sed '1s/.*/ &/' file.txt

sed -s -i '1s/.*/ &/' *.txt

sed '1i\' file.txt

sed -s -i '1i\' *.txt

echo -n ' ' | cat - file

awk 'BEGIN{printf(" ")}1' data.txt

awk 'BEGIN{print""}1' data.txt

awk 'NR==1{a=$0; print ""; next} NR==2{print a; print $0; next}1' file

awk 'FNR==1{a=$0; print ""; next} FNR==2{print a; print $0; next}1' file1 file2 file3

perl -i -ne 'if(eof){$.=0;}if($.==1){print "n";}print' temp temp2