linux shell之文件锁

这个脚本是将文件锁得，不过我对这脚本还有一些疑惑的地方，暂且不讨论，等以后回头再来谈

#!/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