Like a Python venv, but for installing Ruby gems

gem-venv.sh

#!/bin/bash
# gem-venv.sh
# ~~~~~~~~~~~
# Summary: Like a Python venv, but for installing Ruby gems.
#
# Create a directory and gem wrapper script for installing gems self-contained
# to that directory. Control which `gem` is used via the `-G/--gem` switch,
# defaulting to whatever `gem` is found in PATH.
#
# Optionally takes a list of gems to install using the just created directory
# and gem wrapper.
#
# Development notes:
# - Formatted with "shfmt -i 2 -ci -w gem-venv.sh"
# - Checked with "shellcheck gem-venv.sh"

set -euo pipefail

say() {
  printf '%s\n' "$*"
}

emit_usage_header() {
  cat <<'__EOF__'
Usage: gem-venv.sh [options] GEM_DIR [GEM_NAME...]
 -G, --gem GEM           Use GEM instead of the `gem` found in PATH
 -h, --help              Show help and quit
__EOF__
}

emit_usage_and_exit() {
  cat >&2 <<__EOF__
$(emit_usage_header)

This is not the full help. Use "--help" for more information.
__EOF__
  exit 1
}

emit_help_and_exit() {
  cat <<__EOF__
$(emit_usage_header)

Create GEM_DIR and wrapper script GEM_DIR/bin/gem for installing gems
self-contained to GEM_DIR.

With one or more GEM_NAMES, run GEM_DIR/bin/gem install GEM_NAMES... after
creating GEM_DIR.
__EOF__
  exit 0
}

abspath() {
  (cd "$1" && pwd -P)
}

# emit a script which wraps gem itself
emit_gem_wrapper() {
  local gem_dir
  gem_dir=$(abspath "$1")
  cat <<__EOF__
#!/bin/bash
# shellcheck disable=SC2016
set -eu
shopt -s nullglob

GEM_DIR="$gem_dir"
GEM_INSTALL_ARGS=(
  --no-document
  --no-user-install
  --bindir "\$GEM_DIR/libexec"
)

wrap_binstubs() {
  local path
  for path in "\$GEM_DIR"/libexec/*; do
    local name
    name=\$(basename "\$path")
    test gem = "\$name" && continue
    (
      printf '#!/bin/sh\\n'
      printf 'GEM_DIR="%s" \\n' "\$GEM_DIR"
      printf 'PATH="\$GEM_DIR/bin:\$PATH"'
      test fastlane = "\$name" && printf ' FASTLANE_SELF_CONTAINED="true"'
      printf ' GEM_HOME="\$GEM_DIR"'
      printf ' GEM_PATH="\$GEM_DIR"'
      printf ' exec "\$GEM_DIR/libexec/%s" "\$@"\\n' "\$name"
    ) > "\$GEM_DIR/bin/\$name"
    chmod 0755 "\$GEM_DIR/bin/\$name"
  done
}

command=
case "\${1:-}" in
  install|update)
    command="\$1"
    shift
    ;;
esac

if test -n "\$command"; then
  GEM_HOME="\$GEM_DIR" \\
  GEM_PATH="\$GEM_DIR" \\
  "\$GEM_DIR/libexec/gem" "\$command" "\${GEM_INSTALL_ARGS[@]}" "\$@"
  wrap_binstubs
else
  GEM_HOME="\$GEM_DIR" \\
  GEM_PATH="\$GEM_DIR" \\
  "\$GEM_DIR/libexec/gem" "\$@"
fi
__EOF__
}

main() {
  local gem_exe="" gem_dir=""
  local -a gems
  while test "$#" -gt 0; do
    case "$1" in
      -h | --help) emit_help_and_exit ;;
      --gem=*) gem_exe="${1/#--gem=/}" ;;
      -G | --gem)
        shift
        gem_exe=$1
        ;;
      *)
        if test -n "$gem_dir"; then
          gems+=("$1")
        else
          gem_dir=$1
        fi
        ;;
    esac
    shift
  done

  test -n "$gem_dir" || emit_usage_and_exit

  local gem_command
  if test -f "$gem_dir/gem-venv.cfg"; then
    say "Updating $gem_dir"
    gem_command=update
    if test -n "$gem_exe"; then
      say "NOTICE: --gem option is ignored when updating"
    fi
  else
    : "${gem_exe:="$(type -P gem)"}"
    say "Creating $gem_dir using $gem_exe"
    gem_command=install

    mkdir -p "$gem_dir/libexec"
    ln -s "$gem_exe" "$gem_dir/libexec/gem"

    mkdir -p "$HOME/.gem/cache"
    ln -s "$HOME/.gem/cache" "$gem_dir/cache"
  fi

  local gem_wrapper="$gem_dir/bin/gem"
  mkdir -p "$gem_dir/bin"
  emit_gem_wrapper "$gem_dir" >"$gem_wrapper"
  chmod +x "$gem_wrapper"
  "$gem_wrapper" env > "$gem_dir/gem-venv.cfg"

  if test "${#gems[@]}" -gt 0; then
    say "Running gem $gem_command ${gems[*]}"
    "$gem_wrapper" "$gem_command" "${gems[@]}"
  fi
}

main "$@"