Tail Grep - Print surrounding lines until pattern is matched

Question

So here is my requirement.

I am tailing a log file and grepping on it.
I want to get some context on every grep result..
But the context should be "till a pattern is matched" and not the number of lines (which is what grep -A/-B/-C offer).

For example
Say here is my log..

[l] is prefixed to every log line. Also there will be prefix of [logTypeA] or [logTypeB]

[l][logTypeA] - Log line 1
[l][logTypeB] - Log line 2 
[l][logTypeA] - Log line 3 
.... 

Random data about Log line 3
....

[l][logTypeB] - Log line 4

Now my if my tail command was tail -f log_file.log | grep "[logTypeA]", I'd get an output of

[l][logTypeA] - Log line 1
[l][logTypeA] - Log line 3 

But I need contextual information for my grep result, and that context is NOT some number of lines, but rather till a particular pattern is matched (in this case [l])..

In the example I want my grep result to be

[l][logTypeA] - Log line 1
[l][logTypeA] - Log line 3 
.... 

Random data about Log line 3
....

From here (How to show lines after each grep match until other specific match?), I tried sed command on my tail like

tail -f log_file.log | sed '/\[logTypeA\]/,/\[l\]/p'

But that doesn't seem to work.

Any ideas?

Answer 1

This is an extremely common pattern with Awk. Collect related lines into "records", print the record when you have collected all of it if it matches a particular condition.

tail -f file |
awk '/\[l]/ { if (p && stored) print stored; stored = ""; p=0 }
    /\[logTypeA]/ { p=1 }
    { stored = stored (stored ? ORS : "") $0 }
    END { if (p) print stored }'

The END condition doesn't really make sense with a never-ending stream from tail -f but I include it for good measure, and to avoid pesky test failures when the last record you want to test with should be printed, but won't be without the END clause.

Answer 2

I would probably end up doing this with a perl one-liner. You can do it in sed but it feels like it's beginning to hit up against the "unintentional turing-completeness" part of sed.

Perl one-liner (the perl -ne is really the take away here)

echo  '[l] boring\nboring data\n[l] boring\n[l]interesting\ndata\ndata\n[l]boring' |  perl -ne '
    if (/\[l\].*interesting/ ) { print $_; $collect=1 ; }
    elsif (/\[l\]/) {$collect=0 }
    elsif ($collect) {print $_}'

For reference, here is the sed one-liner that does the same thing. (Features of sed used: b to achieve a switch, T to achieve conditional execution).

echo  '[l] boring\nboring data\n[l] boring\n[l\]interesting\ndata\ndata\n[l]boring' |  sed -nE '/\[l\].*interesting/ {
p;
s/.*/collect/ ; x ; # store collect marker in the pattern space
b; # terminate processing
}

/\[l\]/ {
s/.*// ; x; # clear hold flag
b
}

/./ {
x;
s/collect/collect/;
T; x; p # print if we are collecting
}
'