Friday, September 1, 2017

Linux: How to Setup and Get Started wtih cron

When you need to run maintenance jobs routinely in Linux, cron comes in handy. cron is a job scheduler which will automatically perform tasks according to a set schedule. The schedule is called the crontab, which is also the name of the program used to edit that schedule.

cron Daemon to execute scheduled commands
crontab Schedule a command to run at a later time

In this article, we will show you how to setup and get started with cron in Oracle Linux Server 6.7.


The cron service (daemon) runs in the background and constantly checks the following file/directories:
  • /etc/crontab file
  • /etc/cron.*/ directories
  • /var/spool/cron/ directory
    • Each user can have their own crontab, and though these are files in /var/spool/ , they are not intended to be edited directly.
Crontab is the program used to install, deinstall or list the tables used to drive the cron. For example, to display the current crontab, you can do:

# crontab -l

# HEADER: This file was autogenerated at Wed Jan 13 22:49:06 +0000 2016 by puppet.
# HEADER: While it can still be managed manually, it is definitely not recommended.
# HEADER: Note particularly that the comments starting with 'Puppet Name' should
# HEADER: not be deleted, as doing so could cause duplicate cron jobs.
# Puppet Name: cron.puppet.apply
48 * * * * /usr/local/pdit/bin/puppet-apply > /dev/null 2>&1
00 0 * * * /etc/cron.daily.random/

Configuration Files:

You can control access to the crontab command by using two files in the /etc directory:[2]
  • cron.deny
  • cron.allow
These files permit only specified users to perform crontab command tasks such as creating, editing, displaying, or removing their own crontabfiles. Read [2] for more details.

Who can access to
crontab command?
Does Not Exists
cron.deny ExistsOnly users listed in
All users except those listed in
Does Not ExistsOnly users with superuser privilege

How to Edit Crontab Entries?

To edit a crontab entries, use
crontab -e
By default this will edit the current logged-in user's crontab.

After changing the crontable file, you don't need to restart cron. Cron will examine the modification time on all crontabs and reload those which have changed. Thus cron need not be restarted whenever a crontab file is modified.

[ramesh@user1 ~] $ crontab -e
# clean up Monitoring Tables weekly
0 0 * * 5 /scratch/user1/scripts/db/ > /dev/null 2>&1 
"/tmp/crontab.XXXXSERJLH" 2L, 112C

[Note: This will open the crontab file in Vim editor for editing.
Please note cron created a temporary /tmp/crontab.XX... ]
When you save the above temporary file with :wq, it will save the crontab and display the following message indicating the crontab is successfully modified.

"crontab.XXXXSERJLH" 2L, 112C written
crontab: installing new crontab
To edit crontab entries of other Linux users, login to root and use:
crontab -u {username} -e

Syntax of crontab (Field Description)

The syntax is:

1 2 3 4 5 /path/to/command arg1 arg2

1 2 3 4 5 /root/

1: Minute (0-59)
2: Hours (0-23)
3: Day (0-31)
4: Month (0-12 [12 == December])
5: Day of the week(0-7 [7 or 0 == sunday])
/path/to/command – Script or command name to schedule
cron also provides a number of operators that allow you to specify more complex repetition intervals. You can read [9] for more details.

Triggering JFR from Cron job

Below crontab entry will trigger jfr in every 45 mininutes for 900 seconds interval.
*/45 * * * * :

LOG_FILE="${LOG_DIR}/PRODOSB_${NODE}_`date '+%Y%m%d%H%M%S'`.jfr"

PID=`ps -ef | grep ${SERVER}_${NODE} |grep 'Dweblogic' | grep -v grep | awk '{print $2}'`

if [ ! -z "${PID}" ];then

${JDK_HOME}/bin/jcmd ${PID} JFR.start duration=900s filename=${LOG_FILE}



Auditing collects data at the kernel level that you can analyze to identify unauthorized activity. The entries in the audit rules file, /etc/audit/audit.rules, determine which events are audited. In the below example, we have set up a rule to audit crontab activities.
# cat /etc/audit/audit.rules
# This file contains the auditctl rules that are loaded
# whenever the audit daemon is started via the initscripts.
# The rules are simply the parameters that would be passed
# to auditctl.

-a always,exit -F path=/usr/bin/crontab -F perm=x -F auid>=500 -F auid!=4294967295 -k privileged

Each rule is a command-line option that is passed to the auditctl command. You should typically configure this file to match your site's security policy.


Rsyslogd is a system utility providing support for message logging. It is configured via the rsyslog.conf file, typically found in /etc. For example, in the below statement, it directs all cron messages to the file /var/log/cron.


# Log cron stuff
cron.* /var/log/cron

How to Debug?

If you suspect that your cron job was not executed correctly, here are the steps that you could take to debug:
  • Check the local user's email which will contain the output of cron jobs
    • Read [10] to find out where the email is and how to open and  read it
  • Add the following at the top of your bash script:
    • #!/bin/bash -x 
    • Next time when your script runs, it will show all the commands it executes
  • Check if there are mail messages in /var/spool/mail/root that indicate that mail to your user isn't getting delivered
    • Consider restarting sendmail after fixing your issues by doing:[14]
      • /etc/init.d/sendmail restart


  1. HowTo: Add Jobs To cron Under Linux or UNIX?
  2. Controlling Access to the crontab Command
  3. Configuring and Using Auditing
  4. Linux Crontab: 15 Awesome Cron Job Examples
  5. /usr/local : Local hierarchy
  6. How to schedule a biweekly cronjob?
  7. Configuring and auditing Linux systems with Audit daemon
  8. auditctl - Unix, Linux Command
  9. Schedule Tasks with Cron
  10. What is the “You have new mail” message in Linux/UNIX?
  11. How to check if a cron job ran
  12. 25 simple examples of Linux find command
  13. Stop Cron Daemon from Sending Email for Each Job
  14. How to stop and restart sendmail daemon

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