grep '[^[:blank:]]' < file.in > file.out

grep '[:blank:]'

$ awk NF file

sed -e 's/^[[:blank:]]*$//' source_file > newfile

sed -e '/^[[:blank:]]*$/d' source_file > newfile

sed '/^$/d' in > out

ex -s +'v/S/d' -cwq test.txt

ex -s +'bufdo!v/S/d' -cxa *.txt

cat test.txt | ex -s +'v/S/d' +%p +q! /dev/stdin