Cron is a time-based job scheduler, it is configured it to run commands at given times or intervals. Each User has a cron table which defines what to execute and at what interval. crontab command is used to create, modify and view cron jobs.
Configuration files and directories
– Cron is controlled by a set of files called crontabs.
– There is the master file in /etc/crontab, along with crontab files for the users in /var/spool/cron/. In the latter directory, the files are given the same name as a user’s username.
– The /etc/crontab file automatically executes items in several subdirectories at regular periods. The scripts places in various directories – /etc/cron.* are run as per the time interval given below. All the scripts in these directories are run with the root privilege.
| Directory | Time |
|---|---|
| /etc/cron.hourly | First minute of every hour |
| /etc/cron.daily | Between 3:05 AM to 10.55 PM each day |
| /etc/cron.weekly | Between 3:25 AM and 11:10 PM after 7 days since last execution |
| /etc/cron.monthly | Between 3:45 AM and 11:30 PM after a month since last execution |
– All the sysadmin needs to do is to place a shell script or a link to an executable in one of the directories and it will automatically be run at the appropriate time.
crontab syntax
Setting up a user-level crontab is somewhat different. The files in /var/spool/cron are not edited directly. Instead, a program called crontab is used to manipulate them. The syntax of the crontab command is :
Usage: crontab [options] file crontab [options] crontab -n [hostname] Options: -u [user] define user -e edit user's crontab -l list user's crontab -r delete user's crontab -i prompt before deleting -n [host] set host in cluster to run users' crontabs -c get host in cluster to run users' crontabs -s selinux context -x [mask] enable debugging
How to edit a crontab
The best way to edit a crontab is using the command crontab -e. Another way of doing it is :
1. su to the user whose cron you want to change 2. crontab -l > file [ copy the crontab to a file ]. 3. vi file [ make changes to the file as per your need ] 4. crontab file [ this makes the "file" as new crontab ]
There is no need to restart the cron daemon after this.
Interpreting the time and date fields
Each cron command has 5 time and date fields, followed by a user name [optional], and if this is the system crontab file, it will be followed by a command. Commands are executed when the time specified by the time/date fields matches the current time.
field allowed values ----- -------------- minute 0-59 hour 0-23 day of month 0-31 month 0-12 (or names, see below) day of week 0-7 (0 or 7 is Sun, or use names)
A field may be an asterisk (*), which always stands for first to last. So when used in the month field, it means every month from 0 (Jan) to 12 (Dec).
Example Cron job:
# Example of job definition: .---------------- minute (0 - 59) | .------------- hour (0 - 23) | | .---------- day of month (1 - 31) | | | .------- month (1 - 12) OR jan,feb,mar,apr ... | | | | .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat | | | | | 0 1 * * * [user-name] [command to be executed]
Examples of setting cron jobs
Below are few examples of crontab usages to understand how to schedule a task :
Example : runing a job five minutes after midnight, every day :
5 0 * * * /home/oracle/scan_asm_devices.sh
Example : running a job at 5:30pm on the 1st of every month :
30 17 1 * * mail -s "It's 5:30pm"
Example : Running a job at 4:05 every Monday.
5 4 * * mon echo "run at 5 after 4 every monday"
User access control
– To allow users to access the crontabs, /etc/cron.allow and /etc/cron.deny files can be used to allow or deny access respectively. Simple put one username in either of the 2 files to allow or deny the access to the crontab.
– If the /etc/cron.allow file exists then the /etc/cron.deny file will not be used.
– In the default installation only an empty file /etc/cron.deny will exist.
– If neither of the files exists then only root user will be given access to schedule a job through cron.