MacOS does have sed -i, but it works a bit differently than GNU sed -i.

Here is how GNU's works:

$ echo a > input && sed -i 's/a/b/' input && cat input
b

Here is what happens on MacOS:

$ echo a > input && sed -i 's/a/b/' input && cat input
sed: 1: "input
": command i expects \ followed by text
$ cat input
a

Here is how to do the equivalent thing on MacOS:

$ echo a > input && sed -i '' 's/a/b/' input && cat input

What is happening here is that MacOS requires -i be given a backup suffix. The MacOS man page recommends against a zero-length extension like the above example, but doesn't stop you from doing it. They want you do do something like this:

$ echo a > input && sed -i '.bak' 's/a/b/' input && cat input && cat input.bak
b
a

This difference has caused me quite a bit of annoyance at work where most people use MacOS. :)

command -v gsed && sed() { gsed \"\$@\"" -- "$@"; } might better to allow forgotting to use${SED}.