为什么在管道中读取和写入同一个文件会产生不可靠的结果? [英] Why does reading and writing to the same file in a pipeline produce unreliable results?

问题描述

我有一堆包含许多空行的文件，我想删除任何重复的空行以使阅读文件更容易.我写了以下脚本:

I have a bunch a files that contain many blank lines, and want to remove any repeated blank lines to make reading the files easier. I wrote the following script:

#!/bin/bash
for file in * ; do cat "$file" | sed 's/^ +//' | cat -s > "$file" ; done

然而，这产生了非常不可靠的结果，大多数文件变成完全空的，只有少数文件具有预期的结果.更重要的是，每次我重试时，工作的文件似乎都会随机更改，因为每次运行时都会正确编辑不同的文件.怎么回事?

However, this had very unreliable results, with most files becoming completely empty and only a few files having the intended results. What's more, the files that did work seemed to change randomly every time I retried, as different files would get correctly edited in every run. What's going on?

注意:这更像是一个理论问题，因为我意识到我可以使用以下解决方法:

Note: This is more of a theoretical question, because I realize I could use a workaround like:

#!/bin/bash
for file in * ; do 
    cat "$file" | sed 's/^ +//' | cat -s > "$file"-tmp
    rm "$file"
    mv "$file"-tmp "$file"
done

但这似乎不必要地令人费解.那么为什么直接"方法如此不可靠?

But that seems unnecessarily convoluted. So why is the "direct" method so unreliable?

推荐答案

发生不可预测性是因为管道中的两个阶段 cat "$file" 和 cat 之间存在竞争条件 ->$文件".

The unpredictability happens because there's a race condition between two stages in the pipeline, cat "$file" and cat -s > "$file".

第一个尝试打开文件并从中读取，而另一个尝试清空文件.

The first tries to open the file and read from it, while the other tries to empty the file.

• 如果它在读取之前被清空，你会得到一个空文件.
• 如果它在清空之前被读取，你会得到一些数据(但文件在不久之后被清空并且结果被截断，除非它很短).

如果你有 GNU sed，你可以简单地执行 sed -i 'expression' *

If you have GNU sed, you can simply do sed -i 'expression' *

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