There's no reason why this functionality should be in sed when it can be implemented on its own and re-used. IIRC there's an an example script to for that in _The Unix Programming Environment_. Better to use sudo than have everything implement its own sudo-like functionality, etc.
See "UNIX Style, or cat -v Considered Harmful" by Rob Pike (http://harmful.cat-v.org/cat-v/) for a longer discussion of this.
I checked - it's pgs. 152-156. They call their utility "overwrite".
"...Many other commands could also use a -o command. For example, sed could edit a file in place: ... It would be impractical to modify all such commands to add the option. Furthermore, it would be bad design: it is better to centralize functions, as the shell does with the > operator."
Yes, thank you. Forgot to qualify that -i is only in GNU sed.
But I think it is in GNU sed for a reason -- it is a practical shortcut. The only thing I use sed for is to do search and replaces. If the command spanned 2 lines or had pipes and redirects a lot of people just wouldn't remember it.
My most useful one-liner:
It means replace in-place all matches of pat with rep in file.To make a backup of old file use -i.bak instead of -i where .bak would be the suffix of the backup file.