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I'am trying to debug a server that throws a lot of logs on stdout/stderr. I need to redirect only last N lines to a file. Something like a rotating buffer feature there in tcpdump's -C & -W flags. It would be nice if I could view the log while the server is still running and throwing the logs (I could cp it in another file to view it). Is there a utility that does this? From my little understanding of logrotate tool, it should be run repeatedly. I don't think it fits my need. What I would like is:

serverd -d | $TOOL -n 100 srv.log

...where at any time srv.log contains last 100 lines outputted from serverd.

share|improve this question
Related question: Keep log file size fixed without logrotate – Caleb Aug 5 '11 at 14:32
Why exactly do you want this? – user606723 Aug 5 '11 at 19:42
@user606723: As I had mentioned, I am trying to debug a server and it outputs a lot of messages. Redirecting to a file makes it very large. At any time (typically when an issue occurs), I want to see last X number of lines only. Sometimes the server is on embedded device that doesn't have file system space. – Manish Aug 6 '11 at 14:07

You can fetch only the last N lines of any file or input stream using tail.

command | tail -n 100 > file

However it sounds like you want a rotating stream of the last 100 lines always in a log file. This is not easily doable. You can regularly truncate the log file by deleting lines or you can use a system like logrotate to cull old data, but there is not an easy way to keep a 100 line FIFO type log file.

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don't forget about command | trail -fn 100 (perhaps not a solution, but it could be useful) -f is for follow. This command will print out 100 lines and then wait and block on the file writes and output anything new that gets written. – user606723 Aug 5 '11 at 19:38
@user606723, I believe that you meant to write tail rather than trail? – Michael Kjörling Aug 5 '11 at 20:11
oh, yeah, oops. – user606723 Aug 5 '11 at 20:39
@user606723: As far as I know, on a command like you show the -f flag is meaningless. Tail will run until the pipeline is closed then exit with or without the flag. In the case of reading a file I think that would accomplish almost the opposite of what the OP is after ... it would read a minimum of 100 lines and then wait for more. He wants to always truncate at 100 lines. The point here is that there is NOT a good answer to this question, I just threw a reminder in about tail so he could use it to skim off of other log files if he really did just want the last 100 lines of something. – Caleb Aug 5 '11 at 20:44
as I said, "perhaps not a solution, but could be useful." When I said "solution", I meant "answer". It's hard to say what his actual goal is here... I was just trying to make him aware of something else that could be useful. – user606723 Aug 5 '11 at 20:48

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