Insert new line if different number appears in a column

Question

I have a column

1
1
1
2
2
2

I would like to insert a blank line when the value in the column changes:

1
1
1
                <- blank line
2
2
2

Answer 1

I would recommend using awk:

awk -v i=1 'NR>1 && $i!=p { print "" }{ p=$i } 1' file

On any line after the first, if value of the "i"th column is different to the previous value, print a blank line. Always set the value of p. The 1 at the end evaluates to true, which means that awk prints the line. i can be set to the column number of your choice.