How can I test if line is empty in shell script?

try this

while read line;
do 

    if [ "$line" != "" ]; then
        # Do something here
    fi

done < $SOURCE_FILE

bash:

if [[ ! $line =~ [^[:space:]] ]] ; then
  continue
fi

And use done < file instead of cat file | while, unless you know why you'd use the latter.

Since read reads whitespace-delimited fields by default, a line containing only whitespace should result in the empty string being assigned to the variable, so you should be able to skip empty lines with just:

[ -z "$line" ] && continue

cat i useless in this case if you are using while read loop. I am not sure if you meant you want to skip lines that is empty or if you want to skip lines that also contain at least a white space.

i=0
while read -r line
do
  ((i++)) # or $(echo $i+1|bc) with sh
  case "$line" in
    "") echo "blank line at line: $i ";;
    *" "*) echo "line with blanks at $i";;
    *[[:blank:]]*) echo "line with blanks at $i";;
  esac
done <"file"