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copied!<p>If it's not a fixed-record-length file and you don't do some sort of indexing on the line starts, your best bet is to just use:</p> <pre><code>head -n N filespec | tail -1 </code></pre> <p>where <code>N</code> is the line number you want.</p> <p>This isn't going to be the best-performing piece of code for a 3Gb file unfortunately but there are ways to make it better.</p> <p>If the file doesn't change too often, you may want to consider indexing it. By that I mean having <em>another</em> file with the line offsets in it as fixed length records.</p> <p>So the file:</p> <pre><code>0000000000 0000000017 0000000092 0000001023 </code></pre> <p>would give you an fast way to locate each line. Just multiply the desired line number by the index record size and seek to there in the index file.</p> <p>Then use the value at that location to seek in the main file so you can read until the next newline character.</p> <p>So for line 3, you would seek to 33 in the index file (index record length is 10 characters plus one more for the newline). Reading the value there, <code>0000000092</code>, would give you the offset to use into the main file.</p> <p>Of course, that's not so useful if the file changes frequently although, if you can control what happens when things get appended, you can still add offsets to the index efficiently. If you <em>don't</em> control that, you'll have to re-index whenever the last-modified date of the index is earlier than that of the main file.</p> <hr> <p>And, based on your update:</p> <blockquote> <p>Update: If it matters, all the lines have the same length.</p> </blockquote> <p>With that extra piece of information, you don't need the index - you can just seek immediately to the right location in the main file by multiplying the record length by the record length (assuming the values fit into your data types).</p> <p>So something like the pseudo-code:</p> <pre><code>def getline(fhandle,reclen,recnum): seek to position reclen*recnum for file fhandle. read reclen characters into buffer. return buffer. </code></pre>

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