https://swiftenv.fuller.li/en/latest/integrations/travis-ci.html

swiftenv-install.sh

#!/usr/bin/env bash
# Automatically installs swiftenv and run's swiftenv install.
# This script was designed for usage in CI systems.

git clone -b support-installing-previews --depth 1 https://github.com/ikesyo/swiftenv.git ~/.swiftenv
export SWIFTENV_ROOT="$HOME/.swiftenv"
export PATH="$SWIFTENV_ROOT/bin:$SWIFTENV_ROOT/shims:$PATH"

if [ -f ".swift-version" ] || [ -n "$SWIFT_VERSION" ]; then
  swiftenv install -s
else
  swiftenv rehash
fi