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.
Related
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 have a shell script that calls five other scripts from it. The first script creates 50 qsub jobs in the cluster. Individual job execution time varies from a couple of minutes to an hour. I need to know when all the 50 jobs get finished because after completing all the jobs I need to run the second script. How to find whether all the qsub jobs are completed or not? One possible solution can be using an infinite loop and check job status by using qstate command with job ID. In this case, I need to check the job status continuously. It is not an excellent solution. Is it possible that after execution, qsub job will notify me by itself. Hence, I don't need to monitor frequently job status.
qsub is capable of handling job dependencies, using -W depend=afterok:jobid.
e.g.
#!/bin/bash
# commands to run on the cluster
COMMANDS="script1.sh script2.sh script3.sh"
# intiliaze JOBID variable
JOBIDS=""
# queue all commands
for CMD in $COMMANDS; do
# queue command and store the job id
JOBIDS="$JOBIDS:`qsub $CMD`"
done
# queue post processing, depended on the submitted jobs
qsub -W depend=afterok:$JOBIDS postprocessing.sh
exit 0
More examples can be found here http://beige.ucs.indiana.edu/I590/node45.html
I never heard about how to do that, and I would be really interested if someone came with a good answer.
In the meanwhile, I suggest that you use file tricks. Either your script outputs a file at the end, or you check for the existence of the log files (assuming they are created only at the end).
while [ ! -e ~/logs/myscript.log-1 ]; do
sleep 30;
done
I have a Cron Job scheduled to execute a command a few times a day. There are cases where the cron job isn't needed but will automatically run. If that happens the following error message shows:
PM2 [ERROR] Script already launched, add -f option to force re execution
Note: The Cron Job runs PM2 in reference to a script.
Is there any negative effect to having the cron job run even if the script is already running?
Please provided detailed information or references. Not just your opinion please.
Avoid erroneous error messages by writing a wrapper script that is run from cron instead. Inside the wrapper script, only run your job if it is not already running by querying the process table.
Assuming ksh, here's a snippet (I'm a tad rusty so syntax may need to be tweaked):
# Running will be non-zero if no match found
running=$(ps|grep MY_PROGRAM)
if [[ "$running" -gt 0 ]]; then
# run your program
else
# log its already running
fi
Not sure what detailed information or references there could be for a situation like this. It's not like someone commissions a study to look at this.
Assuming your command is intelligent enough to only allow one execution at a time (which appears to be the case judging by the error message you posted) then the only ill effect is a few CPU clock cycles (I think).
I have a script that runs unknown period of time that depends on its input. It can run one hour when little data available, or it can run for 8 hours if much data is to be processed.
I need to run it periodically, particularly 2 hours after previous run was completed.
Is there an utility to do that?
Use 'at' instead of 'cron' and at the end of your script add:
at now +2 hours $*
This means that each occurrence is chained - so if it terminates abnormally the next instance won't be scheduled - but I don't think there's a more robust solution without adding a lot of code/complexity.
I don't like the at solution proposed, so here another solution:
Use cron to launch your every two hours
Upon startup, your application(*) checks if there's a pidfile.
2.1 if it is present, then there may be another instance running: read contents of the file (pid) and see if that pid is the pid of an existing process, a zombie process or something else. If it is the pid of a running, existing process, then exit. If it is the pid of a zombie process then the previous job ended unexpectedly and then you have to delete the pidfile and go to step 3. Otherwise.
After deleting pidfile, you create a new one and put your pid into it. Then proceed to do your job.
*: In order not to add complexity, this application i cited could also be a simple wrapper that spawns your code using exec.
This solution can also be scripted quite easily.
Hope it helps,
SnoopyBBT
If it looks complicated, here is another, dirtier solution:
while true ; do
./your_application
sleep 7200
done
Hope this helps,
SnoopyBBT
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/