Add a string to a text file from terminal

find and sed are your weapons of choice:

find /mydat/ -exec sed '1i 50' {} \;

That will stick 50 followed by a new line on the beginning of the file.

Alternatively if you don't need recursion or complex selectors for find you can drop find completely:

sed '1i 50' *

To edit a file, you need an editor. ed and ex are examples of command based editors, which is useful for editing files from a script. Here's an example inserting a line to every file with .txt extension in /mydata, using ed:

#!/bin/bash
for file in /mydata/*.txt; do
    printf '%s\n' 0a 50 . w | ed -s "$file"
done

That'll handle all kinds of odd characters in the filenames too, unlike all the examples using for-loops with ls in the answers given so far.

Here's a link describing how to use ed: http://bash-hackers.org/wiki/doku.php?id=howto:edit-ed

For getting to grips with bash, I strongly recommend reading http://mywiki.wooledge.org/BashGuide