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Remove duplicate empty lines using sed

Use sed to remove duplicate empty lines from a file and replace them with a single empty line.

Bash
remove_duplicate_empty_lines() {
    sed '/^$/N;/^\n$/D'
}
Bash
sed -i "" '/^$/N;/^\n$/D' input.txt

Example:

Bash
$ cat --number file.txt
     1    1
     2
     3
     4
     5    5
     6    6
     7
     8
     9    9
 
$ cat file.txt | sed '/^$/N;/^\n$/D' > modified.txt
 
$ cat --number modified.txt
     1    1
     2
     3    5
     4    6
     5
     6    9