转载

在Linux上使用flock命令控制程序的异步执行

最近我常常需要同时ssh给若干台电脑做许多需要等待,而且可以同时进行的工作。例如:

  • 让远端电脑同时更新套件
  • 同时传送小档案给远端的电脑(时间大部分在ssh认证)

然而之后的动作又需要在确认上述工作完毕之后,才能继续进行。

过去我都是这样做:

# 前面的工作 update_pkg_on_machine_1 update_pkg_on_machine_2 update_pkg_on_machine_3 # ... 后面的工作

这样虽然可以确保工作同时进行完毕,但是就是很慢…

另一种可能的方法是:

# 前面的工作 update_pkg_on_machine_1 & update_pkg_on_machine_2 & update_pkg_on_machine_3 & sleep 10 # ... 后面的工作

这样子虽然可以同时进行工作,但是如果10秒内工作还没完成,接下来的工作可能就会出错了。

而工作要在多少秒之内做完,其实是很难掌握的。

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

我们先看一下 flock 在ubuntu lucid上的说明 :

NAME        flock - Manage locks from shell scripts  SYNOPSIS        flock [-sxon] [-w timeout] lockfile [-c] command...         flock [-sxon] [-w timeout] lockdir [-c] command...         flock [-sxun] [-w timeout] fd DESCRIPTION        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 <hpa@zytor.com>.  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。

在Linux上使用flock命令控制程序的异步执行

程序员高效开发利器:编程水杯

举例来说,如果我们写一个shell script放在 $HOME 底下:

#! /bin/bash sleep 10 date

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:

A

wush@router:~$ flock /tmp/demo.lock ./test.sh

B

wush@router:~$ flock /tmp/demo.lock ./test.sh

其中A程序先抢到 /tmp/demo.lock 的拥有权,然后执行 test.sh 。而B程序等到A程序结束之后(A归还 /tmp/demo.lock 的拥有权),才拿到 /tmp/demo.lock 的拥有权。所以B程序自然比A程序慢10秒。

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!

解决简介中的问题

所以透过组合flock,我可以同时执行若干个工作,并且等到他们结束之后再继续执行接下来的工作:

# 前面的工作 flock -s lock_path update_pkg_on_machine_1 & flock -s lock_path update_pkg_on_machine_2 & flock -s lock_path update_pkg_on_machine_3 & flock -x lock_path echo "all done!" # ... 后面的工作

关键在于 flock -x lock_path xxx 会因为shared和exclusive互斥的关系,而不能共存。因此就会等到上面的工作都结束(归还 lock_path 的拥有权)之后才执行。

原文  http://www.techug.com/linux-flock-lock
正文到此结束
Loading...