10-filelock

#!/bin/bash
# filelock--A flexible file locking mechanism

retries="10"            # Default number of retries
action="lock"           # Default action
nullcmd="`which true`"     # Null command for lockfile

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 # Output a multi-line error message to stdout.
  cat << EOF >&2
Usage: $0 [-l|-u] [-r retries] LOCKFILE
Where -l requests a lock (the default), -u requests an unlock, -r X 
specifies a max number of retries before it fails (default = $retries).
EOF
  exit 1
fi

# Ascertain if we have the lockfile command.

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