blocks|key|952749|text|更好的解决方案之一(也是最安全的)是使用TempFile创建一个临时文件，并在创建完成后将其移动到原始位置(使用FileUtils)：|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|952750|+++require+'fileutils'
+++require+'tempfile'

++++t_file+=+Tempfile.new('filename_temp.txt')
++++File.open("filename.txt",+'r')+do+%7Cf%7C
++++++f.each_line{%7Cline%7C+t_file.puts+line.split(",")[0].to_s+}
++++end
++++t_file.close
++++FileUtils.mv(t_file.path,+"filename.txt")|code-block|syntax|javascript|952751|entityMap|0|LINK|mutability|MUTABLE|url|http://www.ruby-doc.org/stdlib/libdoc/tempfile/rdoc/classes/Tempfile.html|1|http://www.ruby-doc.org/stdlib/libdoc/fileutils/rdoc/classes/FileUtils.html^0|K|8|0|1K|9|1|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@$A|T|B|U|1|V]|$A|W|B|X|1|Y]]|C|$]]|$1|D|3|E|5|F|7|Z|8|@]|9|@]|C|$G|H]]|$1|I|3|-4|5|6|7|10|8|@]|9|@]|C|$]]]|J|$K|$5|L|M|N|C|$O|P]]|Q|$5|L|M|N|C|$O|R]]]]

The one of the better solutions(and safest) is to create a temporary file using <a href="http://www.ruby-doc.org/stdlib/libdoc/tempfile/rdoc/classes/Tempfile.html">TempFile</a>, and move it to the original location(using <a href="http://www.ruby-doc.org/stdlib/libdoc/fileutils/rdoc/classes/FileUtils.html">FileUtils</a>) once you are done:

<pre><code> require 'fileutils'
 require 'tempfile'

 t_file = Tempfile.new('filename_temp.txt')
 File.open("filename.txt", 'r') do |f|
 f.each_line{|line| t_file.puts line.split(",")[0].to_s }
 end
 t_file.close
 FileUtils.mv(t_file.path, "filename.txt")
</code></pre>

blocks|key|736708|text|使用代码的文件处理与我们在文本编辑器中编辑文件时所做的操作有很大的不同。操作系统提供的文件操作在这方面相当有限(由于许多原因，部分是历史原因-比如磁带)。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|736709|简而言之，您可能应该创建另一个文件并向其中写入数据(Mike为此提供了代码)，或者将整个文件加载到内存中(如果您的文件很大，这可能不是一个好主意)，并用处理过的数据覆盖它。|736710|仅供练习，这里是您实际如何就地编辑文件。如你所见，不是最美的景象：|736711|File.open('foo',+'r%2B')+do+%7Cfile%7C
++write_pos+=+0
++file.each+do+%7Cline%7C
++++word+=+line.chomp.split(',').first
++++read_pos+=+file.pos
++++file.pos+=+write_pos
++++file.puts+word
++++write_pos+=+file.pos
++++file.pos+=+read_pos
++end
++file.truncate+write_pos
end|code-block|syntax|javascript|736712|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|N|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|O|8|@]|9|@]|A|$]]|$1|F|3|G|5|H|7|P|8|@]|9|@]|A|$I|J]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

File processing using code differs substantially from what we are doing when we, for example, edit the file in a text editor. File operations offered by operating systems are quite limited in that matter (due to numerous, partly historical reasons - think magnetic tapes).

In short, you should probably create another file and write data to it (Mike provided code for that), or load entire file in memory (which can be bad idea if your file is huge) and overwrite it with processed data.

