Shell Programming and Scripting

That was probably because Corona's suggestion needed another =-sign:

Code:

awk '$1=="N" { P=1; next } $1=="$$" { P=0 } P' file

the only way I could make sense of why that works is awk prints out whatever line that are between the two patterns, without much directionality.

Code:

/pattern2/ {on=0} on

when pattern2 is found, store the lines but do not print, is that correct?

Code:

 /pattern1/ {on=1}

when pattern1 is found turn on print. But pattern 1 occurs before pattern 2 in the table. So what is it printing? I think awk works its way down the table as it executes the command, so that code doesn't make much logical sense to me. Or does it not work like that?

No. See below.

Yes.
Please try diving in deeper into awk by reading man pages or other literature. awk in very general terms does the following: execute the BEGIN action (if exists), then read input line after line, and apply the program steps given sequentially to each line. Program steps consist of pattern {action} pairs, in priciple. Whenever a pattern evaluates to TRUE, execute the action. The default action is {print}. So in your case

Code:

...                # read line
/pattern2/ {on=0}  # does closing pattern occur in the line --> store THE FACT (implies not printing the closing line)
on                 # default print if on (on == TRUE), don't if FALSE
/pattern1/ {on=1}  # does opening pattern occur in the line --> store THE FACT (implies not having printed the opening line)
...

So, as you can see, by carefully arranging the steps you can tailor the output to your needs.

BTW - you could, of course, append the down-the-line awk processing that you mentioned before into the above awk program...

After much deliberation I can begin to make sense of the code

My final question is, as Awk reads in the first line of input and execute the following commands for the first line

Code:

.../pattern2/ {on=0}on/pattern1/ {on=1}...

What is the default value of

Code:

on

as it has not been initialised? As it reads the first line of input pattern2 is not matched so

Code:

{on=0}

is not executed, then how does Awk evaluate

Code:

on

(no initialised value?) before it goes down to

Code:

/pattern1/ {on=1}

which will be true?

I did try to dive into manuals and online resources but I read little stuffs here and little stuffs there it is difficult to get a complete picture.

But many thanks for your help.

For awk, uninitialized variables are 0 or "". This was implicitly taken into account by my proposal.