SED/AWK - Delete all lines until empty line is found after pattern match
 

Dear All,

I am sitting on a CentOS 5.4 server and trying to delete all lines in a file until a blank line is found after a string is matched.

For E.g. I have file 123
I need to modify this file as below; such that all lines are deleted after matching string hello until a blank line is found (not delete the blank line)
I have googled and tried to match lines after string hello until a blank line is found.
But I need just inverse of it.

Any suggestions ? Thanks in advance.

Hi All,

I have found a partial solution.

I redirected the file and use the awk one liner but it DOES NOT preserves blank lines :scratch:

How about:

Hi.

I admire this concise and loop-less solution. It would help awk-learners such as me to have a few words of explanation. Thanks!

Daniel B. Martin

Fantastic
 

Awesome Grail.
You have been my saviour number of times. Many many thanks for this perfect one liner. :hattip:

@Daniel - sorry about that ... forget some times as just come up with solution and move on (my bad)

Ultimately the while loop must have an action associated so a blank set of braces will execute nothing for each loop. As the loop ends when it finds a line that is empty, when it leaves
the loop the 1 at the end will also print this line (which the user wanted)

Please let me know if you need further information and thanks again for the reminder ;)

This is helpful. Allow me to rephrase and elaborate the operation of the awk and then you add/correct as need be.
The awk spins down the "123" input file, printing each line as it goes, until it encounters a line containing the search string hello. At that point it enters a while loop inside the {braced clause}. Each iteration of the while uses a getline to read another line from "123" and tests the newly-read line with $0 != "" to see if it is *not* a null string. If not a null string, it executes a no-op (the empty braces). If a null string, it leaves the braced clause, prints the line, and resumes searching for hello.

My Dougherty & Robbins "sed & awk" book doesn't tell about getline. In context, it reads the next line. How and why is the getline and-ed with the logical result of $0 != ""?

Daniel B. Martin

Hi, Daniel.

Quote from `info gawk getline':
So the while loop stops either at end of file or at empty line.

Thank you, firstfire, that makes sense.

Please consider this input file which is the same as that posted by OP but for two lines added at the end.
OP wants to suppress all lines beginning with one containing hello and up to but not including a blank line. The last record (bogus) should not appear in the output but it does. Am I being too picky?

Daniel B. Martin

But there's no blank line after bogus, which is not the OPs spec for deletion...

You are right. I interpret the OP's requirement in an algorithmic fashion:
1) Start suppressing printing whenever you see hello.
2) Stop suppressing printing whenever you see a blank line.

There was no assurance that each and every hello had a blank line "partner."

This is another grain of evidence which shows that writing an airtight spec is more difficult than it might seem.

Daniel B. Martin

Actually I would disagree with your assumption, due to the following entries:
The second infers that not only is there a blank matching line but that it is not to be removed.

The other point to remember is that any programming is only ever as good as the information supplied, hence based on example data and information the current script is a worthy solution.
Of course anything could be added to the data to cause anyone one solution to fail.

You are right, and I'm not trying to be annoying. Real-world data often has defects in format or content. It's impossible to code for every possible "glitch" but it's also a mistake to imagine there will be none.

Daniel B. Martin