Shell Programming and Scripting

lockfile:-
This utility is part of the procmail package (www.procmail.org). It creates a lock file, a semaphorefile that controls access to a file, device, or resource. The lock file serves as a flag that this
particular file, device, or resource is in use by a process (it is "busy"), and this permits only restricted
access (or no access) to other processes.
lockfile /home/bozo/lockfiles/$0.lock
# Creates a write-protected lockfile prefixed with the name of the script.
Lock files are used in such applications as protecting system mail folders from simultaneously being
changed by multiple users, indicating that a modem port is being accessed, and showing that an
instance of Netscape is using its cache. Scripts may check for the existence of a lock file created by a
certain process to check if that process is running. Note that if a script attempts to create a lock file
that already exists, the script will likely hang.
Normally, applications create and check for lock files in the /var/lock directory.
A script can test for the presence of a lock file by something like the following.
appname=xyzip
# Application "xyzip" created lock file "/var/lock/xyzip.lock".
if [ -e "/var/lock/$appname.lock" ]
then #+ Prevent other programs & scripts
# from accessing files/resources used by xyzip.
...


flock:-

Much less useful than the lockfile command is flock. It sets an "advisory" lock on a file and then
executes a command while the lock is on. This is to prevent any other process from setting a lock on
that file until completion of the specified command.
flock $0 cat $0 > lockfile__$0
# Set a lock on the script the above line appears in,
#+ while listing the script to stdout.
Unlike lockfile, flock does not automatically create a lock file.