what is diff bentween xargs with braces and without in linux

xargs rm will invoke rm with all arguments as parameter departed with spaces.

xargs -i{} rm {} will invoke rm {} for each of the argument and {} will be replaced by the current argument.

If you have 2 arguments a.txt and b.txt, xargs rm will call this

rm a.txt b.txt

But xargs -i{} rm {} will call

rm a.txt
rm b.txt

This is because -i option implies -L 1 option which mean the command rm will take only 1 line each time. And here each line contains only 1 argument.

Check this Ideone link to get more idea about it.

With braces it will spawn one rm process per file. Without the braces, xargs will pass as many filenames as possible to each rm command.

Compare

ls | xargs echo

and

ls | xargs -i echo '{}'

-i option (equivalent to --replace) creates a sort of placeholder where xargs stores the input it just received. In your second command, the placeholder is "{}", it works like find -exec option. Once defined, xargs will replace this placeholder with the entire line of input. If you don´t like the "{}" name, you can define your own:

ls | xargs -iPLACEHOLDER echo PLACEHOLDER

In your case, both commands are producing the same result. In the second form, you are just making explicit the default behaviour with the -i option.