exist, then you must not be listed in the cron.deny file in order to use this command.

If neither of these files exists, only the super user will be allowed to use this command.

 


To List the cronjobs for a user
 

Example:
Prompt:>crontab -l

The -l option causes the current crontab to be displayed on standard output.

To see cronjobs og other user you can use -u option but you must be superuser to do so.

Prompt:>crontab -u <user> -l


Delete the cronjobs for a user
 

Prompt:>crontab -r

The -r option causes the current crontab (cronjobs of current user)to be removed.

The first form of this command is used to install a new crontab from some named file or standard input if the pseudo-file-
name ``-'' is given.

 


The format to create a new cron:

Field Meaning
Minute Any minute (0-59
Hour Hours from 0 to 23
Day Any day of month (1-31)
Month Any Month (1-12)
Day of week Any day of week (0-6)
CMD The command to be executed(Sunday =0/7)

 


 

To edit the crontab of a user:
Prompt:> crontab -e
The crontab editor will open. Add your jobs here line by line. While editing the crontab file, if there is any error in the crontab file, it wont allow to save the file. It will ask to go to edit the file again.

 

Eg.
20 16 13 3 4 ls -l

The above will execute ls -l at the time 16:20 on 13th of March on day 4 of the week at your home directory. 


 

Prompt:> crontab -l
# DO NOT EDIT THIS FILE - edit the master and reinstall.
# (/tmp/crontab.XXXXFIDLBr installed on Thu Mar 13 16:19:00 2014)
# (Cron version V5.0 -- $Id: crontab.c,v 1.12 2004/01/23 18:56:42 vixie Exp $)
20 16 13 3 4 ls -l


To check if the cronjobs executed or are running
You will get a mail about crons running or executed at here /var/mail/yourId

 
If you are a root user, you can see and edit the cron job of other users.
 
crontab -u someUser -e
The above command will view/edit the corn jobs of someUser.

Example 2:
Suppose you want to delete some log files from your home directory at every friday of the month at 23:59. How will you do this? Simply type below line in your crontab:
 
59 23 * * 5 rm *.log
 
Example 3:
Suppose you want to start a script at 06:10 pm at the end of each month, you can use below cron:
10 18 * * * /path-of-script/script.sh

 To run a command every one minute
Though this could not be the requirement, but just for understanding, below line in crontab will cause the CMD to run every minute.
* * * * * CMD

 


To run a command or script every 5 minute continuously

*/n in the first field (Minute field) means that the command will be execcuted every n minutes. For eg.

*/5 * * * * CMD

So, the above cron will run the CMD every 5 minutes.


To schedule a script/command to execute twice or thrice a day.

0 8,10 * * * CMD

The above cron means the CMD will execute at 8:00 AM as well as 10:00 AM everyday(on weekend also)

To schedule a script/command to execute in period ranging from one to another.

Lets say we want to execute a command starting at 8: 00 AM till 11:00 AM. So, the value '8-11' in the hour field means the command will be executed at 8 AM, 9 AM, 10 AM and at 11 AM.

Eg.

0 8-11 * * * CMD

Now lets see some special cronjobs:

 These special time specification "nicknames" are supported, which replace the 5 initial time and date fields, and are prefixed by the '@' character:

Special Crons Meaning Symbol Equivalent
@reboot  Run once, at startup.  
@yearly Run once a year. 0 0 1 1 * CMD
@annually Run once a year. 0 0 1 1 * CMD
@monthly Run once a month. 0 0 1 * * CMD
@weekly Run once a week. 0 0 * * 0 CMD
@daily  Run once a day. 0 0 * * * CMD
@hourly Run once an hour. 0 * * * * CMD
 
 

Category: Open System-Linux | Views: 2496 | Added by: | Rating: 5.0/1

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