LINUX flock 控制异步执行

    利用flock来管理工作状态

  我过去在自修作业系统的时候，有学到mutex这个东西，而flock就是可以在shell上使用的mutex。

flock的官方说明

NAME       flock - Manage locks from shell scriptsSYNOPSIS       flock [-sxon] [-w timeout] lockfile [-c] command...       flock [-sxon] [-w timeout] lockdir [-c] command...       flock [-sxun] [-w timeout] fdDESCRIPTION       This  utility  manages  flock(2) locks from within shell scripts or the       command line.       The first and second forms  wraps  the  lock  around  the  executing  a       command,  in  a  manner  similar  to  su(1)  or  newgrp(1).  It locks a       specified file or directory, which  is  created  (assuming  appropriate       permissions), if it does not already exist.       The  third form is convenient inside shell scripts, and is usually used       the following manner:       (         flock -s 200         # ... commands executed under lock ...       ) 200>/var/lock/mylockfile       The mode used to open the file doesn’t matter to flock; using >  or  >>       allows  the  lockfile  to  be  created  if  it  does not already exist,       however, write permission is required; using < requires that  the  file       already exists but only read permission is required.       By  default,  if  the  lock cannot be immediately acquired, flock waits       until the lock is available.OPTIONS       -s, --shared              Obtain a shared lock, sometimes called a read lock.       -x, -e, --exclusive              Obtain an exclusive lock, sometimes called a write  lock.   This              is the default.       -u, --unlock              Drop  a  lock.   This  is  usually not required, since a lock is              automatically dropped when the file is closed.  However, it  may              be  required  in  special  cases,  for  example  if the enclosed              command group may have forked a background process which  should              not be holding the lock.       -n, --nb, --nonblock              Fail  (with  an  exit  code  of  1) rather than wait if the lock              cannot be immediately acquired.       -w, --wait, --timeout seconds              Fail (with an exit code of 1) if the  lock  cannot  be  acquired              within  seconds seconds.  Decimal fractional values are allowed.       -o, --close              Close the file descriptor on  which  the  lock  is  held  before              executing  command.   This  is  useful if command spawns a child              process which should not be hold ing the lock.       -c, --command command              Pass a single command to the shell with -c.       -h, --help              Print a help message.AUTHOR       Written by H. Peter Anvin <[e-mail]>.COPYRIGHT       Copyright © 2003-2006 H. Peter Anvin.       This is free software; see the source for copying conditions.  There is       NO  warranty;  not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR       PURPOSE.SEE ALSO       flock(2)AVAILABILITY       The flock command is part of the util-linux-ng package and is available       from ftp://ftp.kernel.org/pub/linux/utils/util-linux-ng/.   

重点说明

透过flock，程序会先尝试取得某个lock（通常代表某个档案）的拥有权之后才执行，执行的时候会握有该lock的拥有权，并且在结束之后才释出拥有权。

举例来说，如果我们写一个shell script放在$HOME底下：

#! /bin/bashsleep 10date

储存成test.sh并且打开执行权限（chmod 700 test.sh）

此时如果我们打开两个shell, 并且约同时执行：

flock /tmp/demo.lock ~/test.sh

这时会发生什麽事情呢？

使用者应该会看到两个shell都停住，一个等10秒后印出时间，一个再过10秒后印出时间：

flock的参数

除了预设的行为之外，我们可以透过参数来调整flock的行为。和预设行为上最主要的差异在于，当无法获得lock_path的拥有权时，接下来的动作会不同。

• flock -n lock_path xxx：当无法获得拥有权的时候，直接中止程序，不执行xxx。
• flock -s lock_path xxx：把lock_path当成一个shared lock，同时能被多个程序拥有。所以大家都可以马上执行，而且同时拥有lock_path
• flock -x lock_path xxx：把lock_path当成一个exclusive lock，同时只能被一个程序拥有。

注：一个lock_path不能同时为shared和exclusive！