是bash>重定向原子? [英] Is bash > redirection atomic?
问题描述
我的crontab工作很奇怪.我的crontab作业执行以下操作:
I've weird problem with my crontab job. My crontab job does the following:
program > file
有时候，文件中充满了我无法解释的随机数据.
Sometimes however file gets filled with random data that I can't explain.
我想知道这是否可能是先前的crontab作业需要花费更长的时间运行，并且以某种方式将file中的结果与当前的crontab作业混合?
I wonder if it could be the previous crontab job that's taking longer to run and it somehow mixes its results in file with current crontab job?
总的来说，我的问题是:>操作是原子的吗?这意味着如果两个程序执行> file，那么最后一个要完成的程序将其数据存储在file中?
Overall my question is: is > operation atomic? Meaning if two programs do > file, then the last one to finish will have its data in file?
推荐答案
不，它不是原子的.甚至一点点原子.
No, it's not atomic. Not even a little bit atomic.
重定向有两件事:
-
它将按名称打开文件 ，并在必要时创建文件.
It opens the file by name, creating it if necessary.
它会截断文件.
然后，启动实用程序，并将其stdin分配给打开的文件.
After that, the utility is started, with its stdin assigned to the opened file.
如果两个脚本或多或少同时执行此操作，则它们都将最终写入同一文件，但是由于它们将具有独立的文件描述符，因此每个进程将覆盖另一个进程的输出，从而导致大量的交错字节，其中一些来自一个进程，而另一些来自另一个进程.
If two scripts do that more or less at the same time, they will both end up writing the same file, but since they will have independent file descriptors, each process will overwrite the other process's output, resulting in a grand interleaving of bytes, some from one process and some from the other.
另一个常见的竞争条件与以下事实有关:在实用程序开始执行之前，文件已被外壳截断.因此，即使该实用程序仅向该文件写入一行，读取该文件的并发实用程序也可能会发现该文件为空.
Another common race condition relates to the fact that the file is truncated (by the shell) before the utility starts executing. Consequently, even if the utility only writes a single line to the file, it is possible that a concurrent utility which reads the file will find that it is empty.
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