Overview

Did you ever want to run a program every few seconds under a linux, unix, bsd or osx cron script? Here's an elegant script that does just that. You can get 1,2,3,4,5,6,10,12,15,20,30 second resolution. Just run it once a minute under crond and it just works.

Features:

Run once per minute under cron
Launches multiple programs in parallel
Multiple time periods supported simultaneously
Use of timeout kills programs that don't terminate within delay time
It just works
Simple, elegant, free

I'm now posting two versions of the program. The first version is simpler and is good for running as many as 30 times a minute. There are 2 issues however. crond as it turns out isn't real precise and it starts programs 1 to 1.5 seconds after the minute. Also - since the program call itself over and over during the minute there is some drift. There's enough drift so you can't really use it to run something every second. But every 2 seconds or greater is fine.

Simple Example

 #! /bin/sh
 
 # Run all programs in a directory in parallel
 # Usage: run-parallel directory delay
 # Copyright 2013 by Marc Perkel
 # docs at http://wiki.junkemailfilter.com/index.php/How_to_run_a_Linux_script_every_few_seconds_under_cron"
 # Free to use with attribution
 
 if [ $# -eq 0 ]
 then
    echo
    echo "run-parallel by Marc Perkel"
    echo
    echo "This program is used to run all programs in a directory in parallel" 
    echo "or to rerun them every X seconds for one minute."
    echo "Think of this program as cron with seconds resolution."
    echo
    echo "Usage: run-parallel [directory] [delay]"
    echo
    echo "Examples:"
    echo "   run-parallel /etc/cron.20sec 20"
    echo "   run-parallel 20"
    echo "   # Runs all executable files in /etc/cron.20sec every 20 seconds or 3 times a minute."
    echo 
    echo "If delay parameter is missing it runs everything once and exits."
    echo "If only delay is passed then the directory /etc/cron.[delay]sec is assumed."
    echo
    echo 'If "cronsec" is passed then it runs all of these delays 2 3 4 5 6 10 12 15 20 30'
    echo "resulting in 30 20 15 12 10 6 5 4 3 2 executions per minute." 
    echo
    exit
 fi
 
 # If "cronsec" is passed as a parameter then run all the delays in parallel
 
 if [ $1 = cronsec ]
 then
    for interval in 2 3 4 5 6 10 12 15 20 30
    do
       $0 $interval &
    done
    exit
 fi
 
 # Set the directory to first parameter and delay to second parameter
 
 dir=$1
 delay=$2
 
 # If only parameter is 2,3,4,5,6,10,12,15,20,30 then automatically calculate 
 # the standard directory name /etc/cron.[delay]sec
 
 if [[ "$1" =~ ^(2|3|4|5|6|10|12|15|20|30)$ ]]
 then
    dir="/etc/cron.$1sec"
    delay=$1
 fi
 
 # Exit if directory doesn't exist or has no files
 
 if [ ! "$(ls -A $dir/ 2> /dev/null)" ]
 then
    exit
 fi
  
 # Sleep if $counter is set
 
 if [ ! -z $counter ]
 then
    sleep $delay
 fi
 
 # Run all the programs in the directory in parallel
 # Use of timeout ensures that the processes are killed if they run too long
 
 for program in $dir/* ; do
    if [ -x $program ] 
    then
       if [ 0$delay -gt 0 ] 
       then
          timeout $delay $program &> /dev/null &
       else
          $program &> /dev/null &
       fi
    fi
 done
 
 # If delay not set then we're done
 
 if [ -z $delay ]
 then
    exit
 fi
 
 # Add delay to counter
 
 counter=$(( $counter + $delay ))
 
 # If minute is not up - call self recursively
 
 if [ $counter -lt 60 ]
 then
    . $0 $dir $delay &
 fi
 
 # Otherwise we're done

You can then create the directories needed to put your programs in that you want to run. You don't have to create the directories that you aren't going to use.

mkdir /etc/cron.2sec
mkdir /etc/cron.3sec
mkdir /etc/cron.4sec
mkdir /etc/cron.5sec
mkdir /etc/cron.6sec
mkdir /etc/cron.10sec
mkdir /etc/cron.12sec
mkdir /etc/cron.15sec
mkdir /etc/cron.20sec
mkdir /etc/cron.30sec

To run every minute you can edit your /etc/crontab file and add:

* * * * * root /usr/local/sbin/run-parallel cronsec

Keep in mind that the programs that you are trying need to be written to complete in the allotted time or they will be killed before completing. Also keep in mind that on a heavily loaded system that on smaller intervals there could be some time drift over the 1 minute period with unknown results. This is especially true of the 1 second interval.

