Editor: nano

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By default nano quietly adds linebreaks to the file that you're working on, not just word-wraps it for display in the editor. This can cause trouble with generating patches, and because it's indistinguishable from word-wrapping in the editor, it can take you a while to diagnose it. To prevent this from happening, when you call nano, do it with the -w flag, which will turn off auto-linebreaking:

nano -w filename

To tell nano to not wrap lines by default, add (or uncomment) this line in /etc/nanorc or ~/.nanorc:

set nowrap