append first line

I like to append the first line of a file with some value.

file1.dat

 
2111111111 23498ljljkj
2222222222 234kjkjs9
2333333333 98234lkjs
2444444444 0923skdfj

i want to append some value eg: 1200910270040625 to the file1.dat how to do this in KSH

I should have the outfile file2.txt

 
1200910270040625
2111111111 23498ljljkj
2222222222 234kjkjs9
2333333333 98234lkjs
2444444444 0923skdfj
echo "1200910270040625" > file2.txt
cat file1.dat >> file2.txt
echo "1200910270040625"| cat - file1.dat > file2.txt
cat - file1.dat <<< "1200910270040625" > file2.txt
{ echo "1200910270040625";cat file1.dat; }>file2.txt
(echo "1200910270040625";cat file1.dat)>file2.txt

see an example here Ex 2.