Shell script replied multiple process for file has successfully created Post: 302963309

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Top Forums Shell Programming and Scripting Shell script replied multiple process for file has successfully created Post 302963309 by Don Cragun on Monday 28th of December 2015 06:03:45 AM

12-28-2015

Registered User

The two step test and create lock file method you're using:

Code:

         if [[ -e ${TMP_LOC}/${FILE_NO}_${INST_NO}.lck ]]
         then
             LOG_PRINT `basename $0` "Duplicate file lck: ${TMP_LOC}/${FILE_NO}_${INST_NO}.lck"
             continue
         else
            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:

Code:

#!/bin/ksh
( 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
fi

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.)

Don Cragun

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shlock

SHLOCK(1)						      General Commands Manual							 SHLOCK(1)

NAME

       shlock - create lock files for use in shell scripts

SYNOPSIS

       shlock -p pid -f name [ -b ] [ -u ] [ -c ]

DESCRIPTION

       Shlock  tries  to  create  a  lock  file named name and write the process ID pid into it.  If the file already exists, shlock will read the
       process ID from the file and test to see if the process is currently running.  If the process exists, then the file will not be created.

       Shlock exits with a zero status if it was able to create the lock file, or non-zero if the file refers to currently-active process.

OPTIONS

       -b     Process IDs are normally read and written in ASCII.  If the ``-b'' flag is used, then they will be written as  a	binary	int.   For
	      compatibility with other systems, the ``-u'' flag is accepted as a synonym for ``-b'' since binary locks are used by many UUCP pack-
	      ages.

       -c     If the ``-c'' flag is used, then shlock will not create a lock file, but will instead use the file to see if the	lock  is  held	by
	      another  program.   If  the  lock  is  valid, the program will exit with a non-zero status; if the lock is not valid (i.e., invoking
	      shlock without the flag would have succeeded), then the program will exit with a zero status.

EXAMPLES

       The following example shows how shlock would be used within a shell script:
	      LOCK=/var/run/innd/LOCK.send
	      trap 'rm -f ${LOCK} ; exit 1' 1 2 3 15
	      if shlock -p $$ -f ${LOCK} ; then
		  # Do appropriate work
	      else
		  echo Locked by `cat ${LOCK}`
	      fi

HISTORY

       Written by Rich $alz <rsalz@uunet.uu.net> after a description of HDB UUCP locking given by Peter Honeyman.  This  is  revision  1.9,  dated
       1996/10/29.

																	 SHLOCK(1)

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