Adding Mac-specific directory to PATH when home is shared
Test the output of uname; it will be Darwin on OS X and Linux on Linux.

if [[ $(uname) == Darwin ]]; then
    export PATH="$HOME/SW/bin_mac:$PATH"
fi


Alternately, you could just add an appropriate directory on each machine, which will be useful if you find yourself needing a separate Linux bin directory as well in the future.

export PATH="$HOME/SW/bin_$(uname):..."


and either mv or ln -s your bin_mac to bin_Darwin; then you can create a bin_Linux directory in the future if you need it, and it'll just work.

(You could also test $HOSTTYPE, which is set by bash, but it contains more information than you're likely to need.)