How to remove multiple blank lines from a file?

You can use uniq to collapse multiple instance of blank lines into one blank line, but it will also collapse lines which contain text if they are the same and below each other.

Case 1:

awk '!NF {if (++n <= 2) print; next}; {n=0;print}'

Case 2:

awk '!NF {s = s $0 "\n"; n++; next}
     {if (n>1) printf "%s", s; n=0; s=""; print}
     END {if (n>1) printf "%s", s}'

Case 1:

perl -i -ane '$n=(@F==0) ? $n+1 : 0; print if $n<=2'

Case 2:

perl -i -ane '$n=(@F==0) ? $n+1 : 0; print $n==2 ? "\n$_" : $n==1 ? "" : $_ '