防止shell脚本重复执行的代码

例如，要求脚本只能顺序访问某个资源，例如磁盘文件等，就可以参考下面的实现。

#!/bin/bash
#
# file locking using bash.
# ver 0.1.6
#
# author : malundao ( malundao@sina.com )
# date   : 2011-08-31  
# ref    : http://unix.derkeiler.com/Newsgroups/comp.unix.shell/2005-09/0472.html
#
# note:
#   shflock_cleanhook() is a user defined function to clean up user-specific sth.
#
# /path/to/lock/. note; directory, not a file.
# should be modified
LOCKPATH="/tmp"
cleanup() {
        shflock_cleanhook
        cd $LOCKPATH
        [ -e lock.pid ] || exit
        read pid >/dev/null 2>&1 <lock.pid
        if [ -n "$pid" ]; then
            if [ "$pid" == "$$" ]; then
                rm -f lock.$pid
                rm -f lock.pid
                exit
            fi 
        fi 
        exit
}
#  trap EXIT ?
trap 'cleanup' HUP INT TERM
getlock() {
        oldpath=`pwd`
        cd $LOCKPATH
        while
                echo $$ > lock.$$
                [ -e lock.pid ]
        do
                rm lock.$$
                read pid >/dev/null 2>&1 <lock.pid
                if [ -n "$pid" ]; then
                        if [ -e /proc/$pid ]; then
                                cd $oldpath
                                return 1 # Lock is taken by others
                        else