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Script that will look the same as Cron


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# 1  
Script that will look the same as Cron

Hi All,.

need your expertise, is there a way to create a script that will run with specific time without using cron. because i dont have access in cron.

TIA
# 2  
You can use loop to run on a specific time of day:
Code:
#!/bin/bash

while true;
do
    TIME=`date | cut -d' ' -f4`
    echo $TIME
    if [[ $TIME == "22:00:00" ]]
    then
            `your command`
            sleep 1s
    fi
done

This User Gave Thanks to Raghav_Singh For This Post:
# 3  
Hi,

If you definitely don't have access to the crontab for the user you want to run the script as, and if you can't get the person who has root on the box in question to set it up for you, then one approach would be to have a script that runs in an infinite loop until a target time arrives, and then runs whatever task it's meant to run at that time.

Here's an example of such a script, showing the code and me starting to run it roughly ten seconds before the appointed time:

Code:
$ cat script.sh 
#!/bin/bash

runtime=`/usr/bin/date -d '2019-08-09 14:22:00' +%s`

while true
do
        nowtime=`/usr/bin/date +%s`

        if [ "$runtime" == "$nowtime" ]
        then
                date
                echo "It's time to run"
                exit 0
        else
                date
                echo "It's not time to run, I'll keep waiting"
        fi

        /usr/bin/sleep 1
done

$ ./script.sh 
Fri Aug  9 14:21:50 BST 2019
It's not time to run, I'll keep waiting
Fri Aug  9 14:21:51 BST 2019
It's not time to run, I'll keep waiting
Fri Aug  9 14:21:52 BST 2019
It's not time to run, I'll keep waiting
Fri Aug  9 14:21:53 BST 2019
It's not time to run, I'll keep waiting
Fri Aug  9 14:21:54 BST 2019
It's not time to run, I'll keep waiting
Fri Aug  9 14:21:55 BST 2019
It's not time to run, I'll keep waiting
Fri Aug  9 14:21:56 BST 2019
It's not time to run, I'll keep waiting
Fri Aug  9 14:21:57 BST 2019
It's not time to run, I'll keep waiting
Fri Aug  9 14:21:58 BST 2019
It's not time to run, I'll keep waiting
Fri Aug  9 14:21:59 BST 2019
It's not time to run, I'll keep waiting
Fri Aug  9 14:22:00 BST 2019
It's time to run
$

So the basic idea is we define our target time in the runtime variable at the top, converting a human-readable date into the UNIX epoch (the number of seconds since the start of 1970). We then in an infinite loop check the current time, compare it to our target time, and do something specific if the two match. If the current time is not the target time, we wait one second, and the loop goes round again until the target time arrives.

Note that this isn't entirely ideal, and you'd need to run this via screen or tmux or something similar to ensure that the script didn't die when your terminal session did. But if you really definitely don't have access to an actual system-level task scheduler, this kind of approach will work in a pinch. You might also want to make the sleep command wait for less than one second, just to be sure you don't ever miss the target second you want your job to actually run, but that's the only other issue that immediately comes to mind.

Hope this helps !
# 4  
woahh.. its possible.. how about if with specific day?
also is it possible to put * * * like the same function in cron?
# 5  
Hi,

Using the approach from my script, you would just tweak the runtime variable accordingly. For instance, in the provided example:

Code:
runtime=`/usr/bin/date -d '2019-08-09 14:22:00' +%s`

the target time is very specifically 14:22:00 on the 9th of August 2019. If you wanted, for instance, to make your code run on 25th December 2020 at midnight, you'd just change this to:

Code:
runtime=`/usr/bin/date -d '2020-12-25 00:00:00' +%s`

and you'd be all set.

--- Post updated at 03:51 PM ---

Hi,

Alternatively, if you wanted a more generic and less specific solution - e.g. a "run this at 14:50 every Friday" kind of situation, without worrying about an actual specific date in the year or specific second - you could do something along these lines:

Code:
#!/bin/bash

runday="Fri"
runhour="14"
runminute="50"

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

        if [ "$runday" == "$nowday" ] && [ "$runhour" == "$nowhour" ] && [ "$runminute" == "$nowminute" ]
        then
                date
                echo "It's time to run"
                exit 0
        else
                date
                echo "It's not time to run, I'll keep waiting"
        fi

        /usr/bin/sleep 60
done

This User Gave Thanks to drysdalk For This Post:
# 7  
Hi,

In that case, you'd really be into the territory of literally re-inventing crontab. For example, you could define an input file format that consisted of runtimes and their associated commands, and have your script read that file and run the commands when the runtimes arrived - in other words, to do pretty much exactly what the crond daemon does.

I've not got time right now to write a full example unfortunately, but if you defined your input format as something like:

Code:
Mon,10,00,/usr/local/bin/foo.sh
Fri,17,00,/usr/local/bin/bar.sh

and had your script read this file line-by-line, breaking out the variables in each line into the days, hour, minutes and commands, you could then run those commands via the same kind of infinite-loop approach as you've seen in the examples given thus far.

If you absolutely don't have access to crontab, and can't get the maintainer of the server to put in the cron entries for you, then this would more or less be the only way you'd get a general-purpose task scheduler going. But you'd probably want to be sure that the maintainer of the server was happy with you doing this, since presumably they've blocked access to cron to prevent people running commands on a set schedule, which is what you'd end up doing anyway here.
This User Gave Thanks to drysdalk For This Post:
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