为什么如果[！$（grep的-q）如果不grep的-q不工作的时候？ [英] Why does if [ !$(grep -q) ] not work when if grep -q does?

问题描述

我遇到了麻烦，如果grep的语句在正常工作。在以下code段中，如果检查总是出现真实的（即，未找到的字），并且程序打印没有发现，即使该字已在〜/。记忆体

I'm having trouble getting grep to work properly in an if statement. In the following code segment, the if-check always comes up true (i.e. the word is not found), and the program prints NOT FOUND, even though the words are already in ~/.memory.

for (( i=0; i<${#aspellwords[*]}; i++)); do
if [ !$(grep -q "${aspellwords[$i]}" ~/.memory) ]; then
   words[$i]="${aspellwords[$i]}"
   printf "\nNOT FOUND\n"
fi
done

然而，当我测试的以下的code代替previous段：

However, when I test the following code in place of the previous segment:

for (( i=0; i<${#aspellwords[*]}; i++)); do
if grep -q "${aspellwords[$i]}" ~/.memory; then echo FOUND IT; fi
done

这工作完全正常，并认为这个词没有任何问题。

It works perfectly fine and finds the word without any issues.

那么，什么是错code的第一段？

推荐答案

许多事情是不对的第一个片段。

A number of things are wrong with that first snippet.

您不想 [...] 如果你想测试返回code。丢弃这些。

You don't want [ ... ] if you want to test the return code. Drop those.

[] 不是如果语法的一部分（你可以从你的第二个片段见）。

[] is not part of the if syntax (as you can see from your second snippet).

[是一个外壳内置和二进制您的系统上。它只是一个返回code退出。 如果...;然后测试返回code ... 。

[ is a shell built-in and binary on your system. It just exits with a return code. if ...; then tests the return code of ....

$（）是命令替换。它取代本身带有的输出的从已运行的命令。

$() is command substitution. It replaces itself with the output from the command that was run.

所以 [！$（grep的...）] 实际上是评估 [！output_from_grep] 和 [文字] 是PTED为 [-n字] 这将是真实的，只要字非空。鉴于！是从来没有非空，将永远是正确的。

So [ !$(grep ...) ] is actually evaluating [ !output_from_grep ] and [ word ] is interpreted as [ -n word ] which will be true whenever word is non-empty. Given that ! is never non-empty that will always be true.

简单地说，在他的评论（斜位）由@thom所示，添加！否定你的第二个片断与它<$ C $之间的空间C>的grep 。

Simply, as indicated by @thom in his comment (a bit obliquely), add the ! negation to your second snippet with a space between it and grep.

这篇关于为什么如果[！$（grep的-q）如果不grep的-q不工作的时候？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！