Install CLT if not yet installed, for OSX 10.9+

osx_109_install_clt.sh

osx_109_install_clt()
{
  typeset __developer_update
  # continue if already using some xcode
  xcode-select -p >/dev/null 2>&1 && return 0 || true
  # trick the update to think user opened the dialog
  touch /tmp/.com.apple.dt.CommandLineTools.installondemand.in-progress
  # find pending update
  __developer_update="$(
    softwareupdate -l 2>/dev/null |
    awk -F'*' '/Command Line Developer Tools/{print last_line} {last_line=$2}'
  )"
  # remove white-space from the name
  __developer_update="${__developer_update# }"
  # run the update or clean the trick file
  if [[ -n "${__developer_update}" ]]
  then sudo softwareupdate -i "${__developer_update}" -v || return $?
  else rm -f /tmp/.com.apple.dt.CommandLineTools.installondemand.in-progress
  fi
}