Crontab Configuration

Scheduling the jobs usung "at" and "cron" :

   cron: job runs multiple times
   at: jobs runs only onetime.

 cron:      
syntax:     crontab   options
-e (edit/add (uses vi editor))
-l (list the jobs)
-r (remove all the jobs)



syntax: crontab -e
                crontab  -u  user-name  -e ( as root to configure a cron job for a specific user)

    mins hrs dayofmonth month dayofweek task
    0-59 0-23 1-31 1-12 0-6 cmd

ex:
45 08 * * *  backup.sh
0 0 * * * script1.sh
*/5   *  *  *  *    mon02.sh
0 * * * *  script2.sh
0 2 * * * automation.sh  

00  8,18  * * * /sbin/reboot

30  9,17  10,20  *  *  /bin/rm  -rf  /tmp/*
00  9,17  *  *  6  /bin/rm  -rf  /tmp/*

NOTE:   crontab is a binary command, all normail users are having access permission on crontab

               which crontab

   1. user listed in  /etc/cron.deny  are not allowed to use crontab
   2. user's listed in /etc/cron.allow are only allowed to use crontab and /etc/cron.deny file is ignored.

cron jobs are managed by the service "crond"

    chkconfig  --list  crond ;  service crond status       ( Before RHEL7)
    chkconfig  crond    on ;  service crond restart        ( Before RHEL7)
 
    systemctl enable crond ; systemctl restart crond       (RHEL7)

cron logs are available in, /var/log/cron
croh jobs are stored in, /var/spool/cron/$USERNAME


task:
1. configure a cron job as root , it runs daily at 5:30PM and executes   /bin/rm -rf /tmp/*

2. configure a cron job as student , it runs daily at 12:30AM and executes   /bin/echo hello

3. restrict "user1" user, to perform cron jobs ( create user1 for checking )


------------------------------------------------------------------
   at:      syntax:       at  options
                               now
now+10min
now+1day
5:00pm  or  17:00
5:00am
23:59  12/31/2013

      ex:     at   23:59   12/31/2013
               at> cmd
at> ctrl+d(to quit)

      atq  ... to list at jobs                                          
      atrm jobid .. to remove jobs
      at  -c jobid ( to see job details )

                        chkconfig   --list  atd
service  atd  status

"at" is a binary command, all normail users are having access permission on "at"

               which at

   1. user listed in  /etc/at.deny  are not allowed to use "at"
   2. user's listed in /etc/at.allow are only allowed to use "at" and /etc/at.deny file is ignored.


task:
1. configure a at job as root , it runs at 31st December 2020 at 23:59 and executes   /bin/echo hi
2. configure a at job as root , it runs today at 2:30AM and executes   /bin/echo hello