How to run a Linux script every few seconds under cron

From Computer Tyme Support Wiki

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

 #! /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 prameter 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 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
 
 if ! [ -d $dir ]
 then
    exit
 fi
 
 # Exit if directory has no files
 
 if ! [ "$(ls -A $dir/)" ]
 then
    exit
 fi
 
 # Sleep if both $delay and $counter are set
 
 if ! [ -z $delay ]
 then
    if ! [ -z $counter ]
    then
       sleep $delay
    fi
 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 [ $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

Then you create a script that you run every minute. Here's an example called 00-minute

/usr/local/sbin/run-parallel /etc/cron.2sec 2 &
/usr/local/sbin/run-parallel /etc/cron.3sec 3 &
/usr/local/sbin/run-parallel /etc/cron.4sec 4 &
/usr/local/sbin/run-parallel /etc/cron.5sec 5 &
/usr/local/sbin/run-parallel /etc/cron.6sec 6 &
/usr/local/sbin/run-parallel /etc/cron.10sec 10 &
/usr/local/sbin/run-parallel /etc/cron.12sec 12 &
/usr/local/sbin/run-parallel /etc/cron.15sec 15 &
/usr/local/sbin/run-parallel /etc/cron.20sec 20 &
/usr/local/sbin/run-parallel /etc/cron.30sec 30 &

You could eliminate the lines of times you aren't going to use but it doesn't matter because if there's no directory or nothing to run in the directory then it just skips that almost instantly. Be sure to create the directories for the time periods you want to use.

To run 00-minute every minute you can edit your /etc/crontab file and add:

* * * * * root /usr/local/sbin/00-minute