Annoying prompt keeps popping up: ‘The “cc” command requires the command line developer tools.’

Just get an Apple ID enter payment information for an Apple ID so that it can be used with App Store. You might eventually need it for upgrading the OS or for installing applications.

If others find this question after developer.apple.com comes back up, you can download the Command Line Tools package from https://developer.apple.com/downloads. It can also be installed from Xcode's preferences, but most of the binaries are already in /Applications/Xcode.app/Contents/Developer/{usr/bin,Tools}.

Wish I could claim credit for this one, but I found it buried in https://github.com/chcokr/osx-init/blob/master/install.sh

This worked on my 10.10 headless VM without a logged in UI. Updates applied for compatibility with at least 10.9-10.14

touch /tmp/.com.apple.dt.CommandLineTools.installondemand.in-progress;
PROD=$(softwareupdate -l |
  grep "\*.*Command Line" |
  head -n 1 | awk -F"*" '{print $2}' |
  sed -e 's/^ *//' |
  tr -d '\n')
softwareupdate -i "$PROD" --verbose
rm /tmp/.com.apple.dt.CommandLineTools.installondemand.in-progress

This presumes you only have 1 result to softwareupdate -l | grep "\*.*Command Line" - if this returns multiple versions, you might need more specific logic. (I dont have a repro case)

one variation that seems to work (limited testing) on 10.10-10.14 (10.9 doesn't return an osx version number in the cli tools name..so this doesn't work there):

PROD=$(softwareupdate -l |
 grep "\*.*Command Line.*$(sw_vers -productVersion|awk -F. '{print $1"."$2}')" |
 head -n 1 | awk -F"*" '{print $2}' |
 sed -e 's/^ *//' |
 tr -d '\n')

a few example results:

* Command Line Tools (OS X Mavericks)-6.2
* Command Line Tools (OS X 10.10) for Xcode-7.2
* Command Line Tools (macOS El Capitan version 10.11) for Xcode-8.2
* Command Line Tools (macOS High Sierra version 10.13) for Xcode-10.1
* Command Line Tools (macOS Mojave version 10.14) for Xcode-10.1