This fish-shell script detects and switches the Swift version. It has been tested with fish-shell version 2.2.0 under Ubuntu Linux 16.04 LTS.

switch_swift.fish

#!/usr/bin/fish

set SWIFT_PATH /opt
set swift_versions (sudo find $SWIFT_PATH -maxdepth 1 -name "swift[0-9][0-9][0-9]" -type d -printf "%f\n" | sort)
set swift_active_version (stat $SWIFT_PATH/swift -c "%N" | sed -r "s/.+(swift[0-9][0-9][0-9]).+/\1/g")

echo -e "\n\e[44;1mI found these Swift versions:\e[0m"

set i 1
for v in $swift_versions
	echo -en "   $i) $v"
	if [ $v = $swift_active_version ]
		echo -n " (currently active)"
	end
	echo
	set i (math $i + 1)
end

read -p "echo -e '\nSwitch to Swift version: '" selected_version

if [ "$selected_version" = "" ]
	echo -e "\e[32mNo changes are made. Bye.\e[0m\n"
	exit
else
	set selected_version (math $selected_version)
	if [ $selected_version -lt 1 -o $selected_version -ge $i ]
		echo -e "\e[31mInvalid selection. Bye.\e[0m\n"
		exit
	end

	echo -en "\e[42mSwitching to version \e[1m$swift_versions[$selected_version]\e[22m... "
	sudo rm -f $SWIFT_PATH/swift
	sudo ln -s $SWIFT_PATH/$swift_versions[$selected_version] $SWIFT_PATH/swift
	echo -e "Done.\e[0m\n"
end