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copied!<p>It is possible that tail can emit 'unparsable lines' - but only if invalid lines are written to the file. Kind of a circular bit of argument, but here's an example of how it could happen:</p> <ul> <li><p>You tail -f on /var/log/syslog</p></li> <li><p>syslog-ng dies in the middle of a block-spanning write (sectors are 512 bytes, your filesystem block size is most likely larger, altho probably not much larger than 4096.. so, syslog has 9k of data buffered up to write out, it gets through 4k byte page and before it can write the next 4k+1k syslog dies. On ext2 at least, that'll end up as a partial write even after fsck. ext3? .. heh. I've been doing embedded for so long I can't remember... But, I'd HOPE not.. But who's to say that the data you're writing is always going to be correct? You might get a non-fatal string formatting error that doesn't include a newline that you're expecting.</p></li> <li><p>You'll then have a partial line that isn't terminated with a newline (or even a \0), and the next time syslog starts up and starts appending it'll just be appending on the end of of the file with no notion of 'valid' records. So the first new record will be garbage, but the next one will be ok.</p></li> </ul> <p>This is easy to exercise..</p> <p>In one window </p> <p><code>tail -f SOMEFILE</code></p> <p>In another window</p> <pre><code>echo FOO >>SOMEFILE echo BAR >>SOMEFILE printf NO_NEWLINE >>SOMEFILE echo I_WILL_HAVE_THE_LAST_LINE_PREFIXED_TO_ME_CAUSING_NERD_RAGE >>SOMEFILE </code></pre> <p>Since Linux's tail uses inotify by default, whatever is reading will get the last line without a newline and wait until the next newline comes along appending NO_NEWLINE to the beginning of what it considers 'the latest line'.</p> <p>If you want to do this 'the pythonic' way, if you're using Linux - use inotify, if you're using OSX or BSD use 'knotty' and negate the use of 'tail' as an input pipe and just watch the file yourself. </p> <p>Tail might do weird things if you use the 'resync on truncate' too - i.e. if the file gets zeroed and restarted in the middle of a read, you might get some weird amount of data on the read since 'tail' will close the previously opened file handle in exchange for the new one. </p>

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