Shell Programming and Scripting

I have a shell script that is building a string that consists of the parts of a command that I want run at the end of the string. So it looks like this:

$PART1=/path/to/command
$PART2="-arg1"
$PART3="-arg2"
and so on.

At the end of the command is a list of files I get from a loop and place in its own string.

So in the end I end up with:
$PART1 $PART2 $PART3 $FILES
as the command I want to run.

The problem is that when the script is run the command ends up being:
/path/to/command -arg1 -arg2 '/path/to/file1 /path/to/file2'
if there are any spaces in the file names which there often are.

How can I get rid of the single quotes and just have the command be:
/path/to/command -arg1 -arg2 /path/to/file1 /path/to/file2

I am doing this as zsh script on OS X.

Thanks.

How is the FILES variable initialized...echo $FILES and post output.

Code:

echo $FILES

$FILES is created in this loop:

while read line; do
FILES+="${line/ /\ }"
done

So when I input the files: Photo 119.jpg and Photo 136.jpg

echo $FILES
Photo\ 119.jpg Photo\ 136.jpg

It is more that when the command is run instead of a set space delimitated file names there are single quotes around the list of files and so the command thinks they are all one file.

I noticed a similar thread that suggested doing:
echo $(command) | sh
Which did work.