Efficient way to add two lines at the beginning of a very large file

Question

I have a group of very large (a couple of GB's each) text files. I need to add two lines at the beginning of each of these files.

I tried using sed with the following command

sed -i '1iFirstLine'
sed -i '2iSecondLine'

The problem with sed is that it loops through the entire file, even if had to add only two lines at the beginning and therefore it takes lot of time.

Is there an alternate way to do this more efficiently, without reading the entire file?

Answer 1

You should try

echo "1iFirstLine" > newfile.txt
echo "2iSecondLine" >> newfile.txt
cat oldfile.txt >> newfile.txt
mv newfile.txt oldfile.txt

Answer 2

This one is perfectly working and its extremely fast too.

perl -pi -e '$.=0 if eof;print "first line\nsecond line\n" if ($.==1)' *.txt

Answer 3

Adding at the beginning is not possible without file rewrite (contrary to appending to the end). You simply cannot "shift" file content as no filesystem supports that. So you should do:

echo -e "line 1\nLine2" > tmp.txt
cat tmp2.txt oldbigfile.txt > newbigfile.txt
rm oldbigfile.txt
mv newbigfile.txt oldbigfile.txt

Note you need enough diskspace to hold both files for a while.