awk and multilines matching (sub-regex)

Question

I am trying to use awk to parse a multiline expression. A single one of them looks like this:

_begin  hello world !
_attrib0    123
_attrib1    super duper
_attrib1    yet another value
_attrib2    foo
_end

I need to extract the value associated to _begin and _attrib1. So in the example, the awk script should return (one per line):

hello world ! super duper yet another value 

The separator used is a tab (\t) character. Spaces are used only within strings.

Answer 1

The following awk script does the job:

#!/usr/bin/awk -f
BEGIN { FS="\t"; }
/^_begin/      { output=$2; }
$1=="_attrib1" { output=output " " $2; }
/^_end/        { print output; }

You didn't specify whether you want a tab (\t) to be your output field separator. If you do, let me know and I'll update the answer. (Or you can; it's trivial.)

Of course, if you want a scary alternative (since we're getting close to Hallowe'en), here a solution using sed:

$ sed -ne '/^_begin./{s///;h;};/^_attrib1[^0-9]/{s///;H;x;s/\n/ /;x;};/^_end/{;g;p;}' input.txt 
hello world ! super duper yet another value

How does this work? Mwaahahaa, I'm glad you asked.

• /^_begin./{s///;h;}; -- When we see _begin, strip it off and store the rest of the line to sed's "hold buffer".
• /^_attrib1[^0-9]/{s///;H;x;s/\n/ /;x;}; -- When we see _attrib1, strip it off, append it to the hold buffer, swap the hold buffer and pattern space, replace newlines with spaces, and swap the hold buffer and pattern space back again.
• /^_end/{;g;p;} -- We've reached the end, so pull the hold buffer into the pattern space and print it.

This assumes that your input field separator is just a single tab.

SO simple. Who ever said sed was arcane?!

Answer 2

This should work:

#!/bin/bash 

awk 'BEGIN {FS="\t"} {if ($1=="_begin" || $1=="_attrib1") { output=output " " $2 }} END{print output}'