How to run a Linux script every few seconds under cron
From Computer Tyme Support Wiki
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.
Even though this software is free if you find it really useful and you want to reward/encourage me you can email me an Amazon Gift Certificate to marc@perkel.com.
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.
The usleep command is required for microsecond resolution sleep times.
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