问题猛砸输出重定向 [英] Problem with Bash output redirection

问题描述

我试图删除除最后一行一个文件的所有行，但下面的命令没有工作，虽然file.txt的不是空的。

I was trying to remove all the lines of a file except the last line but the following command did not work, although file.txt is not empty.

$cat file.txt |tail -1 > file.txt

$cat file.txt

为什么会这样呢？

Why is it so?

推荐答案

从通过管道回到同一文件的文件重定向是不安全的;如果 file.txt的之前设置管道的最后阶段时，由外壳覆盖，尾巴开始读取关闭第一阶段，你最终空的输出。

Redirecting from a file through a pipeline back to the same file is unsafe; if file.txt is overwritten by the shell when setting up the last stage of the pipeline before tail starts reading off the first stage, you end up with empty output.

执行以下操作来代替：

tail -1 file.txt >file.txt.new && mv file.txt.new file.txt

......嗯，其实，不这样做，在生产中code;特别是如果你在一个安全敏感的环境，并以root权限运行，以下是比较合适的：

...well, actually, don't do that in production code; particularly if you're in a security-sensitive environment and running as root, the following is more appropriate:

tempfile="$(mktemp file.txt.XXXXXX)"
chown --reference=file.txt -- "$tempfile"
chmod --reference=file.txt -- "$tempfile"
tail -1 file.txt >"$tempfile" && mv -- "$tempfile" file.txt

另一种方法（避免临时文件，除非＆LT;＆LT;＆LT; 隐式地创建他们你的平台上）如下：

Another approach (avoiding temporary files, unless <<< implicitly creates them on your platform) is the following:

lastline="$(tail -1 file.txt)"; cat >file.txt <<<"$lastline"

（以上实施bash的特异性，但是在回波不情况下工作 - 例如当最后一行包含--version，例如）。

(The above implementation is bash-specific, but works in cases where echo does not -- such as when the last line contains "--version", for instance).

最后，可以使用来自 moreutils海绵：

tail -1 file.txt | sponge file.txt

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