Can I run a cron job more frequently than every minute?

Question

Is it possible to run a cron job every 30 seconds without a sleep command?

Answer 1

If your task needs to run that frequently, cron is the wrong tool. Aside from the fact that it simply won't launch jobs that frequently, you also risk some serious problems if the job takes longer to run than the interval between launches. Rewrite your task to daemonize and run persistently, then launch it from cron if necessary (while making sure that it won't relaunch if it's already running).

Answer 2

Candidate for the most creative misuse of a Linux command:

nohup watch -n 30 --precise yourprog >/dev/null &

If yourprog consists of:

date +%M.%S.%N >> yourprog.out

then yourprog.out might look like:

50.51.857291267
51.21.840818353
51.51.840910204
52.21.840513307
52.51.842455224
53.21.841195858
53.51.841407587
54.21.840629676

indicating a pretty good level of precision.

Here is an explanation of the parts of the command:

• nohup - This keeps the command that follows it, watch in this case, from exiting when the terminal exits.
• watch - This program runs a command repeatedly. Normally the first screenful of output from the command is displayed each time watch runs the command.
• -n 30 - The interval at which to run the command. In this case it's every thirty seconds.
• --precise - Without this option, watch runs the command after interval seconds. With it, each start of the command begins on the interval if possible. If this option were not specified in the example, the times would get later and later by more than 30 seconds each time due to the time it takes to launch and execute the command (yourprog).
• yourprog - The program or command line for watch to execute. If the command line contains characters special to the shell (e.g. space or semicolon) it will need to be quoted.
• >/dev/null - The greater-than redirects the output of the command being run by watch to a file, /dev/null. That file discards any data written to it. This prevents the output from being written to the screen or, since nohup is being used, it prevents output from being sent to a file called nohup.out.
• & - The watch command is run in the background and control is returned to the terminal or parent process.

Note that nohup, the redirection of output and the & background control operator are not specific to watch.

Here is an explanation of the example yourprog script:

• date - Outputs the current date and/or time. It can also set them.
• +%M.%S.%N - This specifies the output format for date to use. %M is the current minute, %S is the current second and %N is the current nanosecond.
• >> yourprog.out - This redirects the output of the date command to a file called yourprog.out. The double greater-than causes the output to be appended to the file on each invocation rather than the previous contents being overwritten.

Edit:

Possibly another thing that could be abused (or perhaps it's a legitimate use) is systemd timers.

See systemd/Timers as a cron replacement and Cron vs systemd timers.

I'll try to post an example soon.

Answer 3

* * * * * /path/to/program
* * * * * sleep 30; /path/to/program

Don't forget to write something into your program so that it exits if a previous instance is already running.

#!/bin/sh

if ln -s "pid=$$" /var/pid/myscript.pid; then
  trap "rm /var/pid/myscript.pid" 0 1 2 3 15
else
  echo "Already running, or stale lockfile." >&2
  exit 1
fi

Of course, this still leaves a very small opportunity for failure, so search google for a better solution applicable to your environment.

Answer 4

Cron is designed to wake up at every minute, so it is not possible to do it without some hacking, for example sleep like you mentioned.

Answer 5

You can do this with third party software.

An option that has worked well for me is frequent-cron

It allows millisecond precision and it gives you the option to defer the next execution until the current one has exited..

Answer 6

my simplest and favorite solution for this task:

cron entry:
* * * * * flock -w0 /path/to/script /path/to/script

script:
while true;do echo doing something; sleep 10s;done

lazy alternative: :)

* * * * * flock -w0 /path/to/log watch -n 10 echo doing >> /path/to/log

or

* * * * * flock -w0 /path/to/log watch -n 10 /path/to/script

pros

• use of flock command avoids running the script by multiple instances in the same time. It can be very important in the most cases.
• flock and watch commands is available on most Linux installs

cons

• stopping of this kind of "Service" needs two to steps
  • comment out the cron entry
  • kill the script or the watch command

Answer 7

I'd have a couple of concerns:

(1) sometimes a system gets busy and cannot start things exactly on the 30 second point, it is then possible that at the same time you are running one job another job would pop and then you have 2 (or more) jobs doing the same thing. Depending on the script, there may be some significant interference here. Thus, coding in such a script should contain some code to insure that only one instance of the given scripting is running at the same time.

(2) The script could possibly have a lot of overhead, and consume more system resources than you might want. This is true if you are competing against a lot of other system activities.

Thus as one poster has put it, in this case I'd seriously consider putting in a daemon running with additional processes to ensure it remains running if its of critical importance to your operations.

Answer 8

Not an exact answer to your question, but I think it could be useful to some people anyway. If you want to run a command every second, you could do it like this. Each minute, it loops through the 60 seconds, runs a script in a sub-shell and sleeps for 1 second. You need to make sure, that you create a locking file (or a similar mechanism) and abort the script.pl accordingly (if you don't want to run it multiple times in parallel).

* * * * * for((i=0; i<60; i++)); do /usr/bin/perl script.pl & sleep 1; done;

Answer 9

One simple strategy for Linux OSes is to make a SystemD service, e.g. /etc/systemd/system/myservice.service:

[Install]
WantedBy=multi-user.target

[Unit]
Description=Bmon
StartLimitIntervalSec=30
StartLimitBurst=2

[Service]
ExecStart=/usr/bin/bash /home/janie/myservice.sh
Restart=always

Then in myservice.sh:

#!/usr/bin/bash
while true
do
  /home/janie/run_every_10_sec
  sleep 10
done

This will accomplish running the command /home/janie/run_every_10_sec every 10 seconds.

To enable myservice.service, run sudo systemctl enable myservice.

Answer 10

A solution, if it's for your own script or if you can wrap it:

1. Get and remember start time.
2. If a lock file, which you will be touching later, is present and script has not been running for 60 seconds, wait a second and check again. (e.g. while/sleep)*
3. If lock file is still present after 60 seconds have elapsed, exit with a stale lock warning.
4. Touch lock file.
5. While script has not been running for 60 seconds, loop your actual task with the desired sleep duration.
6. Delete lock file.
7. Add as minutely cron.
8. Bob's your uncle.

Less of a headache than building and monitoring a daemon.

*If you're using PHP, remember clearstatcache().