Have a comment? You can email me at support@junkemailfilter.com

More Precise Implementation

Maybe I'm too picky but I want it to start right on the minute and have the intervals not be subject to drift/loading errors. So here is a slightly more complex version. The only drawback to this version is that it will take at least a full minute to start and maybe up to 2 minutes. That's because when it is launched it waits out the first minute to start processes right at the top of the minute instead of being late.

This implementation starts on time and has 1 second resolution. Accuracy on my servers seem to be about 15 ms or better.

#! /bin/sh

# Run all programs in a directory in parallel
# Usage: run-parallel directory delay
# Copyright 2013 by Marc Perkel
# docs at http://wiki.junkemailfilter.com/index.php/How_to_run_a_Linux_script_every_few_seconds_under_cron"
# Free to use with attribution

if [ $# -eq 0 ]
then
   echo
   echo "run-parallel by Marc Perkel"
   echo
   echo "This program is used to run all programs in a directory in parallel" 
   echo "or to rerun them every X seconds for one minute."
   echo "Think of this program as cron with seconds resolution."
   echo
   echo "Usage: run-parallel [directory] [delay]"
   echo
   echo "Examples:"
   echo "   run-parallel /etc/cron.20sec 20"
   echo "   run-parallel 20"
   echo "   # Runs all executable files in /etc/cron.20sec every 20 seconds or 3 times a minute."
   echo 
   echo "If delay parameter is missing it runs everything once and exits."
   echo "If only delay is passed then the directory /etc/cron.[delay]sec is assumed."
   echo
   echo 'If "cronsec" is passed then it runs all of these delays 1 2 3 4 5 6 10 12 15 20 30'
   echo "resulting in 60 30 20 15 12 10 6 5 4 3 2 executions per minute." 
   echo
   exit
fi

# Runs like the sleep command but compensates for drift to start on time

function syncsleep ()
{
   usleep $(( $1000000 - (10#$(date +%N) / 1000) ))
}

# If "cronsec" is passed as a parameter then run all the delays in parallel

if [ $1 = cronsec ]
then

   # Wait till start of next minute

   syncsleep $(( 60 - 10#$(date +%S) ))

   for interval in 1 2 3 4 5 6 10 12 15 20 30
   do
      $0 $interval &
   done

   exit
fi

# Set the directory to first parameter and delay to second parameter

dir=$1
delay=$2

# If only parameter is 1,2,3,4,5,6,10,12,15,20,30 then automatically calculate
# the standard directory name /etc/cron.[delay]sec

if [[ "$1" =~ ^(1|2|3|4|5|6|10|12|15|20|30)$ ]]
then
   dir="/etc/cron.$1sec"
   delay=$1
fi

# Exit if directory doesn't exist or has no files

if [ ! "$(ls -A $dir/ 2> /dev/null)" ]
then
   exit
fi

# Sleep if $counter is set

if [ ! -z $counter ]  
then
   syncsleep $delay
fi

# Run all the programs in the directory in parallel
# Use of timeout ensures that the processes are killed if they run too long

for program in $dir/* ; do
   if [ -x $program ] 
   then
      if [ 0$delay -gt 0 ] 
      then
         timeout $delay $program &> /dev/null &
      else
         $program &> /dev/null &
      fi
   fi
done

# If delay not set then we're done

if [ -z $delay ]
then
   exit
fi

# Add delay to counter

counter=$(( $counter + $delay ))

# If minute is not up - call self recursively

if [ $counter -lt 60 ]
then
   . $0 $dir $delay &
fi

# Otherwise we're done

How to run a Linux script every few seconds under cron

From Computer Tyme Support Wiki

Overview

Simple Example

More Precise Implementation

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