Replacement for xargs -d in osx

How about:

echo {1..4} | xargs -n1 -I{} ssh root@www{}.example.com hostname

From man xargs:

-n number
Set the maximum number of arguments taken from standard input for each invocation of utility.

Alternatively, you can always install GNU xargs through Homebrew and the GNU findutils package.

Install Homebrew:

/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install.sh)"

Follow the instructions. Then install findutils:

brew install findutils

This will give you GNU xargs as gxargs, and you can use the syntax you're accustomed to from GNU/Linux. The same goes for other basic commands found in the findutils package such as gfind or glocate or gupdatedb, which have different BSD counterparts on OS X.

You can also use tr in OSX to convert the commas to new lines and use xargs to enumerate through each item as follows:

echo -n 1,2,3,4 | tr -s ',' '\n' | xargs -I{} ssh root@www{}.example.com hostname