Suppose if i have cron tasks running every minute. And if each time, that task takes more than one minute to run, what will happen. Will the next cron wait for the first cron or will it run without any checks.
I want to run a cron task every minute and I don't over lapping cron tasks like that in case of a long running task/situation.
please help.
It depends on what you run. If it's your own script, you can implement a locking/lock checking mechanism to avoid running duplicates.
But that's not cron's job.
Yes, cron will go ahead and start your 1+ minute-running process every minute until something crashes.
You'll want to put a lock of some sort into your job if you can to basically do this at start-up:
if not get_lock()
print "Another process is running"
exit
This, of course, assumes that you own the code running. If you're running a command that you didn't code, then I'd recommend building a shell wrapper that implements the above pseudocoded logic where get_lock() will see if another process like this one is running.
As others have mentioned, CRON will run your script every minute regardless of whether another instance of your script is still running.
If you want to avoid this and don't fancy implementing your own locking mechanism then you could try using a CRON alternative called The Fat Controller which is a daemon that will continually re-run scripts. You can optionally specify an interval between runs and also optionally specify a maximum execution time so if a script goes AWOL then it can be killed.
There's some use cases and more information on the website:
http://fat-controller.sourceforge.net/
Related
I have a background task that needs to be run repeatedly, every hour or so, sending me an email whenever the task emitted non-trivial output.
I'm currently using cron for that, but it's somewhat ill-suited: it forces me to choose exact times at which the command is run, and it doesn't prevent overlap.
An alternative would be to run the script in a loop with sleep 3600 at the end of each iteration but this then needs extra work to make sure the script is always restarted after boot and such.
Ideally, I'd like a cron-like tool where I can give a set of commands to run repeatedly with approximate execution rates and the tool will run them "when convenient" and without overlapping execution of different iterations of a command (or even without overlapping execution of any command).
Short of writing such a tool myself, what would be the recommended approach?
I have an uploader service which needs to run every 5minutes and it definitely finished within 5 minutes so there are never two parallel session.
Wondering what would be a good strategy to run this, either to schedule this as a cron job on host or start a go program with infinite loop which execute the program and sleeps(Golang: Implementing a cron / executing tasks at a specific time)
If your task is...
On Unix
Stand alone
Periodic
Has an acceptable startup time
cron will be better than rolling your own scheduler just for the one service. It will guarantee the process will always run at the correct time and has rudimentary error reporting. There's no need to add a watchdog in case your infinite loop has an error, cron will run the process again in 5 minutes.
If cron is insufficient, look into other job schedulers before rolling your own.
I have an uploader service which needs to run every 5minutes and it definitely finished within 5 minutes so there are never two parallel session.
These are famous last words. I would suggest adding in some form of locking. For example, write your PID to a file in /var/run and check if that process is running. There's even a little pidfile library for Go.
Take a look on Systemd, you can execute a script with timers and set max execution time for the script.
https://wiki.archlinux.org/index.php/Systemd/Timers
I can't seem to get my script to run in parallel every minute via cron on Ubuntu 14.
I have created a cron job which executes every minute. The cron job executes a script that runs much longer than a minute. When a minute expires it seems the new cron execution overwrites the previous execution. Is this correct? Any ideas welcomed.
I need concurrent independent running jobs. The cron job runs a script which queries a mysql database. The idea is to poll a db- if yes execute script in its own process.
cron will not stop a previous execution of a process to start a new one. cron will simply kick off the new process even though the old process is still running.
If you need cron to terminate the previous process, you'll need to modify your script to handle that itself.
You need a locking mechanism to identify that the script is already running.
There are several ways of doing this but you need to be careful to use an atomic method.
I use lock directories as creating a directory is guaranteed to be atomic -
LOCKDIR=/tmp/myproc.lock
if ! mkdir $LOCKDIR >/dev/null 2>&1
then
print -u2 "Processing already running - terminating"
exit 1
fi
trap "rm -rf $LOCKDIR" EXIT
This is a common occurrence. Try adding a check in your script to see if a lockfile already exists. If it does, exit. If not, continue.
Cronjobs are not overrun. They do however have the possibility of overlapping. Unless your script explicitly kills any pre-existing process, it shouldn't be able to stop the previously running script.
However, introducing the concept of lockfiles will save you from all these confusions altogether.
hey getting used to groovy and i wanted to have a loop such as a do while loop in my groovy script which is ran every hour or 2 for until a certain condition inside the loop is met (variable = something). So I found the sleep step but was wondering if it would be ok to sleep for such a long time. The sleep function will not mess up right?
The sleep function will not mess up. But that isn't your biggest problem.
If all your script is doing is sleeping, it would be better to have a scheduler like Cron launch your script. This way is simpler and more resilient, it reduces the opportunities for the script to be accumulating garbage, leaking memory, having its JVM get killed by another process, or otherwise just falling into a bad state from programming errors. Cron is solid and there is less that can go wrong that way. Starting up a JVM is not speedy but if your timeframe is in hours it shouldn't be a problem.
Another possible issue is that the time your script wakes up may drift. The OS scheduler is not obliged to wake your thread up at exactly the elapsed time. Also the time on the server could be changed while the script is running. Using Cron would make the time your script acts more predictable.
On the other hand, with the scheduler, if a process takes longer than the time to the next run, there is the chance that multiple instances of the process can exist concurrently. You might want to have the script create a lock file and remove it once it's done, checking to see if the file exists already to let it know if another instance is still running.
First of all there's not do {} while() construct in groovy. Secondly it's a better idea to use a scheduler e.g. QuartzScheduler to run a cron task.
I am confused whether to use cron job scheduler or use sleep function in the program itself. There are questions on this previously but I seem to have some different requirements form them.
I need some information from the previous run of the program so if I use cron to schedule
job I would have to store that information at some place and re-read it next time(this can make the program less scale-able if the size of this information grows).
I can also use sleep() but that will be using resources.
I will need to re-run the program every 10 mins or so. Which one is better to use.
Is there any other nice way of doing it which I may be missing.
In general you should use cron whenever you can for something like this.
The only problem I could foresee is if your program somehow took longer than 10 minutes to run, cron is going to call the next execution 10 minutes later anyway. This creates a really long race condition basically, where if you did sleep it would only start sleeping after the previous execution ended.
But assuming your program will take less time to run, I say go with cron.