Filtering multi-lines from a log

Question

Should this question be moved to stackoverflow instead?

I often need to read log files generated by java applications using log4j. Usually, a logged message (let's call it a log entry) spans over multiple lines. Example:

INFO  10:57:01.123 [Thread-1] [Logger1] This is a multi-line
text, two lines
DEBUG 10:57:01.234 [Thread-1] [Logger2] This entry takes 3 lines
line 2
line 3

Note that each log entry starts at a new line and the very first word from the line is TRACE, DEBUG, INFO or ERROR and at least one space. Here, there are 2 log entry, the first at millisecond 123, the other at millisecond 234.

I would like a fast command (using a combination of sed/grep/awk/etc) to filter log entries (grep only filters lines), eg: remove all the log entries containing text 'Logger2'.

I considered doing the following transformations:

1) join lines belonging to the same log entries with a special sequence of chars (eg: ##); this way, all the log entries will take exactly one line

INFO  10:57:01.123 [Thread-1] [Logger1] This is a multi-line##text, two lines
DEBUG 10:57:01.234 [Thread-1] [Logger2] This entry takes 3 lines##line 2##line 3

2) grep
3) split the lines back (ie: replace ## with \n)

I had troubles at step 1 - I do not have enough experience with sed.

Perhaps the 3 steps above are not required, maybe sed can do all the work.

Answer 1

There is no need to mix many instruments. Task can be done by sed only

sed '/^INFO\|^DEBUG\|^TRACE\|^ERROR/{
         /Logger2/{
             :1
             N
             /\nINFO\|\nDEBUG\|\nTRACE\|\nERROR/!s/\n//
             $!t1
             D     }
                                    }' log.entry

Answer 2

perl filter for multiline log records (record begin mark)

Use the following perl script as a working prototype.
Usage script_path regular_expression log_files
e.g. script_path "line \d" log_file_1 log_file_2

#!/usr/bin/perl
$pattern = qr/(?^s)$ARGV[0]/; shift; # process filtering expression
# (?^s) - treats matched string as single line
my $line = ''; # accumulates current log file record/paragraph
while(<>) {
 if( /^(TRACE|DEBUG|INFO|ERROR) /o ) { # start of new record
   &flush; # flush/print previous recors
 }
 $line.=$_;
}
&flush;
exit;

sub flush {
  local $_ = $line;
  if( length($_) and /$pattern/ ) {
    print;
  }
  $line = '';
}

Answer 3

Based on one answer at https://stackoverflow.com/questions/9605232/merge-two-lines-into-one this seems to fit the bill

#!/usr/local/bin/bash

PATTERN1='TRACE *';
PATTERN2='DEBUG *';
PATTERN3='INFO *';
PATTERN4='ERROR *';
LINEOUT=""
while read line; do
    case $line in
        $PATTERN1)
                echo $LINEOUT
                LINEOUT="$line"
                        ;;
        $PATTERN2)
                echo $LINEOUT
                LINEOUT="$line"
                        ;;
        $PATTERN3)
                echo $LINEOUT
                LINEOUT="$line"
                        ;;
        $PATTERN4)
                echo $LINEOUT
                LINEOUT="$line"
                        ;;
        "")
                LINEOUT=""
                ;;

        *)      LINEOUT="$LINEOUT ## $line"
                ;;
    esac        
done
echo $LINEOUT

NB: This will add a blank space at the begining of output