最近我常常需要同時ssh給若干臺電腦做許多需要等待，而且可以同時進行的工作。例如：
讓遠端電腦同時更新套件
同時傳送小檔案給遠端的電腦（時間大部分在ssh認證）
然而之後的動作又需要在確認上述工作完畢之後，才能繼續進行。
過去我都是這樣做：
# 前面的工作update_pkg_on_machine_1 update_pkg_on_machine_2 update_pkg_on_machine_3# ... 後面的工作
這樣雖然可以確保工作同時進行完畢，但是就是很慢…
另一種可能的方法是：
# 前面的工作update_pkg_on_machine_1 & update_pkg_on_machine_2 & update_pkg_on_machine_3 &sleep 10# ... 後面的工作
這樣子雖然可以同時進行工作，但是如果10秒內工作還沒完成，接下來的工作可能就會出錯了。
而工作要在多少秒之內做完，其實是很難掌握的。
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
我們先看一下 flock 在ubuntu lucid上的說明 :
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/.