Quick note about sed’s edit in place option

From the sed manpage:

-i extension
   Edit files in-place, saving backups with the specified extension.
   If a zero-length extension is given, no backup will be saved.  It
   is not recommended to give a zero-length extension when in-place
   editing files, as you risk corruption or partial content in situ-
   ations where disk space is exhausted, etc.

This doesn’t work:

sed -i -e's/apples/oranges/' file.txt

The key thing here is that the extension after the -i flag is not optional. If you leave it off, sed assumes you’ll be entering it via stdin, which isn’t allowed and yields this error:

sed: -i may not be used with stdin

The solution is to send a zero-length extension like this:

sed -i '' -e's/apples/oranges/' file.txt

Careful with this, it could be really dangerous with poorly crafted commands.