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 ( [e-mail] )
# 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 $oldpath
                                return 1 # lock is taken by others
                        else
                                #unsafe: rm -f lock.$pid
                                echo warn: please delete stale lock.pid by hand.
                                return 2
                        fi
                else
                        # sleep some seconds,then back to 'while' loop
                        # 11 is a prime number, $$ as a random. 
                        echo sleep $(( $$ % 11 ))
                        sleep $(( $$ % 11 ))
                fi
        done
        # 'ln -s' is an atom op.
        ln -s lock.$$ lock.pid >/dev/null 2>&1
        if [ $? -eq 0 ]; then
                cd $oldpath
                return 0 # we got the lock
        else
                [ -e lock.pid ] || echo warn: please delete hanging lock.pid by hand.
                cd $oldpath
                return 3 # lock is probably taken by others.
        fi
}
putlock () {
    oldpath=`pwd`
    cd $lockpath && rm -f lock.$$ lock.pid
    cd $oldpath
}
## test
shflock_cleanhook() {
    echo i\'m a hook.
}
while true; do
    while ! getlock; do
        #echo wait a second...
        sleep 1
    done
    echo \[$$\] `date` ,now hold lock for 3 seconds...
    sleep 3
    #echo putlock
    putlock
    sleep 1 # yield
done

防止脚本多次执行（一）

while [ `ps x |grep -v grep|grep filename.sh|wc -l` -gt 0 ];
do
        exit
done

把内容放在脚本最上面

filename.sh 替换为你要监控的脚本名字
还有一种方法是 死循环 可以根据不同的环境，不同的脚本内容，使用的方法也同步，方法很简单日后在写出来