bash 使用锁定文件以避免同时运行脚本的两个实例时如何避免竞争条件？
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How to avoid race condition when using a lock-file to avoid two instances of a script running simultaneously?
提问by n-alexander
A typical approach to avoid two instances of the same script running simultaneously looks like this:
避免同一脚本的两个实例同时运行的典型方法如下所示：
[ -f ".lock" ] && exit 1
touch .lock
# do something
rm .lock
Is there a better way to lock on files from a shell-script, avoiding a race condition? Must directories be used instead?
有没有更好的方法来锁定来自 shell 脚本的文件，避免竞争条件？必须使用目录吗？
回答by Barry Kelly
Yes, there is indeed a race condition in the sample script. You can use bash's noclobberoption in order to get a failure in case of a race, when a different script sneaks in between the -ftest and the touch.
是的，示例脚本中确实存在竞争条件。您可以使用 bash 的noclobber选项来在比赛中失败，当不同的脚本潜入-f测试和touch.
The following is a sample code-snippet (inspired by this article) that illustrates the mechanism:
以下是说明该机制的示例代码片段（受本文启发）：
if (set -o noclobber; echo "$$" > "$lockfile") 2> /dev/null;
then
# This will cause the lock-file to be deleted in case of a
# premature exit.
trap 'rm -f "$lockfile"; exit $?' INT TERM EXIT
# Critical Section: Here you'd place the code/commands you want
# to be protected (i.e., not run in multiple processes at once).
rm -f "$lockfile"
trap - INT TERM EXIT
else
echo "Failed to acquire lock-file: $lockfile."
echo "Held by process $(cat $lockfile)."
fi
回答by jpastuszek
Try flock command:
尝试 flock 命令：
exec 200>"$LOCK_FILE"
flock -e -n 200 || exit 1
It will exit if the lock file is locked. It is atomic and it will work over recent version of NFS.
如果锁定文件被锁定，它将退出。它是原子的，可以在最新版本的 NFS 上工作。
I did a test. I have created a counter file with 0 in it and executed the following in a loop on two servers simultaneously 500 times:
我做了一个测试。我创建了一个包含 0 的计数器文件，并在两台服务器上同时循环执行以下 500 次：
#!/bin/bash
exec 200>/nfs/mount/testlock
flock -e 200
NO=`cat /nfs/mount/counter`
echo "$NO"
let NO=NO+1
echo "$NO" > /nfs/mount/counter
One node was fighting with the other for the lock. When both runs finished the file content was 1000. I have tried multiple times and it always works!
一个节点正在与另一个节点争夺锁。两次运行完成后，文件内容为 1000。我尝试了多次，它始终有效！
Note: NFS client is RHEL 5.2 and server used is NetApp.
注意：NFS 客户端是 RHEL 5.2，使用的服务器是 NetApp。
回答by Hank
Lock your script (against parallel run)
锁定您的脚本（防止并行运行）
http://wiki.bash-hackers.org/howto/mutex
http://wiki.bash-hackers.org/howto/mutex
FYI.
供参考。
回答by n-alexander
seems like I've found an easier solution: man lockfile
似乎我找到了一个更简单的解决方案： man lockfile
