A flexible file-locking mechanism

filelock.sh

#!/bin/sh
# filelock - A flexible file-locking mechanism.
retries="10"            # default number of retries
action="lock"           # default action
nullcmd="/bin/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
  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