Custom cron job

[Deleted member 89527]

If I'm right I see erros in the logs about the ? in this cron statement because it is "non standard"

What is the correct way to run something every 45 seconds?

crond[1618]: user admin: parse error at ?

0/45 0 0 ? * * *

 

[ColinTaylor]

Cron can't do that. Cron's smallest interval is 1 minute.

 

[Tech9]

Perhaps this may help:

Crontab.guru - The cron schedule expression generator

An easy to use editor for crontab schedules.

crontab.guru

 

[Ranger802004]

If I'm right I see erros in the logs about the ? in this cron statement because it is "non standard"

What is the correct way to run something every 45 seconds?

The lowest you can go is once every minute.
*/1 * * * *

 

[Deleted member 89527]

This (from Google) won't work either?

*/45 * * * * *

 

[Ranger802004]

This (from Google) won't work either?

That is going to run every 45th minute of every hour, not second.

 

[ColinTaylor]

This (from Google) won't work either?

You can't arbitrarily decide to insert additional fields in front of the others. It's syntactically invalid. Sounds like an answer ChatGPT would have invented.

 

[Tech9]

What is the correct way to run something every 45 seconds?

What do you want to run so often?

 

[Deleted member 89527]

What do you want to run so often?

NAT-PMP keep port open script.

 

[Martinski]

If I'm right I see erros in the logs about the ? in this cron statement because it is "non standard"

What is the correct way to run something every 45 seconds?

NAT-PMP keep port open script.

For such cases where you want to run some task/cmds with a frequency < 60 sec., you can write a shell script in which you set up a while loop that encloses the desired tasks/cmds followed by the interval delay. I have done this before when reading from a FIFO pipe that was set up by another process, and because the FIFO buffer is rather small I had to read it every 2 seconds or so.

Here's a rough draft of what the script would look like:

#!/bin/sh
####################################################################
# RunTaskEveryXseconds.sh
# Run some task/cmds every X seconds until told to exit.
####################################################################
set -u

# Set interval between runs #
intervalSecs=30

ScriptFName="${0##*/}"
LockDirPath="/tmp/var/${ScriptFName%.*}_d.LOCK"
ExitSemFile="/tmp/var/${ScriptFName%.*}_f.EXIT"

_ReleaseLock_()
{ rm -f "$ExitSemFile" ; rm -fr "$LockDirPath" ; }

_AcquireLock_()
{
   if mkdir "$LockDirPath" &>/dev/null
   then return 0 ; else exit 1 ; fi
}

trap "_ReleaseLock_; exit 0" HUP INT QUIT ABRT TERM
_AcquireLock_

echo "Entering while loop..."
while true
do
   echo "[$(date)]: Run some task/process every [$intervalSecs] sec."
   ...
   ## Run desired task/cmds here ##
   ...
   if [ -f "$ExitSemFile" ]
   then echo "Exiting while loop." ; break ; fi

   sleep "$intervalSecs"
done

_ReleaseLock_
exit 0

#EOF#

Then you can run the script in the background from one of the hook scripts (e.g. services-start) depending on the requirements or needs of the process or set of commands you're trying to run.

/jffs/scripts/RunTaskEveryXseconds.sh &

My 2 cents.