010-filelock.sh

#!/bin/sh

# filelock - a flexible file locking mechanism

retries="10"		# default number of retries: 10
action="lock"		# default action
nullcmd="/bin/true"	# null command for lockf

while getopts "lur:" opt; do
  case $opt in
    l ) action="lock"	   ;;
    u ) action="unlock"    ;;
    r ) retries="$OPTARG"  ;;
  esac
done
shift $(($OPTIND - 1))

if [ $# -eq 0 ] ; then
  cat << EOF >&2
Usage: $0 [-l|-u] [-r retries] lockfilename
Where -l requests a lock (the default), -u requests an unlock, -r X 
specifies a maximum number of retries before it fails (default = $retries).
EOF
  exit 1
fi

# ascertain whether we have lockf or lockfile system apps

if [ -z "$(which lockfile | grep -v '^no ')" ] ; then
  echo "$0 failed: 'lockfile' utility not found in PATH." >&2
  exit 1
fi

if [ "$action" = "lock" ] ; then
  if ! lockfile -1 -r $retries "$1" 2> /dev/null; then
    echo "$0: Failed: Couldn't create lockfile in time" >&2
    exit 1
  fi
else	# action = unlock
  if [ ! -f "$1" ] ; then
    echo "$0: Warning: lockfile $1 doesn't exist to unlock" >&2
    exit 1
  fi
  rm -f "$1"
fi

exit 0