pyenv/libexec/pyenv-version-file-write

#!/usr/bin/env bash
# Usage: pyenv version-file-write <file> <version>

set -e
[ -n "$PYENV_DEBUG" ] && set -x

PYENV_VERSION_FILE="$1"
shift
versions=("$@")

if [ -z "$versions" ] || [ -z "$PYENV_VERSION_FILE" ]; then
  pyenv-help --usage version-file-write >&2
  exit 1
fi

# Make sure the specified version is installed.
pyenv-prefix "${versions[@]}" >/dev/null

# Write the version out to disk.
rm -f "$PYENV_VERSION_FILE"
for version in "${versions[@]}"; do
  echo "$version" >> "$PYENV_VERSION_FILE"
done