Use of flock command for whole script Post: 302982536

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Top Forums Shell Programming and Scripting Use of flock command for whole script Post 302982536 by Don Cragun on Thursday 29th of September 2016 03:47:37 PM

09-29-2016

Registered User

As long as you just have a single script that wants to be sure that only one copy of it is running at a time, you can use the shell's no clobber option to create a lock file something like:

Code:

#!/bin/bash
IAm=${0##*/}
LockFile="/tmp/$IAm.lock"

# Set lock and verify that no other copy of this code is already running...
set -C	# set no clobber option

# Create the lock file...  This will fail if the lock file already exists.
if ! echo "PID $$ holds the lock." > "$LockFile"
then	echo "$IAm: Another $IAm is already running." >&2
	cat "$LockFile" >&2
	echo "$IAm: Aborting!" >&2
	exit 1
fi

# We now hold an exclusive lock to the lock file (as long as other processes
# wanting to create this lock file also use set -C when trying to create this
# lock file).
set +C	# clear no clobber option

# Set up to remove the lock file when we're done.
trap 'rm -f "$LockFile"' EXIT

echo "$IAm: $$ running."
sleep 60
echo "$IAm: $$ quitting."

This should work with any POSIX conforming shell.

Just be aware that if you use kill -9 to kill this script, you'll have to manually remove the lock file before you can start another instance of this script.

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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)