ruby 检查文件是否包含字符串
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Check if file contains string
提问by Danny
So I found this question on here, but I'm having an issue with the output and how to handle it with an if statement. This is what I have, but it's always saying that it's true even if the word monitor does not exist in the file
所以我在这里找到了这个问题，但是我遇到了输出问题以及如何使用 if 语句处理它。这就是我所拥有的，但它总是说即使文件中不存在monitor这个词也是如此
if File.readlines("testfile.txt").grep(/monitor/)
do something
end
Should it be something like == "nil"? I'm quite new to ruby and not sure of what the outputs would be.
它应该是 == "nil" 之类的吗？我对 ruby 很陌生，不确定输出会是什么。
采纳答案by Ed S.
Enumerable#grepdoes not return a boolean; it returns an array (how would you have access to the matches without passing a block otherwise?).
Enumerable#grep不返回布尔值；它返回一个数组（否则如何在不传递块的情况下访问匹配项？）。
If no matches are found it returns an empty array, and []evaluates to true. You'll need to check the size of the array in the ifstatement, i.e.:
如果未找到匹配项，则返回一个空数组，并[]计算为true。您需要检查if语句中数组的大小，即：
if File.readlines("testfile.txt").grep(/monitor/).size > 0
# do something
end
The documentation should be your first resource for questions like this.
文档应该是您解决此类问题的第一个资源。
回答by the Tin Man
I would use:
我会用：
if File.readlines("testfile.txt").grep(/monitor/).any?
or
或者
if File.readlines("testfile.txt").any?{ |l| l['monitor'] }
Using readlineshas scalability issues though as it reads the entire file into an array. Instead, using foreachwill accomplish the same thing without the scalability problem:
使用readlines具有可扩展性问题，因为它将整个文件读入数组。相反， usingforeach将完成相同的事情而没有可扩展性问题：
if File.foreach("testfile.txt").grep(/monitor/).any?
or
或者
if File.foreach("testfile.txt").any?{ |l| l['monitor'] }
See "Why is "slurping" a file not a good practice?" for more information about the scalability issues.
有关可扩展性问题的更多信息，请参阅“为什么“slurping”文件不是一个好习惯？
回答by steenslag
Grep will give you an array of all found 'monitor's. But you don't want an array, you want a boolean: is there any 'monitor' string in this file? This one reads as little of the file as needed:
Grep 将为您提供所有找到的“监视器”的数组。但是你不想要一个数组，你想要一个布尔值：这个文件中是否有任何“监视器”字符串？这个根据需要读取尽可能少的文件：
if File.open('test.txt').lines.any?{|line| line.include?('monitor')}
p 'do something'
end
readlinesreads the whole file, linesreturns an enumerator which does it line by line.
readlines读取整个文件，lines返回一个枚举器，它逐行执行。
回答by equivalent8
if anyone is looking for a solution to display last line of a file where that string occurs just do
如果有人正在寻找解决方案来显示该字符串出现的文件的最后一行，请执行
File.readlines('dir/testfile.txt').select{|l| l.match /monitor/}.last
example
例子
file:
文件：
monitor 1
monitor 2
something else
you'll get
你会得到
monitor 2
