为什么 if [ !$(grep -q) ] 在 if grep -q 起作用时不起作用? [英] Why does if [ !$(grep -q) ] not work when if grep -q does?

问题描述

我无法让 grep 在 if 语句中正常工作.在下面的代码段中，if-check 总是为真(即找不到单词)，即使单词已经在 ~/.memory 中，程序也会打印 NOT FOUND.

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 "
NOT FOUND
"
fi
done

但是，当我测试以下代码代替前一段时:

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

它工作得非常好，可以毫无问题地找到这个词.

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

那么第一段代码有什么问题?

推荐答案

第一个片段有很多问题.

A number of things are wrong with that first snippet.

如果你想测试返回码，你不需要 [ ... ] .放下那些.

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

[] 不是 if 语法的一部分(正如您在第二个代码段中所见).

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

[ 是您系统上的内置 shell 和二进制文件.它只是以返回码退出.<代码>如果...;然后测试...的返回码.

[ 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.它用所运行命令的输出替换自己.

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

所以 [ !$(grep ...) ] 实际上是在评估 [ !output_from_grep ] 并且 [ word ] 被解释为 [ -n word ] 只要 word 不为空，它就会为真.鉴于 ! 永远不会是非空的，这将永远是真的.

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，正如@thom 在他的评论中所指出的(有点倾斜)，将 ! 否定添加到您的第二个片段中，并在它和 grep 之间添加一个空格.

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.

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