replace two newlines to one in shell command line - perl

Question

There are lot of questions about replacing multi-newlines to one newline but no one is working for me.
I have a file:
first line
second line MARKER
third line MARKER
other lines
many other lines
I need to replace two newlines (if they exist) after MARKER to one newline. A result file should be:
first line
second line MARKER
third line MARKER
other lines
many other lines
I tried sed ':a;N;$!ba;s/MARKER\n\n/MARKER\n/g' Fail.
sed is useful for single line replacements but has problems with newlines. It can't find \n\n
I tried perl -i -p -e 's/MARKER\n\n/MARKER\n/g' Fail.
This solution looks closer, but it seems that regexp didn't reacts to \n\n.
Is it possible to replace \n\n only after MARKER and not to replace other \n\n in the file?
I am interested in one-line-solution, not scripts.

Answer 1

I think you were on the right track. In a multi-line program, you would load the entire file into a single scalar and run this substitution on it:
s/MARKER\n\n/MARKER\n/g
The trick to getting a one-liner to load a file into a multi-line string is to set $/ in a BEGIN block. This code will get executed once, before the input is read.
perl -i -pe 'BEGIN{$/=undef} s/MARKER\n\n/MARKER\n/g' input

Answer 2

Your Perl solution doesn't work because you are search for lines that contain two newlines. There is no such thing. Here's one solution:
perl -ne'print if !$m || !/^$/; $m = /MARKER$/;' infile > outfile
Or in-place:
perl -i~ -ne'print if !$m || !/^$/; $m = /MARKER$/;' file
If you're ok with loading the entire file into memory, you can use
perl -0777pe's/MARKER\n\n/MARKER\n/g;' infile > outfile
or
perl -0777pe's/MARKER\n\K\n//g;' infile > outfile
As above, you can use -i~ do edit in-place. Remove the ~ if you don't want to make a backup.

Answer 3

awk:
kent$ cat a
first line
second line MARKER
third line MARKER
other lines
many other lines
kent$ awk 'BEGIN{RS="\x034"} {gsub(/MARKER\n\n/,"MARKER\n");printf $0}' a
first line
second line MARKER
third line MARKER
other lines
many other lines

Answer 4

See sed one liners.

Answer 5

awk '
marker { marker = 0; if (/^$/) next }
/MARKER/ { marker = 1 }
{ print }
'

Answer 6

This can be done in very simple sed.
sed '/MARKER$/{n;/./!d}'

Answer 7

This might work for you:
sed '/MARKER/,//{//!d}'
Explanation:
Deletes all lines between MARKER's preserving the MARKER lines.
Or:
sed '/MARKER/{n;N;//D}'
Explanation:
Read the next line after MARKER, then append the line after that. Delete the previous line if the current line is a MARKER line.