Is it possible to run a cron job every 30 seconds without a sleep command?
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).
Candidate for the most creative misuse of a Linux command:
nohup watch -n 30 --precise yourprog >/dev/null &
yourprog consists of:
date +%M.%S.%N >> yourprog.out
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,
watchin 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
watchruns the command.
-n 30- The interval at which to run the command. In this case it's every thirty seconds.
--precise- Without this option,
watchruns 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- The program or command line for
watchto 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
watchto a file,
/dev/null. That file discards any data written to it. This prevents the output from being written to the screen or, since
nohupis being used, it prevents output from being sent to a file called
watchcommand is run in the background and control is returned to the terminal or parent process.
nohup, the redirection of output and the
& background control operator are not specific to
Here is an explanation of the example
date- Outputs the current date and/or time. It can also set them.
+%M.%S.%N- This specifies the output format for
%Mis the current minute,
%Sis the current second and
%Nis the current nanosecond.
>> yourprog.out- This redirects the output of the
datecommand 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.
Possibly another thing that could be abused (or perhaps it's a legitimate use) is systemd timers.
I'll try to post an example soon.
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.
* * * * * /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.
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..
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.
my simplest and favorite solution for this task:
* * * * * flock -w0 /path/to/script /path/to/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
* * * * * flock -w0 /path/to/log watch -n 10 /path/to/script
- use of
flockcommand avoids running the script by multiple instances in the same time. It can be very important in the most cases.
watchcommands is available on most Linux installs
- stopping of this kind of "Service" needs two to steps
- comment out the cron entry
- kill the script or the
A solution, if it's for your own script or if you can wrap it:
- Get and remember start time.
- 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)*
- If lock file is still present after 60 seconds have elapsed, exit with a stale lock warning.
- Touch lock file.
- While script has not been running for 60 seconds, loop your actual task with the desired sleep duration.
- Delete lock file.
- Add as minutely cron.
- Bob's your uncle.
Less of a headache than building and monitoring a daemon.
*If you're using PHP, remember clearstatcache().