prepending text dynamically to a file
 

I have a command line program (not mine) that checks the first line of a file for a certain string. For example, lets say it looks for three x's. If it doesn't find those three x's, it exits. Now I want to run this program on some files but they all don't have the three x's at the top. Is there a way to dynamically prepend that text to the file before it is passed to the program?

I have a way currently, but it doesn't do it "on the fly". I have to make temporary files first and then feed it into the command line program.

Let's say the command line program is called `check`

$ ./check example_file
Error: could not find three x's at the top of file
$ echo "xxx" > header
$ cat header example_file > example_file_prepended
$ ./check example_file_prepended
Success: three x's found


So I was wondering if there was some way to dynamically prepend to the file, before the check command line program reads the file.

Does that make sense?

There are probably a hundred different ways to do what you want. You could certainly do it with sed or awk, for example, though I don't know the syntax for them offhand. But a quick and dirty way I've used is something like this:

Edit: here's a solution using sed:
The -i flag in sed makes it modify the original file in place, 1i\ means insert text at line one, and is followed by the text you want to insert.