Shell Programming and Scripting

p will be set to 2 if the line matching any pattern begins with <?xml..., or 1 otherwise. When printing it is counted down until it reaches 0, thus printing 2 or 1 line, resp.
The n = 2 doesn't seem to be necessary for the correct output; maybe a relict from other attempt to the solution?

Try also (shamelessly stealing from Don Cragun's proposal)

Code:

find "$log_dir" -type f -name '*.log' -exec awk '
FNR == NR       {patterns[$0]
                 next
                }
/^<\?xml v.*n/  {getline X; $0 = $0 RS X
                }
                {for (i in patterns)
                   if ($0 ~ i) print
                }
'  "$pattern_file" {} +

Hi Rüdiger,
Note that the last line of the sample input in post #1 was:

Code:

<?xml version="1.0" process id is 987skj29

If 987skj29 had been included in the pattern file my code would print that line while the code you suggested in post #9 would not. Of course both of our scripts might pick up the 1st (perhaps non-matching line) from a subsequent log file if that sample line had not been in the last log file processed in a batch by find -exec. To avoid that problem, I probably should have included an additional three lines in my script, changing:

Code:

/^<[?]xml version/ {
	n = 2
}

to:

Code:

FNR == 1 {
	p = 0
}
/^<[?]xml version/ {
	n = 2
}

to stop an erroneous printing of a continuation line when we switch to a new input log file.

Hi pred55,
With the sample input you provided in post #1, the script I suggested produces the output:

Code:

<?xml version="1.0" processid is 123abc17
read successfully at 20161109093456
message id aazzkk110 is 123abc17
message id aakjahsdk110 is 234cdf19
<?xml version="1.0" processid is 235ifg20
read successfully at 20161109093456

exactly matching the output you requested in post #1.

Please post a sample of a sequence of lines in your real data (from pattern.txt and from one of your *.log files that was not processed correctly by the code I suggested.

Thanks for pointing a weakness out that would need additional measures to prevent errors from happening. But: it WOULD print out that line but with a false second line - the last read X which would keep its value when getline failed.
Possible remedies: Check the getline status if (1 == getline X) $0 = $0 RS X or set X = "".

Thank you all for your suggestions, I got the results.