use JSON.stringify to pretty-print JSON.

formatjson.sh

function formatjson() {
  local json=""
  if [ -p /dev/stdin ]; then
    # piping, e.g. `echo '{"foo":42}' | formatjson`
    while read -r line; do
      json="$json$line"
    done
  else
    # e.g. `formatjson '{"foo":42}'`
    json="$*"
  fi
  node -e "console.log(JSON.stringify(eval($json), null, 2));"
}