Cronjob Activation - Shell Programming and Scripting

Raynon · #1 (**permalink**) 02-03-2008

Hi All,

I have set a cronjob to run a script at a frequency of 10 mins.
However, if the script takes more than 10 mins to complete, how will the cronjob behaves?
1) Does it ignore the current process and restart the whole process again?
2) Or does it stop to trigger the script again unless previous process has been completed?

Smiling Dragon · #2 (**permalink**) 02-03-2008

Quote:

Originally Posted by Raynon

Hi All,

I have set a cronjob to run a script at a frequency of 10 mins.
However, if the script takes more than 10 mins to complete, how will the cronjob behaves?
1) Does it ignore the current process and restart the whole process again?
2) Or does it stop to trigger the script again unless previous process has been completed?

Depends on your cron implementation but generally it will run another copy of the script 10 mins after the first one is started - regardless of how long the first run takes.

Solutions:

Make your script reentrant - ie write it such that it running multiple times at once isn't a problem.
Make your script aware of how many times it's currently running - ie create a lock file when it starts up and check for this every time you start.
Don't use cron, have the script run continuously and manage it's own scheduling - ie sleep 600
Don't use cron, get a better scheduler - depending on your enviornment, this could be something like anacron or control-m.

Raynon · #3 (**permalink**) 02-04-2008

Quote:

Originally Posted by Smiling Dragon

Depends on your cron implementation but generally it will run another copy of the script 10 mins after the first one is started - regardless of how long the first run takes.

Solutions:

Make your script reentrant - ie write it such that it running multiple times at once isn't a problem.
Make your script aware of how many times it's currently running - ie create a lock file when it starts up and check for this every time you start.
Don't use cron, have the script run continuously and manage it's own scheduling - ie sleep 600
Don't use cron, get a better scheduler - depending on your enviornment, this could be something like anacron or control-m.

Hi,

Running the script continously means to say that i have to leave a command prompt window always open. Once closed, the process will be ended, and also another disadvantage is that a infinite looping script will eat up alot of resources, wouldn;t it ?
So i still think cron is the more practical one. But i have no idea about anacron or control-m. Can you enlightened me on that ?

Can you give an example of script being re-entrant ?
I was thinking if i could make the script check for the process to see if the previous process have been completed, can any experts give me some examples of this ? I am using csh by the way.

Smiling Dragon · #4 (**permalink**) 02-06-2008

Quote:

Originally Posted by Raynon

Running the script continously means to say that i have to leave a command prompt window always open. Once closed, the process will be ended

Not at all, just move it to the background, no different to all the other daemonised stuff you are running on your system. If it likes to write to STDOUT, redirect this to a log file (or /dev/null if you prefer).

Quote:

Originally Posted by Raynon

and also another disadvantage is that a infinite looping script will eat up alot of resources, wouldn;t it ?

Much as before, no problems here either. Just use sleep at the bottom of the loop to stop it thrashing.

Quote:

Originally Posted by Raynon

So i still think cron is the more practical one.

Your call of course.

Quote:

Originally Posted by Raynon

But i have no idea about anacron or control-m. Can you enlightened me on that ?

Anacron is a slighty more advanced version of cron, may or may not do what you want. Free (I think)
Control-M is a BMC product that manages scheduling at an enterprise level, it supports job dependancies, understands limited resources and works across multiple platforms. Costs a bit.

Google for more info on these

Quote:

Originally Posted by Raynon

Can you give an example of script being re-entrant ?

Reentrant just means that it can be run multiple times at once without going wonky. One example:
Not rentrant:

Code:

#!/bin/sh
/usr/bin/do_some_stuff > /var/log/did_some_stuff.log
do some other things
if grep 'it worked' /var/log/did_some_stuff.log
then
  /usr/sbin/assume_we_are_good_to_go
else
  echo "Argh"
fi

Reentrant:

Code:

#!/bin/sh
/usr/bin/do_some_stuff > /var/log/did_some_stuff.$$.log
do some other things
if grep 'it worked' /var/log/did_some_stuff.$$.log
then
  /usr/sbin/assume_we_are_good_to_go
else
  echo "Argh"
fi
rm /var/log/did_some_stuff.$$.log

The first example could end up reading back the log of a different instance of itself. The second version includes the current process ID in the log file which prevents this happening.

Another, simpler, way is to just look in the process table for another copy of the script, if found exit immediatly.

Quote:

Originally Posted by Raynon

I was thinking if i could make the script check for the process to see if the previous process have been completed, can any experts give me some examples of this ? I am using csh by the way.

(Assuming system V as opposed to bsd)

Code:

#!/bin/csh
set numprocs=`ps -ef | grep -v grep | grep $scriptname | wc -l | awk '{ print $1 }'`
if ($numprocs == 1) then
    # carry on
else
   echo "Another instance is already running, exiting..."
   exit 0
endif

Raynon · #5 (**permalink**) 02-12-2008

Hi ,

I am refering to this particular exmaple that you have provided me.

Code:

#!/bin/csh
set numprocs=`ps -ef | grep -v grep | grep $scriptname | wc -l | awk '{ print $1 }'`
if ($numprocs == 1) then
    # carry on
else
   echo "Another instance is already running, exiting..."
   exit 0
endif

What is the significance of the numprocs?
If it equals to 1 , why does it show that the script is not running anymore?

I tried a simple csh script below and it doesn;t really work.
When it is sleeping, the name of the script which is "myscriptname" is not reflected during " ps -ef ", it only shows " usr 28951 28941 0 15:00:39 pts/7 0:00 sleep 100 ".

Can you help ?

Code:

#!/bin/csh

echo xxx
echo ppp 
echo zzz

sleep 100

Smiling Dragon · #6 (**permalink**) 02-13-2008

Quote:

Originally Posted by Raynon

I am refering to this particular exmaple that you have provided me.

Code:

#!/bin/csh
set numprocs=`ps -ef | grep -v grep | grep $scriptname | wc -l | awk '{ print $1 }'`
if ($numprocs == 1) then
    # carry on
else
   echo "Another instance is already running, exiting..."
   exit 0
endif

What is the significance of the numprocs?
If it equals to 1 , why does it show that the script is not running anymore?

Because one copy is running, the one that's currently executing. It's looking for another version of itself (ie 2 or more) in case there are other copies still running.

Quote:

Originally Posted by Raynon

I tried a simple csh script below and it doesn;t really work.
When it is sleeping, the name of the script which is "myscriptname" is not reflected during " ps -ef ", it only shows " usr 28951 28941 0 15:00:39 pts/7 0:00 sleep 100 ".

Works for me, when I run a csh script with a sleep in it, I see the sleep, _and_ the main script running.

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