Control only one instance of script for particular command line argument

karank · 09-07-2014, 02:19 PM

Hello

I want to allow only one instance of shell to execute but with one particular command line argument
Thus if the same shell script is executing with another command line argument it shall be be allowed

Ex.

while following is executing

Code:

./outer.sh argument_1

This shall not be allowed

Code:

./outer.sh argument_1

However folowing shall be allowed

Code:

./outer.sh argument_2

AS of now I have got it worked by following methods [1] & [2]; But I want to achieve it using method [3], which is not working for me

[1]
Here I am checking-inserting-deleting the 'command line argument' in a file(say flag or lock file)
However I am looking for better option rather than depending on file external file to control this

Code:

#!/bin/bash

arg=$1

grep ${arg} /tmp/arg.txt

if [ $? -eq 0 ]; then
    echo "another instance of the script is already running"
    exit
	
   if [ $(pidof -x outer.sh| wc -w) -gt 2 ]; then
        echo "another instance of the script is already running"
       exit
   fi
fi

echo ${arg} >> /tmp/arg.txt

while read p; do
sleep 60
    /tmp/inner.sh $p
done < $1

sed -i "/$arg/d" z.txt

[2]

Code:

#!/bin/bash
scriptname=`basename $0`


#if [ `ps -ef|grep ${scriptname}|grep "log1.txt"|awk '{print $2}'|wc -l` -gt 2 ]; then

if [ `ps -ef|grep ${scriptname}|grep ${1}|awk '{print $2}'|wc -l` -gt 2 ]; then
     echo "another instance of the script is already running"
exit
fi

while read p; do
sleep 60
    /tmp/inner.sh $p
done < $1

[3]
The following works only when the "command line argument" is hardcoded in the shell script but not as a variable

Hardcoding the command line argument of inner script in calling i.e. outer script works (as below)

Code:

cat outer.sh
#!/bin/bash

scriptname=`basename $0`

pidfile=/tmp/${scriptname}.pid

if [ -f ${pidfile} ]; then

   OLDPID=`cat ${pidfile}`

# here the command line argument "log1.txt" is hardcoded

   RESULT=`ps -ef | grep ${OLDPID} | grep ${scriptname} | grep "log1.txt"`

   if [ -n "${RESULT}" ]; then

     echo "another instance of the script is already running"

     exit 255

   fi
fi



PID=`ps -ef | grep ${scriptname} | head -n1 |  awk ' {print $2;} '`

echo ${PID} > ${pidfile}

while read p; do

sleep 60

    /tmp/inner.sh $p

done < $1

if [ -f ${pidfile} ]; then
    rm ${pidfile}
fi

Please suggest

Thanks in Advance

Kind Regards
Karan

smallpond · 09-07-2014, 02:49 PM

All three of your solutions have the same race condition. If another instance is started between doing the test and creating the blocker, then you will have two instances running. You need an atomic operation which both tests and blocks at the same time. Maybe something like:

Code:

BLOCKER_FILE=something based on argument
PID=$$
# Create my pid file
echo $$ >/var/run/$PID
# Try to create a link to it
ln -s /var/run/$PID $BLOCKER_FILE
if [ $? -ne 0 ] ; then
    # failed to create $BLOCKER_FILE
    echo "Already running"
    unlink /var/run/$PID
    exit 1
fi

# Do stuff
...
# All done
unlink $BLOCKER_FILE
unlink $PID