Why is `xargs bash -ic echo` not the same thing as `xargs echo` or `xargs`?

Question

I expect the command

ls -d doc/* | grep -P "<some_pattern>" | xargs bash -ic echo

to do the same as:

ls -d doc/* | grep -P "<some_pattern>" | xargs echo

that is, to give me the files matched by ls -d doc/* | grep -P "<some_pattern>", only separated by spaces instead of newlines.

But I get only a newline as output instead.

Why is this? How can I fix the first command to do what I want?

By the way, I’m realling using zsh instead of bash, but neither will work.

I actually need this to print a bunch of files whose file names are matched by the pattern specified by "grep" and should be printed using an .zshrc-aliased command.

Answer 1

from the bash manpage

   -c string If the -c option is present,  then  commands  are  read  from
             string.   If  there  are arguments after the string, they are
             assigned to the positional parameters, starting with $0.

so...

 $ echo a b c d e f g | xargs bash -ic echo

 $ echo a b c d e f g | xargs bash -ic 'echo $0 $@'
 a b c d e f g