给shell提供一把锁，让bash的进程同步。

有时候需要跟shell提供一把锁来实现多个相关shell进程间的同步。下面例子也许就是你想要的， 未经严格测试， 如果有什么bug. 别找我呀。

#!/bin/sh


function lock
{
    while ! mkdir $1 2> /dev/null;
    do
        dd bs=1 count=1 <& 3 > /dev/null 2>/dev/null;
    done;
}

function unlock
{
    if ! rmdir $1;
    then
        echo "bad call unlock";
    fi;
    dd bs=1 count=1 >& 4 < /dev/zero 2>/dev/null;
}

function gotter
{
    let use_idx=0;
    exec 3<_sleep;
    exec 4>_wakeup;
    lock _lock;
    export use_idx=`cat _refcount`;
    while [ $use_idx -lt 1000 ];
    do
        echo $(($use_idx+1)) > _refcount;
        unlock _lock;
        echo $1.lock $use_idx;
    sleep 1;
           lock _lock;
           export use_idx=`cat _refcount`;
    done;
    rmdir $1.lock;
    unlock _lock;
    exec 4>&-;
    exec 3<&-;
    echo $1.lock is finish
}

rm -f _lock;
echo 0 > _refcount;
mkfifo _wakeup _sleep;

for i in a b c d e f;
do
    mkdir $i.lock;
    (gotter $i &);
done;

exec 3>_sleep;
exec 4<_wakeup;

for i in a b c d e f;
do
    while [ -d $i.lock ];
    do
        dd bs=1 count=1 >&3 <&4 2>/dev/null;
    done;
done;

exec 4<&-
exec 3<&-

rm _sleep _wakeup;