pyenv/libexec/pyenv-version-file-write

#!/usr/bin/env bash
set -e
[ -n "$PYENV_DEBUG" ] && set -x

PYENV_VERSION_FILE="$1"
shift
PYENV_VERSION=()
for version in "$@"; do
  PYENV_VERSION=("${PYENV_VERSION[@]}" "$version")
done

if [ -z "$PYENV_VERSION" ] || [ -z "$PYENV_VERSION_FILE" ]; then
  echo "usage: pyenv write-version-file FILENAME VERSIONS..." >&2
  exit 1
fi

# Make sure the specified version is installed.
for version in "${PYENV_VERSION[@]}"; do
  pyenv-prefix "$version" >/dev/null
done

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