How to run a Linux script every few seconds under cron

From Computer Tyme Support Wiki

Revision as of 16:29, 11 January 2014

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 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

 #! /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
    $0 2 &
    $0 3 &
    $0 4 &
    $0 5 &
    $0 6 &
    $0 10 &
    $0 12 &
    $0 15 &
    $0 20 &
    $0 30 &
    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/)" ]
 then
    exit
 fi
  
 # Sleep if both $delay and $counter are set
 
 if [ ! -z $delay ] && [ ! -z $counter ]
 then
    sleep $delay
 fi
 
 # Set counter to 0 if not set
 
 if [ -z $counter ]
 then
    counter=0
 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 1 ] 
       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