Cron Job-Shell-Commands-part6

The crontab is a list of commands that you want to run on a regular schedule,.

 Linux Crontab Format
 MIN HOUR DOM MON DOW CMD
 
 For eg : 30 08 10 06 * ls -latr /home/  ==>30 – 30th Minute 08 – 08 AM 10 – 10th Day 06 – 6th Month (June) * – Every day of the week .
 
 crontab -l ==> To view the crontab entries as the user root .
 crontab -u user-name -l ===>> View the Crontab entry for other user .
 
 To edit crontab entry : crontab -e
 * * * * *  ls -latr /home/   ==> The * means all the possible unit — i.e every minute of every hour through out the year. 
 
This example checks the status of docker service(including weekends) during the working hours 9 a.m – 6 p.m
00 09-18 * * * /usr/local/bin/script-to-check-if-docker-running. 
OR
crontab -l
* * * * * /root/hello.sh > ~/cron-test.txt

script-to-check-if-docker-running.sh
#!/bin/bash
service="docker"
/bin/systemctl -q is-active "$service.service"
status=$?
if [ "$status" == 0 ]; then
    echo "OK"
else
    echo "Not ok"
fi

chmod 777 script-to-check-if-docker-running.sh

To verify if cronjob is running :
 grep "script-to-check-if-docker-running.sh" /var/log/cron
 OR,
 cat ~/cron-test.txt
 
Cron special keywords and its meaning

Keyword    Equivalent
@yearly    0 0 1 1 *
@daily     0 0 * * *
@hourly    0 * * * *
@reboot    Run at startup.