Constant time text file modification

Question

I have a set of fairly large files (about 50 megabytes each, and at least a hundred of them), but I need to insert a small header (about 2 dozen lines) onto each one for processing purposes. I was hoping to write a script in either bash or python to do it, but I can't find a constant-time function that will let me insert at the front of a text file. If it is not constant time, I think it'll take too long to complete. Does anyone have experience with this issue?

Answer 1

Similar to Uwe's answer, but if your processing tool can only accept parameters as filenames, you can fake one up with mkfifo(1).

For example, in bash...

echo 'My header' > header.txt
echo 'My content' > content.txt
mkfifo fakefile.txt
cat header.txt content.txt > fakefile.txt &
cat fakefile.txt

...would stream the contents of the two files, rather than creating a new file.