Hello,

OK, here's a quick hacked-together version of a script that more-or-less does what crond would do. As Neo has mentioned however, this is not the best idea, as you're kind of needlessly re-inventing something that already exists and which will definitely do a far better job than this shell script. If the customer/client/maintainer/whoever actively doesn't want you running scheduled tasks on the server, then they're not going to be happy about you doing it no matter how you go about doing it. Alternatively if you have a hard requirement to run scheduled tasks and the customer just doesn't want you installing software, then you need to have a conversation with your customer explaining that they need to install crond to enable you to do the work that you've been asked to do, otherwise you can't do it. If the job you need to do has a 100% requirement for scheduled tasks, then whoever provides this system either has to install or let you install crond, or they need to accept that the job can't be done since they won't let you have the tools you need to do it.

Anyway - bearing in mind all the above caveats (and the potential no doubt for bugs and issues that could be lurking with this approach), here's a quickly knocked-together script that would do more or less what you need.

Code:

$ cat cron.tab
Fri,16,13,echo "It's thirteen minutes past four !"
Fri,16,15,echo "It's quarter past four !"
$ cat script3.sh
#!/bin/bash

crontab=/home/unixforum/282472/cron.tab

while true
do
        while read cron
        do
                runday=`echo "$cron" | /usr/bin/awk -F, '{print $1}'`
                runhour=`echo "$cron" | /usr/bin/awk -F, '{print $2}'`
                runminute=`echo "$cron" | /usr/bin/awk -F, '{print $3}'`
                runcommand=`echo "$cron" | /usr/bin/awk -F, '{print $4}'`

                nowday=`/usr/bin/date +%a`
                nowhour=`/usr/bin/date +%H`
                nowminute=`/usr/bin/date +%M`

                if [ "$runday" == "$nowday" ] && [ "$runhour" == "$nowhour" ] && [ "$runminute" == "$nowminute" ]
                then
                        /usr/bin/date
                        $runcommand
                fi
        done < "$crontab"

        /usr/bin/sleep 60
done

$ date
Fri Aug  9 16:11:14 BST 2019
$ ./script3.sh
Fri Aug  9 16:13:16 BST 2019
"It's thirteen minutes past four !"
Fri Aug  9 16:15:16 BST 2019
"It's quarter past four !"
^C
$