tr -s ["\012"] < test.txt > newtest.txt

sed -n '/./p' temp_file5 > newtest.txt

grep . temp_file5 > newtest.txt

grep -v '^$' temp_file5 > newtest.txt

cat test.txt | tr -s [:space:]

sed 's/__*/_/g' test.txt     //这里，_表示一个空格