Installing crontab using bash script

Question

I have created a script to install two scripts on to the crontab.

#!/bin/bash

 sudo crontab -l > mycron
 #echo new cron into cron file

 echo "*/05 * * * * bash /mnt/md0/capture/delete_old_pcap.sh" >> mycron #schedule the delete script
 echo "*/12 * * * * bash  /mnt/md0/capture/merge_pcap.sh" >> mycron     #schedule the merge script

#install new cron file
 crontab mycron
rm mycron

The script runs, and add the two lines to the crontab. But if I run the script again, it adds those lines again , thus I will have four lines saying the same stuff. I want the install script to run such that, the lines inserted to the crontab do not repeat. How can I do that

Answer 1

I would recommend using /etc/cron.d over crontab.

You can place files in /etc/cron.d which behave like crontab entries. Though the format is slightly different.

For example
/etc/cron.d/pcap:

*/05 * * * * root bash /mnt/md0/capture/delete_old_pcap.sh
*/12 * * * * root bash  /mnt/md0/capture/merge_pcap.sh

The difference in the format is adding the user to run the job as after the time specification.

Now you can simply check if the file exists, and if you overwrite it, it doesn't matter.

 

Note that it's possible your cron daemon might not have /etc/cron.d. I do not know which cron daemons have it, but vixie cron is the the standard cron daemon on linux, and it does.

Answer 2

You could instead declare a function:

add() {
  grep -Fq "$1" mycron || echo "$1" >> mycron
}

and invoke it by saying:

add "*/05 * * * * bash /mnt/md0/capture/delete_old_pcap.sh"

This would append the line only if doesn't exist in the file.

Answer 3

from bash I use

    crontab -l | { cat; echo "*/10 * * * * /script/script.sh > /dev/null 2>&1"; } | crontab -

Also use this script to add cron entries on remote servers

    cronok="##";
    cronok+=`ssh $host 'crontab -l'`;
    pattern="reboot.sh"

    if [[ "$cronok" == *${pattern}* ]]; then
         echo "found cron  at [$host]"
      else
        echo "Cron at [$host] not found adding now"
       `ssh $host 'crontab -l | { cat; echo "*/10 * * * * /root/reboot.sh > /dev/null 2>&1"; } | crontab -'`
       echo "finished cron"
    fi

Answer 4

You can write a script and add it to crontab to stop it:

####You should calculate tiem you want kill your script.
#######code of calculating........
pid=`ps ax |egrep myscript |awk {'print $1'}`
kill -9 $pid
#OR
kill $pid

Answer 5

For those who don't have /etc/cron.d, guard comment is your friend, it's simple and easy to implement:

#!/bin/bash

guard_comment='# == my custom scheduler =='

sudo crontab -l >mycron

grep -qF "$guard_comment" mycron && exit 0

# echo new cron into cron file
echo "$guard_comment" >>mycron
echo "*/05 * * * * bash /mnt/md0/capture/delete_old_pcap.sh" >>mycron #schedule the delete script
echo "*/12 * * * * bash  /mnt/md0/capture/merge_pcap.sh" >>mycron     #schedule the merge script

# install new cron file
crontab mycron
rm mycron