How to run a Linux script every few seconds under cron
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Overview
Did you ever want to run a program every few seconds, or even fractions of a second, under a linux, unix, bsd or osx cron script? Here's an elegant script that does just that.
Features:
- Run once per minute under cron
- Launches multiple programs in parallel
- Multiple time periods supported simultaneously just by the directory name.
- It just works
- Simple, elegant, free
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.
Precise Cron with microsecond resolution
Tthe existence of the directory creates the schedule. This version supports microsecond resolution assuming your computer can handle it. It is the most accurate on start time and actually us a more simple implementation. Also the number of executions per minute does not have to to evenly divisible by 60. If you want to run something 17 times a minute - no problem. You can also run faster than once a second.
Even though this has microsecond resolution probably 2400 per minute (40/sec) is as fast as you can practically go.
Keep in mind that this program takes up to 2 minutes to start.
#! /bin/sh # Microsecond Cron # Usage: cron-ms start # Copyright 2014 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 basedir=/etc/cron-ms if [ $# -eq 0 ] then echo echo "cron-ms by Marc Perkel" echo echo "This program is used to run all programs in a directory in parallel every X times per minute." echo "Think of this program as cron with microseconds resolution." echo echo "Usage: cron-ms start" echo echo "The scheduling is done by creating directories with the number of" echo "executions per minute as part of the directory name." echo echo "Examples:" echo " /etc/cron-ms/7 # Executes everything in that directory 7 times a minute" echo " /etc/cron-ms/30 # Executes everything in that directory 30 times a minute" echo " /etc/cron-ms/600 # Executes everything in that directory 10 times a second" echo " /etc/cron-ms/2400 # Executes everything in that directory 40 times a second" echo exit fi # If "start" is passed as a parameter then run all the loops in parallel # The number of the directory is the number of executions per minute # Since cron isn't accurate we need to start at top of next minute if [ $1 = start ] then for dir in $basedir/* ; do $0 ${dir##*/} 60000000 & done exit fi # Loops per minute and the next interval are passed on the command line with each loop loops=$1 next_interval=$2 # Sleeps until a specific part of a minute with microsecond resolution. 60000000 is full minute usleep $(( $next_interval - 10#$(date +%S%N) / 1000 )) # Run all the programs in the directory in parallel for program in $basedir/$loops/* ; do if [ -x $program ] then $program &> /dev/null & fi done # Calculate next_interval next_interval=$(($next_interval % 60000000 + (60000000 / $loops) )) # If minute is not up - call self recursively if [ $next_interval -lt $(( 60000000 / $loops * $loops)) ] then . $0 $loops $next_interval & fi # Otherwise we're done
Kind of amazing that you can do all this in less than 50 lines of code.
Companion Program with 1 minute resolution
This is a companion program to run a program every X minutes. you can of course do this in crontab but this makes it easier under some circumstances. This program allows you to:
- Set the timing in number of minutes just be the directory name and put as many programs in that directory as you want to start.
- The number of minutes does not need to divide equally into 60.
- The number of minutes isn't limited to 60. You can run a program every 61 minutes or every 923 minutes if you want.
Both of these programs can be started at the same time from a single crontab entry. (see below) One is for minutes and up. The other is for seconds and down. Between these two you have a very powerful time based execution extension to compliment what cron does.
#! /bin/sh # Minute Cron # Usage: cron-min start # Copyright 2014 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 # Run this script under Cron once a minute basedir=/etc/cron-min if [ $# -gt 0 ] then echo echo "cron-min by Marc Perkel" echo echo "This program is used to run all programs in a directory in parallel every X minutes." echo echo "Usage: cron-min" echo echo "The scheduling is done by creating directories with the number of minutes as part of the" echo "directory name. The minutes do not have to evenly divide into 60 or be less than 60." echo echo "Examples:" echo " /etc/cron-min/1 # Executes everything in that directory every 1 minute" echo " /etc/cron-min/5 # Executes everything in that directory every 5 minutes" echo " /etc/cron-min/13 # Executes everything in that directory every 13 minutes" echo " /etc/cron-min/75 # Executes everything in that directory every 75 minutes" echo exit fi for dir in $basedir/* ; do minutes=${dir##*/} if [ $(( ($(date +%s) / 60) % $minutes )) -eq 0 ] then for program in $basedir/$minutes/* ; do if [ -x $program ] then $program &> /dev/null & fi done fi done
Launching Both Programs with Cron
To launch both of the above programs from cron every minute edit your /etc/crontab file and add:
* * * * * root /usr/local/sbin/cron-min ; /usr/local/sbin/cron-ms start