利用flock來管理工作狀態

  我過去在自修作業系統的時候，有學到mutex這個東西，而flock就是可以在shell上使用的mutex。

flock的官方說明

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 <[email protected]>.

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/.   

重點說明

舉例來說，如果我們寫一個shell script放在$HOME底下：

#! /bin/bash
sleep 10
date

儲存成test.sh並且開啟執行許可權（chmod 700 test.sh）

此時如果我們開啟兩個shell, 並且約同時執行：

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

這時會發生什麼事情呢？

使用者應該會看到兩個shell都停住，一個等10秒後印出時間，一個再過10秒後印出時間：

flock的引數

除了預設的行為之外，我們可以透過引數來調整flock的行為。和預設行為上最主要的差異在於，當無法獲得lock_path的擁有權時，接下來的動作會不同。

• flock -n lock_path xxx：當無法獲得擁有權的時候，直接中止程式，不執行xxx。
• flock -s lock_path xxx：把lock_path當成一個shared lock，同時能被多個程式擁有。所以大家都可以馬上執行，而且同時擁有lock_path
• flock -x lock_path xxx：把lock_path當成一個exclusive lock，同時只能被一個程式擁有。

注：一個lock_path不能同時為shared和exclusive！