Bash if on single line

You would write it as such:

if [ -f "/usr/bin/wine" ]; then export WINEARCH=win32; fi

Note that this could also be written (as suggested by @glennjackman):

[ -f "/usr/bin/wine" ] && export WINEARCH=win32

I also find that just typing any complex if then else command, hit enter, and then after it executes just hit the up arrow. The command line will repeat the last typed command of course but in this case it puts it all on one line as you require. It's a cheat way, but it's effective.