If you try to execute any command in linux/unix terminal, it is for one time execution and it will get executed once it is invoked.
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at :
To see pending jobs:
To see job details:
To allow and Deny the user to create crontab:
/etc/at.deny : To deny the user to create “at” job, we need to make an entry of username into /etc/at.denny
Now system wont allow kiran to create at jobs, it will show permission denied.
Note:
5 5.
6. Weekly starting time ; execute a job
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Suppose if you have a command/script which should be executed at particular time/period OR at regular intervals of time... is it possible ?
Yes. it is possible!
We have a commands "at" and "crontab" which were used for scheduling one time job and regular interval of time (recurring jobs) respectively.
For ex: If you want to remove files in /tmp at evening 4pm today. You can automate this using "at" job . i.e., One time scheduler.
Suppose if the same task should be performed on everyday at 4pm , you can make use of "crontab" which is used to schedule the recurring jobs.
Scheduling jobs:
at ⇒ One time job scheduler ; Executes commands at specified time.
crontab ⇒ Recurring jobs ; it is a deamon to execute scheduled commands.
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Creating jobs using “at” :
Syntax: #at time [here time is mandatory]
at >commands
at >commands
at >ctrl+d [ctrl+d is for save]
Note: If you dont specify any time, then it will give grabled time error.
[root@myserver ~]# at
Garbled time
[root@myserver ~]#
Few examples:
#at 8:31 [it will schedule the job at 8:31am to remove all files under /tmp]
#at 8:31 [it will schedule the job at 8:31am to remove all files under /tmp]
at> rm –rf /tmp/*
at> ctrl+d
Sample at job scheduling:
#at 7am
#at 11:30
#at noon
#at tea time → schedules job at evening 4pm or 16:00
#at now+10mits → after 10mins from current time
#at 8pm + 3days
#at now + 2hours
#at 7pm monday
#at 8am tomorrow
#at 8pm december 25
To see pending jobs:
# atq (or) # at -l
To remove jobs:
# at -d jobno (OR) #atrm jobno
ex: # atrm 4 → 4th job will be removed/deleted
ex: # atrm 4 → 4th job will be removed/deleted
To see job details:
# at -c jobno
Few important points to remember:
Few important points to remember:
- If you set the job for the which is already passed then it will be scheduling at next day Ex: current time is 10:00am if we scheduled at 7:00am then it will run on tomorrow 7:00am
- Editing (or)modifying at job is not possible after we set it. We need to delete job and recreate it.
- If the system is down but job exist at the time then once the syatem is up, immediately the job will run
- We can see our own job using # atq
- Other user jobs cannot be seen. only the root user can see all jobs of users.
- After executing “at” job it will be deleted.
- Non redirected output will be redirected to mail
echo "helloooo"
then the output ont be displayed on terminal rather the outpout will be sent to user mail.
if you want to display the output on terminal, then use terminal name
eg: echo "helloooo" > /dev/tty2 [to verify tty name : use tty command]
To allow and Deny the user to create crontab:
/etc/at.deny : To deny the user to create “at” job, we need to make an entry of username into /etc/at.denny
Vi /etc/at.deny
kiran
:wq!
:wq!
Now system wont allow kiran to create at jobs, it will show permission denied.
# /etc/at.allow : Block/restrict all current and future users otherthan one user.
Note: by default /etc/at.allow will be empty.
Be careful, if you update /etc/at.allow with any name, remaining all users will be denied to use at jobs.
Note: by default /etc/at.allow will be empty.
Be careful, if you update /etc/at.allow with any name, remaining all users will be denied to use at jobs.
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crontab:
Recurring Jobs:
crontab: It is used to schedule the jobs on daily basis.
It stands for cron tables
It consists fo 6 fields. Out of which 5 are time and date related and last filed (6th filed) is either command/script to be executed.
below is the commad to know crontab fileds:
# cat /etc/crontab
Here
Minutes - 0 to 59
Hours - 0 to 23
Day of Month - 1-31
Month - jan-oct or 1-12
Day of Week - 0-sun ; 1-mon; 6-sat
Commands:
crontab -e
|
To edit the crontab if exist or create a new one
|
crontab -l
|
To display existing crontab.
|
crontab -r
|
To remove existing crontab.
|
crontab -e -u
user
|
To set/edit the crontab for another user. (Only root
can do this)
|
- An " * " in a fiels represtent all possible values.
- Multiple values may be seperated by " , " (comma)
- Ranges can be seperated by " - "
- Seconds is not possible. Default is atleast a minute. ( you can write script to schedule for a second)
- If we specify */5 in a minutes fields , it means every 5 minutes
- 0-10/2 in a minutes fields means, every 2 minutes in 1 to 10 minutes
- Root can set crontab for any users , normal users can set crontab for only himself
Examples:
1. Every month; Every monday & Friday ; at 10:40am ; execute a job.
45 10 * * 1,5 commands
|
2. January & March; Monday to Friday; 13:40 ; execute a job
40 13 * 1,3 1-5 commands
|
3.Every new year starting time ; execute a job
0 0 1 1 * commands
|
4. Every Month starting time; execute a job
0 0 1 * * commands
|
5. Every Day starting time; execute a job
0 0 * * * commands
|
0 0 * * 1 commands
|
7.Every 2 hours ; execute a job
0 */2 * * * commands
|
Few more easy commands:
Fields
|
Usage
|
Entries
|
@reboot
|
Execute the command
immediately after reboot.(once system is UP)
|
@reboot commands
|
@hourly
|
To run once an hour
|
0 * *
*
* commands
|
@daily
|
To run once a day
|
0 0 *
* * commands
|
@weekly
|
To run once a week
|
0
0 * * 0 commands
|
@monthly
|
To run once a month
|
0 0 1 * * commands
|
@yearly
|
To run once a year
|
0
0 1 1 * commands
|
Usages of crontab:
To take regular backups, scheduled updates, copying/moving/deleting files, displaying notifications etc.,
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Thanks 😊
