防止shell指令碼重複執行

利用鎖機制，讓一個特定的shell指令碼，每次只能運行一個執行個體。具體來說，獲得鎖的指令碼執行個體，能夠繼續往下執行臨界區代碼；沒有獲得鎖的執行個體，則只能等待。

例如，要求指令碼只能順序訪問某個資源，例如磁碟檔案等，就可以參考下面的實現。

#!/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" == "$$" ]; then                rm -f lock.$pid                rm -f lock.pid                exit             fi          fi          exit}#  trap EXIT ? trap 'cleanup' HUP INT TERMgetlock() {        oldpath=`pwd`        cd $LOCKPATH        while                echo $$ > 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 # yielddone