Sorry, need more robust effort; use quotes around $f, like

Code:

if [ "" = "$(cmp xx/'$f' xx_Arc/'$f' 2>&1)" ]

Since the whole is already in double quotes, a single quote does not upset that quoting and it does not prevent expanding $f, since the single quote, in double quotes, is a literal for itself, not a live quote, yet. The expanded $f is in single quotes, so spaces are OK unless it gets passed through some shell again not properly quoted. For instance, a shell script should accept parameters as "$1" not barefoot, in case of meta-characters.

Now, if you have a file with a quote character in the name, anothe flavor of dealing with it is to convert anymets-characterslike space and quote into '?', a wild card for a single character but otherwise not a space or quote. In this case, just stick a '| tr ' ' '?' " before the "| while read " and all the spaces become '?'. Just make sure that the last use of it is barefoot so the shell can expand it and virtually quote it as $1 or whatever to the C program. In actuality, by that time it is converted to a null terminated string pointed to by some member of the argv[] array of character pointers. Quoting become start here and null terminate there in machine language. The C open() call and such can deal with the embedded spaces just fine, it is the shell that divides it into two or more arguments when it finds a $IFS character.