crontab problem: multiple processes at the same time

["Luis P. Mendes" <luis_lupe@netvisao.pt>]

Hi,

I scheduled a job to run a script each two minutes that verifies if a
process (python script) is running or not.

If it's not running, it should start a new process of the same python
script.

The problem is that crond sometimes starts three or more processes
within seconds of each other.

Why does this happen?  How to prevent this behavior?

System: Debian Sarge


Here's the data:

$ crontab -l
SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=lupe
HOME=/home/lupe
LOGNAME=/home/lupe/scripts/segur.log

#m  h  dom mon dow
1-59/2 * *   *   mon-fri  /home/lupe/mus/controlador.sh


$ cat controlador.sh
set +v
# get pid of 'gerir.py'
FICHEIRO='gerir.py'
PROGRAMA=$(ps aw | grep $FICHEIRO | grep -v T)
PID=$(echo $PROGRAMA | awk '{ print $1 }')
#echo "PID: " $PID

# still running or halted/suspended?
F='/home/lupe/mus/predefinidos.cfg'
DATA_F=$(ls -l $F | awk '{ print $6 " " $7 }')
DATA_F=$(date +%s --date="$DATA_F")
DATA_ACTUAL=$(date +%s)
let diferenca_segundos=$DATA_ACTUAL-$DATA_F
PRAZO=240       # 4 minutes

if [[ $diferenca_segundos -gt $PRAZO && -n $PID ]]
       then    # service has to start again
               kill -9 $PID
               sleep 3
               cd /home/lupe/mus/
               exec /usr/bin/python /home/lupe/mus/$FICHEIRO &

elif [[ $diferenca_segundos -gt $PRAZO && -z $PID ]]
       then
               cd /home/lupe/mus/
               exec /usr/bin/python  /home/lupe/mus/$FICHEIRO &
fi


$ ps auxw | grep python
lupe      4821  0.1  3.3 28480 25956 ?       S    00:53   0:03
/usr/bin/python /home/lupe/mus/gerir.py
lupe      5407  0.3  3.3 28492 25976 ?       S    01:29   0:01
/usr/bin/python /home/lupe/mus/gerir.py
lupe      5436  0.5  3.3 28472 25916 ?       S    01:31   0:01
/usr/bin/python /home/lupe/mus/gerir.py
lupe      5549  1.1  3.3 28468 25912 ?       S    01:33   0:00
/usr/bin/python /home/lupe/mus/gerir.py
lupe      5584  0.0  0.0  2048  772 pts/4    R+   01:34   0:00 grep
python

Previously to posting, I googled and found out that Via chipsets (or
some of them) do set the system clock time forward and backward on a
strange way that could cause this problem.  I have Via chipsets on my
motherboard: VT8237 PCI Bridge, VT8374 P4X400 Host Controller, and
others.  Any further ideas on this?

Luis P. Mendes