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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flock

FLOCK(1)							   User Commands							  FLOCK(1)

NAME

       flock - manage locks from shell scripts

SYNOPSIS

       flock [-sxon] [-w timeout] lockfile [-c] command...

       flock [-sxon] [-w timeout] lockdir [-c] command...

       flock [-sxun] [-w timeout] fd

DESCRIPTION

       This utility manages flock(2) locks from within shell scripts or the command line.

       The  first and second forms wraps the lock around the executing a command, in a manner similar to su(1) or newgrp(1).  It locks a specified
       file or directory, which is created (assuming appropriate permissions), if it does not already exist.

       The third form is convenient inside shell scripts, and is usually used the following manner:

       (
	 flock -n 9 || exit 1
	 # ... commands executed under lock ...
       ) 9>/var/lock/mylockfile

       The mode used to open the file doesn't matter to flock; using > or >> allows the lockfile to be created if it does not already exist,  how-
       ever, write permission is required; using < requires that the file already exists but only read permission is required.

       By default, if the lock cannot be immediately acquired, flock waits until the lock is available.

OPTIONS

       -s, --shared
	      Obtain a shared lock, sometimes called a read lock.

       -x, -e, --exclusive
	      Obtain an exclusive lock, sometimes called a write lock.	This is the default.

       -u, --unlock
	      Drop  a  lock.   This  is  usually  not required, since a lock is automatically dropped when the file is closed.	However, it may be
	      required in special cases, for example if the enclosed command group may have forked a background process which should not be  hold-
	      ing the lock.

       -n, --nb, --nonblock
	      Fail (with an exit code of 1) rather than wait if the lock cannot be immediately acquired.

       -w, --wait, --timeout seconds
	      Fail (with an exit code of 1) if the lock cannot be acquired within seconds seconds.  Decimal fractional values are allowed.

       -o, --close
	      Close  the  file	descriptor  on	which the lock is held before executing command.  This is useful if command spawns a child process
	      which should not be holding the lock.

       -c, --command command
	      Pass a single command to the shell with -c.

       -h, --help
	      Print a help message.

AUTHOR

       Written by H. Peter Anvin <hpa@zytor.com>.

COPYRIGHT

       Copyright (C) 2003-2006 H. Peter Anvin.
       This is free software; see the source for copying conditions.  There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICU-
       LAR PURPOSE.

SEE ALSO

       flock(2)

AVAILABILITY

       The flock command is part of the util-linux package and is available from ftp://ftp.kernel.org/pub/linux/utils/util-linux/.

util-linux							   February 2006							  FLOCK(1)