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    <p>There are important differences beetween those 3 choices.</p> <h1><code>File.open("file").each_line { |line| puts line }</code></h1> <ul> <li><code>File.open</code> opens a local file and returns a file object</li> <li>the file stays open until you call <code>IO#close</code> on it</li> </ul> <h1><code>open("file").each_line { |line| puts line }</code></h1> <p><code>Kernel.open</code> looks at the string to decide what to do with it.</p> <pre><code>open(".irbrc").class # =&gt; File open("http://google.com/").class # =&gt; StringIO File.open("http://google.com/") # =&gt; Errno::ENOENT: No such file or directory - http://google.com/ </code></pre> <p>In the second case the <code>StringIO</code> object returned by <code>Kernel#open</code> actually holds the content of <a href="http://google.com/" rel="noreferrer">http://google.com/</a>. If <code>Kernel#open</code> returns a <code>File</code> object, it stays open untill you call <code>IO#close</code> on it.</p> <h1><code>IO.foreach("file") { |line| puts line }</code></h1> <ul> <li><code>IO.foreach</code> opens a file, calls the given block for each line it reads, and closes the file afterwards.</li> <li>You don't have to worry about closing the file.</li> </ul> <h1><code>File.read("file").each { |line| puts line }</code></h1> <p>You didn't mention this choice, but this is the one I would use in most cases.</p> <ul> <li><code>File.read</code> reads a file completely and returns it as a string.</li> <li>You don't have to worry about closing the file.</li> <li>In comparison to <code>IO.foreach</code> this makes it clear, that you are dealing with a file.</li> <li>The memory complexity for this is O(n). If you know you are dealing with a small file, this is no drawback. But if it can be a big file and you know your memory complexity can be smaller than O(n), don't use this choice.</li> </ul> <p>It fails in this situation:</p> <pre><code> s= File.read("/dev/zero") # =&gt; never terminates s.each … </code></pre> <h1>ri</h1> <p>ri is a tool which shows you the ruby documentation. You use it like this on your shell.</p> <pre><code>ri File.open ri open ri IO.foreach ri File#each_line </code></pre> <p>With this you can find almost everything I wrote here and much more.</p>
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    1. POFile.open, open and IO.foreach in Ruby, what is the difference?
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    1. USjohannes
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    1. POFile.open, open and IO.foreach in Ruby, what is the difference?
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    1. COKudos for the polite `ri` reminder - a gentleman's RTFM.
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      1. USFábio Batista
    2. COThanks for the pointer to ri. It could be more friendly, though. "ri File.open" doesn't find anything for me. Trying to find "open" I eventually find Kernel#orig_open and then try looking up "Kernel#open" only to be told there are multiple matches without any apparent way to just give me the docs for exactly Kernel#open instead of searching for Kernel#open*.
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    3. COOn my system `ri File.open` works.
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