Replacing strings in files with "shell"macos system

The Shell's handling of the problem is simple, but most of the online search is done under Linux. As follows:

Sed-i ' s/[match string []/[Replace string]/g ' [filename]

And my system is MacOS, using this command has been unsuccessful, are quick to question the life of ...

Until you see this sentence:

SED requires a backup file for the original file when used on the command line of the MAC system

Sed-i '. Bak ' s/test string/new Test string/g ' *.txt

If you do not want to set up a backup file, enter nothing in the first single quotation mark:

Sed-i "' S/test string/new Test string/g ' *.txt

Replacing strings in files with "shell"macos system