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How do I read prior lines in a text file?

I want to compare a line in a file with zero(0) and if its more than zero i want to print the prior two lines, can u please help me in solving out this... please

Dave's Answer:

I read this a couple of times and think I know what you're asking, but it depends on whether I'm correct in understanding the format of the data file you're talking about and the specific task you face.

Let's consider a data file like this:

a critical line of data
and a second one, followed by
7
and another line of data

Let's look at the task "check every line to see if it's a number, then test to see if it's greater than zero. If it is, show the prior two lines".

Honestly, the trickiest part is to test a line of text to see if it's a number or not. Here's how I solve that:

lineWithoutDigits="$(echo $input | sed 's/[^0-9]//g')"

if [ "$lineWithoutDigits" = "$input" ] ; then
if [ $input -gt 0 ] ; then

The logic here is that if we remove every character in the line that's not a digit, then we can test to see if the line is still intact (meaning it's a line with just a numeric value) and then the secondary test, to see if it's greater than zero.

That shows how to test to see if an unknown line is a numeric value greater than zero, but what about going back two lines too? That's not too bad:

while read input ; do
twoback="$oneback"
oneback="$last"
last="$input"
done < $in

See how that works? Just keep track of two previous lines...

Here's the entire script:

#!/bin/sh

in="datafile"

twoback="" ; oneback=""

while read input
do
twoback="$oneback" ; oneback="$last"
lineWithoutDigits="$(echo $input | sed 's/[^0-9]//g')"
if [ "$lineWithoutDigits" = "$input" ] ; then
if [ $input -gt 0 ] ; then
echo "$twoback"
echo "$oneback"
fi
fi
last="$input"
done < $in
exit 0

So there's your answer. I think. :-)

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