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 <[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/.