Comment all lines in a text file?

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http://unix.stackexchange.com – In a text file I need to comment out all lines by adding a ";" as first character of each line. What is a good way to do this? I thought of Vim's visual block mode, but I couldn't find a "select all" option and marking several hundred lines manually also isn't great ;-) Any idea? I have nano, vi and vim at hand, I would prefer one of those for this task. (HowTos)