仅在ruby中尚不存在的情况下，如何打开文件进行写入 [英] How do open a file for writing only if it doesn't already exist in ruby

问题描述

我想打开一个文件进行写入，但前提是该文件尚不存在.如果文件存在，我想引发一个异常.这是最好的方法吗?

I want to open a file for writing but only if it doesn't already exist. If the file exists I want to raise an exception. Is this the best way to do it?

filename = 'foo'
raise if File.exists? filename
File.open(filename, 'w') do |file| 
  file.write contents
end

在没有比赛条件的情况下，最惯用的方法是什么?

What is the most idiomatic way to do this without the race condition?

推荐答案

经过进一步研究，似乎可以使用File :: CREAT和File :: EXCL模式标志.

After doing some further research, it seems you can use the File::CREAT and File::EXCL mode flags.

filename = 'foo'
File.open(filename, File::WRONLY|File::CREAT|File::EXCL) do |file|
  file.write contents
end

在这种情况下，如果文件存在，则open将引发异常.运行一次后，该程序成功运行，没有错误，创建了一个名为foo的文件.在第二次运行时，程序将发出以下消息:

In this case, open will raise an exception if the file exists. After running once, this program succeeds without error, creating a file named foo. On the second run, the program emits this:

foo.rb:2:in `initialize': File exists - foo (Errno::EEXIST)
    from foo.rb:2:in `open'
    from foo.rb:2

来自man open:

       O_WRONLY        open for writing only
       O_CREAT         create file if it does not exist
       O_EXCL          error if O_CREAT and the file exists

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