Just for practice, here's how you could actually edit file in-place. As you can see, not the prettiest sight:

<pre><code>File.open('foo', 'r+') do |file|
 write_pos = 0
 file.each do |line|
 word = line.chomp.split(',').first
 read_pos = file.pos
 file.pos = write_pos
 file.puts word
 write_pos = file.pos
 file.pos = read_pos
 end
 file.truncate write_pos
end
</code></pre>

blocks|key|736772|text|我想你误解了这句话的意思|type|unstyled|depth|inlineStyleRanges|entityRanges|data|736773|file.each_line+{+%7Cline%7C+line+=+line.split(",")[0].to_s+}|code-block|syntax|javascript|736774|确实如此。它取一行，用逗号将其拆分，取第一个值，将其转换为字符串(它已经是)，将结果赋给块局部变量“line”。后来怎么样了？|736775|它继续到下一行，前一行什么也没做--都没了。请看其他答案如何解决这个问题。|736776|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|P|8|@]|9|@]|A|$]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

I think you misunderstand what this line

<pre><code>file.each_line { |line| line = line.split(",")[0].to_s }
</code></pre>

really does. It takes a line, splits it on a comma, takes the first value, turns it to a string (which it was already), assigns the result to the block-local variable 'line'. And then? 
It goes on to the next line, and nothing is done with the previous one - it's all gone. See the other answers how to remedy this.

blocks|key|736801|text|就地修改文件的另一种方法是使用-i开关|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|736802|ruby+-F","+-i.bak+-ane+'puts+$F[0]'+file|code-block|syntax|javascript|736803|entityMap^0|F|2|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|P|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|Q|8|@]|D|@]|E|$]]]|L|$]]

Another way to modify the file inplace is to use the <code>-i</code> switch

<pre><code>ruby -F"," -i.bak -ane 'puts $F[0]' file
</code></pre>

blocks|key|952911|text|公认的答案的问题是它修改了文件权限和所有权(请注意这一点)。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|952912|另一种方法是在Ruby中使用就地编辑(而不是从命令行)：|952913|#!/usr/bin/ruby

def+inplace_edit(file,+bak,+&block)
++++old_stdout+=+$stdout
++++argf+=+ARGF.clone

++++argf.argv.replace+[file]
++++argf.inplace_mode+=+bak
++++argf.each_line+do+%7Cline%7C
++++++++yield+line
++++end
++++argf.close

++++$stdout+=+old_stdout
end

inplace_edit+'test.txt',+'.bak'+do+%7Cline%7C
++++line+=+line.gsub(/search1/,"replace1")
++++line+=+line.gsub(/search2/,"replace2")
++++print+line+unless+line.match(/something/)
end|code-block|syntax|javascript|952914|如果您不想创建备份，请将“.bak”更改为“”。|952915|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|N|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|O|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|P|8|@]|9|@]|A|$]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

The problem with the accepted answer is that it modifies file permissions and ownership (pay attention to that).

Another approach is to use inplace editing inside Ruby (not from the command line):

<pre><code>#!/usr/bin/ruby

def inplace_edit(file, bak, &amp;block)
 old_stdout = $stdout
 argf = ARGF.clone

 argf.argv.replace [file]
 argf.inplace_mode = bak
 argf.each_line do |line|
 yield line
 end
 argf.close

 $stdout = old_stdout
end

inplace_edit 'test.txt', '.bak' do |line|
 line = line.gsub(/search1/,"replace1")
 line = line.gsub(/search2/,"replace2")
 print line unless line.match(/something/)
end
</code></pre>

If you don't want to create a backup then change '.bak' to ''.

I'm trying to find a simple way of editing each line in a file, and I'm having some trouble understanding how to use the <code>File</code> class to do so.

The file I want to edit has several hundred lines with comma separated values in each line. I'm only interested in the first value in each line, and I want to delete all values after the first one. I tried to do the following:

<pre><code>File.open('filename.txt', 'r+') do |file|
 file.each_line { |line| line = line.split(",")[0] }
 file.write
 file.close
end
</code></pre>

Which doesn't work because <code>File.write</code> method requires the contents to be written as an argument.

Could someone enlighten me as to how I could achieve the desired effect?

Edit each line in a file in Ruby

我正在尝试找到一种简单的方法来编辑文件中的每一行，但是我在理解如何使用File类来实现这一点时遇到了一些问题。我要编辑的文件有几百行，每行都有逗号分隔值。我只对每一行的第一个值感兴趣，并且我想删除第一个值之后的所有值。我试着做了以下事情：File.open('filename.txt', 'r+') do |file|...

问在Ruby中编辑文件中的每一行
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