This is a simple one-liner:     perl -ne 'print $prev, $_ if /foobar/; $prev = $_' file

However, if there are several consecutive matches it will print duplicated data.

This one doesn't produce duplicates: 

    perl -ne 'BEGIN{$re=qr/foo/}if(/$re/){print$p if$p!~/$re/;print$_}
+;$p=$_' file

[download]

If you have a grep that supports it you can also do this:

    grep -B 1 foobar file

And finally, this is a more generalised method of printing a matched line and the n-1 previous lines, with and without duplicates: 

    perl -ne '$i=$.% 2; $a[$i]=$_; print @a[$i+1..$#a,0..$i] if /foo/'
+ file
    perl -ne '$i=$.% 2; $a[$i]=$_; print @a[$i+1..$#a,0..$i] and @a=()
+ if /foo/' file

[download]

--
John.

In reply to Re: grabbing previous line in a log by jmcnamara
in thread grabbing previous line in a log by adonai

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.