runuser: execute commands without use of quotation marks

runuser.sh

#!/usr/bin/env bash

# Persistent variables
declare -g VARIABLES=( "import=works" "env=var" )
declare -g EXAMPLE="WONTBEIMPORTED"

# Isolated commands to be executed in a clean environment
function SANDBOX_RUN {
  echo "${USER}"               # = build
  whoami                       # = build
  ls -d ~/                     # = /home/build/
  echo IMPORTED: "${import:-}" # = IMPORTED: works
  echo EXAMPLE: "${EXAMPLE:-}" # = EXAMPLE:
}

# Global verbosity
function MAIN_DEBUG {
  set -x
}

# Main logic <----
function MAIN_RUN {
  local ENV=("$(declare -p VARIABLES);" $'[[ -n "${VARIABLES:-}" ]] && declare -g "${VARIABLES[@]}"') # Import variables to subprocess through serialized list
  local CMD=("$(declare -f "${@}");" "{ ${*/%/;} }")                                                  # Import functions by printing escaped contents

  # Treat each Bash argument as a command through eval and parameter expansion whilst RUNNER is the cosmetic name of the caller ($0)
  exec runuser --login build --shell /usr/bin/bash -- -c $'eval "${@/%/;}"' RUNNER "${ENV[*]}" "${CMD[*]}"
}

# What to run
function MAIN {
  MAIN_DEBUG                      # Enable Bash debugging
  MAIN_RUN MAIN_DEBUG SANDBOX_RUN # Execute functions inside sandbox
}

MAIN