#! /bin/bash

file="$@"
realpath "$file" 2>/dev/null || eval realpath $(echo $file | sed 's/ /\\ /g')

This makes up for the shortcomings of realpath, store it in a shell script fullpath. You can now call:

$ cd && touch a\ a && rm A 2>/dev/null 
$ fullpath "a a"
/home/user/a a
$ fullpath ~a\ a
/home/user/a a
$ fullpath A
A: No such file or directory.

#! /bin/bash

file="$@"
realpath "$file" 2>/dev/null || eval realpath $(echo $file | sed 's/ /\\ /g')

This makes up for the shortcomings of realpath, store it in a shell script fullpath. You can now call:

$ cd && touch a\ a && rm A 2>/dev/null 
$ fullpath "a a"
/home/user/a a
$ fullpath ~/a\ a
/home/user/a a
$ fullpath A
A: No such file or directory.