How can i pass a variable into a string to be used as part of a non-file commandline parameter?

Question

The commandline command i want to run is an mkvmerge command:

mkvmerge arguments: [-o] [output_filename] [-a] [tracknumer] [-S] [source_filename]

So i have a variable command and it takes on the values of either -a 1, -a 1 -S, or -S depending on certain conditions being met, so i have this if statement in my bash script to run the command

if [[ ! -z $command ]]; then
    mkvmerge -o "${file}"$command "$sourcefile"
fi

The reason i have $command right next to ${file} without a space is because $command contains a single leading whitespace. The issue that i'm having is that when i run the script, it is assuming $command to be part of the filename and not as additional parameters that are meant to follow after the filename.