How long can the line be?

There's no perlrun switch that makes this easy, but regular old IF-ELSIF and a state variable will get the job done.

No doubt this could be golf'd into submission, but left reasonably verbose here for clarity.

The bigger question (to me) is, "why?" My system grep implements Perl regular expressions, plus there have been multiple Perl grep implementations such as grepp -- Perl version of grep. There is a point at which the convenience of a one-liner can't make up for the power of a purpose-built utility. Maybe this isn't that point, but it'd be getting awfully close, for me.

I'll golf it readable (untested):

perl -ne '
    print, $aft++, next if /PAT/;
    $aft or next;
    print "$_---\n";
    $aft = 0;'
[download]

This may not be the best or most efficient way to do it, but it does appear to work (written on multiple lines for readability):

Code:

Example output:

I started with a single if (m/.../ or $flag) statement, but in testing found that it missed the following line if the search value appeared in consecutive lines. If you want more following context, change the mod value to N+2, where N is the number of lines you want to see including the matching one (i.e., if you want 1 extra line, set to 3, 2 extra to 4, etc.).

Hope that helps.

perl -ne 'print $_.=<> if /string/' testfile
[download]

Nice, but it misses the line following a second consecutive line matching the pattern

# eg this works as expected
test1
string
test2
test3

# this one fails to print test2
test1
string
string
test2
test3
[download]

Cheers,
R.

Pereant, qui ante nos nostra dixerunt!

It also hangs if the last line matches. So much for working off the top of my head.

sroux:

I normally don't do one-liners, so I thought I'd play with it a bit. Here's my shot at it:

Update: here's another:

Update: ...but the second one is broken, as kschwab mentions...

...roboticus

When your only tool is a hammer, all problems look like your thumb.

The latter misses the last line of the file if it has matching text...I like the first one though.

Here is my humble variation:

perl -ne "print if /error/ || $last=~/error/; $last=$_;" test_log.txt
[download]