How to get a unix script to run every 15 seconds? - unix

Question

I've seen a few solutions, including watch and simply running a looping (and sleeping) script in the background, but nothing has been ideal.
I have a script that needs to run every 15 seconds, and since cron won't support seconds, I'm left to figuring out something else.
What's the most robust and efficient way to run a script every 15 seconds on unix? The script needs to also run after a reboot.

Answer 1

If you insist of running your script from cron:
* * * * * /foo/bar/your_script
* * * * * sleep 15; /foo/bar/your_script
* * * * * sleep 30; /foo/bar/your_script
* * * * * sleep 45; /foo/bar/your_script
and replace your script name&path to /foo/bar/your_script

Answer 2

I would use cron to run a script every minute, and make that script run your script four times with a 15-second sleep between runs.
(That assumes your script is quick to run - you could adjust the sleep times if not.)
That way, you get all the benefits of cron as well as your 15 second run period.
Edit: See also #bmb's comment below.

Answer 3

Modified version of the above:
mkdir /etc/cron.15sec
mkdir /etc/cron.minute
mkdir /etc/cron.5minute
add to /etc/crontab:
* * * * * root run-parts /etc/cron.15sec > /dev/null 2> /dev/null
* * * * * root sleep 15; run-parts /etc/cron.15sec > /dev/null 2> /dev/null
* * * * * root sleep 30; run-parts /etc/cron.15sec > /dev/null 2> /dev/null
* * * * * root sleep 45; run-parts /etc/cron.15sec > /dev/null 2> /dev/null
* * * * * root run-parts /etc/cron.minute > /dev/null 2> /dev/null
*/5 * * * * root run-parts /etc/cron.5minute > /dev/null 2> /dev/null

Answer 4

Won't running this in the background do it?
#!/bin/sh
while [ 1 ]; do
echo "Hell yeah!" &
sleep 15
done
This is about as efficient as it gets. The important part only gets executed every 15 seconds and the script sleeps the rest of the time (thus not wasting cycles).

Answer 5

I wrote a scheduler faster than cron. I have also implemented an overlapping guard. You can configure the scheduler to not start new process if previous one is still running. Take a look at https://github.com/sioux1977/scheduler/wiki

Answer 6

Use nanosleep(2). It uses structure timespec that is used to specify intervals of time with nanosecond precision.
struct timespec {
time_t tv_sec; /* seconds */
long tv_nsec; /* nanoseconds */
};

Answer 7

#! /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
# 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

Answer 8

Since my previous answer I came up with another solution that is different and perhaps better. This code allows processes to be run more than 60 times a minute with microsecond precision. You need the usleep program to make this work. Should be good to up to 50 times a second.
#! /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

Answer 9

To avoid possible overlapping of execution, use a locking mechanism as described in that thread.