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ady6831983
Asked: 2018-10-02 08:12:10 +0800 CST

how to write into cronjob from a shell script

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I would like to automate cronjob (meaning schedule a job to be done from a shell script). So i have script that does this

#!/bin/ksh
timestamp=$(date +%H%M%S)
if [ "$timestamp" -eq 230000 ]; then
 write to cronjob to execute the same script @ 11:00 PM
fi

so the output should be like below, is this possible?

00 23 2 10 2 /home/test/run_cron.sh
command-line bash cron ksh

3 Answers

  1. I assume you only want to be added to crontab ONCE. Therefore one must remove the script from the crontab first: 

    #!/bin/ksh
myname="$0"  

(crontab -l | grep -E -v "$myname";echo " 0 23 \* \* \* $myname") |\
    crontab -
    • 2

  2. Hello and welcome to Ask Ubuntu!

    You can write to crontab via stdin:

    echo  '0 23 * * * /home/test/run_cron.sh' | crontab -

    The downside is that this clears all the previous crontab entries.

    If you must keep already present entries (and not start from scratch), something like this should work:

    echo "$(crontab -l ; echo  '0 23 * * * /home/test/run_cron.sh')" | crontab -
    • 1

  3. From the comments and your updated question I read you want to run your script not every day at 11pm but only once the next day at 11pm. While this is possible with cron I suggest using an at job instead.

    When adding a cronjob for this, we would need to calculate the next day's date (to setup something like 0 23 2 10 * … for Oct 2) and then again remove that cronjob the next day. As said: possible but cumbersome. Hence, an at job. You may need to install the at command via sudo apt install at.

    The syntax of at is a bit surprising: the command to be run is not given as a parameter but read from stdin, so we need to pipe it into at. Some examples:

    echo "ls -l > out1.txt" | at now + 2 minutes
echo "ls -l > out2.txt" | at 23:00
echo "ls -l > out3.txt" | at tomorrow

    In your case:

    echo "./test-script.sh" | at 23:00

    This will run test-script.sh the next time it is 11pm.

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