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Shell script replied multiple process for file has successfully created

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# 1  
Old 12-28-2015
Shell script replied multiple process for file has successfully created

Hi All,

I have written the following code
         FILE_NO=$(echo $LINE|awk -F"|" '{print $1}'|tr "'" '+'|sed 's/\(.*\)\(++\)\(.*\)\(++\)/\3/')
         INST_NO=$(echo $LINE|awk -F"|" '{print $2}'|tr "'" '+'|sed 's/\(.*\)\(++\)\(.*\)\(++\)/\3/')

         if [[ -e ${TMP_LOC}/${FILE_NO}_${INST_NO}.lck ]]
             LOG_PRINT `basename $0` "Duplicate file lck: ${TMP_LOC}/${FILE_NO}_${INST_NO}.lck"
            touch ${TMP_LOC}/${FILE_NO}_${INST_NO}.lck
            if [[ $? -ne 0 ]]
              LOG_PRINT `basename $0` "Unable to create lck file : ${TMP_LOC}/${FILE_NO}_${INST_NO}.lck"
            LOG_PRINT `basename $0` "Successfully lck file : ${TMP_LOC}/${FILE_NO}_${INST_NO}.lck created"

         LOG_PRINT `basename $0` "Processing OUTPUT PARAM : $LINE"
         i=$( expr $i + 1 )

But When I run 30 instance (same time via cron) of the script multiple output comes on the script stating that "Successfully lck file created" with same name.

Please help

Moderator's Comments:
Mod Comment Please use code tags for your data and code, thanks

Last edited by vbe; 12-28-2015 at 05:12 AM..
# 2  
Old 12-28-2015
We are missing some code to understand...e.g. how is written the loop?
then how do you run 30 instances? you call the script 30 times or is it the loop?...
# 3  
Old 12-28-2015

I run 30 instance of script with crontab or TWS on same time.
loop is picking up the information from Some DATABASE which return set of parameter.
and i have to use it once for each instance of script.
# 4  
Old 12-28-2015
The two step test and create lock file method you're using:
         if [[ -e ${TMP_LOC}/${FILE_NO}_${INST_NO}.lck ]]
             LOG_PRINT `basename $0` "Duplicate file lck: ${TMP_LOC}/${FILE_NO}_${INST_NO}.lck"
            touch ${TMP_LOC}/${FILE_NO}_${INST_NO}.lck
            if [[ $? -ne 0 ]]

has an obvious race condition when you have 30 scripts trying to create a lock file in the same namespace. All 30 scripts can test and find that the lock file does not exist before any of them get to the touch command to create the lock file. And, if the lock file exists when touch runs, touch will not report an error if the file already exists.

If you're using cron to create 30 jobs doing the same thing and then having them fight each other to create separate lock files; why not have the 30 different cron jobs pass a parameter to each job saying which lock file that job should use. Then there won't be any contention and there won't be any race condition.

If that can't be done and your scripts really have to fight each other to create separate locks, use the shell's set -C command to turn on file redirection no-clobber mode and just use a redirection to test for the existence of the lock file and create it atomically if it didn't already exist. For example:
( set -C; 2>| /dev/null > lock) 
if [ $? -eq 0 ]
then	echo "PID $$ got the lock." > lock
	echo we have the lock
else	echo we do NOT have the lock

Although this was written and tested with a Korn shell, this will work with any shell that meets basic set options and output redirections. (Note that the 2>| /dev/null in the subshell overrides the no-clobber mode on the redirection of stderr so you won't get a diagnostic from the shell if the lock file has been created by another process.)

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