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


 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


5 Answers 5


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

*/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.

  • This is not a solution if the user does not have root priv. If I am a user with no root priv, I need to use crontab -e, to edit my /var/spool/cron rules.
    – Dave
    Mar 14, 2022 at 18:27
  • @Dave you're asking a different question. This answer perfectly addresses the question that's been asked. Feel free to ask your own - and reference this question if you think it would provide context Mar 14, 2022 at 23:50
  • Aye. The original question uses sudo, which implies root privileges.
    – phemmer
    Mar 15, 2022 at 0:01

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.

  • can you please tell what grep -Fq "$1" does? Feb 27, 2014 at 12:35
  • And the script give error grep: input file `mycron' is also the output Feb 27, 2014 at 12:43
  • @JishnuUNair -F would make grep interpret the pattern as fixed string.
    – devnull
    Feb 27, 2014 at 12:50
  • @JishnuUNair Are you sure that you copied the above text correctly?
    – devnull
    Feb 27, 2014 at 12:50
  • yes, I have copied it correctly. Feb 27, 2014 at 12:59

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+=`ssh $host 'crontab -l'`;

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

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
kill $pid

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


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

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