Shell Programming and Scripting

BASH Gurus: Anyone know how to append continuous output command appending to a file, but limit that file to no more than 20 lines? The program I have running is simply monitoring my UDP port 53 for incoming packets endlessly. I just need to keep this file from going over 20 lines. Once the file reaches 21 lines, the 1st line of the file should be removed and so on.

I have used sed, which does this perfectly:

Code:

sed "1d" log.txt

Here is the command below, when ran, it continuously logs the incoming packets

Code:

netcat -ul -p 53 >> log.txt

I just don't know how to combine these in one command (piping would be great). Open to any ideas.

First post on the forums, thanks and hello to all!

Assuming that log.txt contains 20 lines of text when you start the following script:

Code:

netcat -ul -p 53 | while IFS='' read -r line
do	sed 1d log.txt > _log.txt
	{ cat _log.txt; printf '%s\n' "$line"; } > log.txt
done

might come close to doing what you want.

I am very grateful for your reply, that did it perfectly! I simply used a

Code:

LINECOUNT=$(wc -l < "log.txt")

to grab the number of lines in the file and put on space in any remaining. Easy enough to parse out the spaces and display data

I cannot thank you enough Don, many, MANY thanks!

While Don Cragun's fine proposal works brilliantly, it preserves the line count on hand, be it 8 or 800. Try piping into this (shamelessly stolen from Don Cragun) to make log.txt exactly 20 lines long, no matter what its size be:

Code:

while IFS='' read -r line
  do    tail -19 log.txt > _log.txt
        { cat _log.txt
          printf '%s\n' "$line"
        } > log.txt
  done