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 $oldpath
                                return 1 # Lock is taken by others
                        else
                                #unsafe: rm -f lock.$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

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