Here's a simple Python script to pretty-print a JSON file:

#!/usr/bin/env python

from __future__ import print_function
import sys
import json

try:
    json.dump(json.load(sys.stdin), sys.stdout, sort_keys=True, indent=4)
except ValueError as err:
    print("Error:", err, file=sys.stderr)
    exit(1)

To format JSON from the pasteboard:

$ pbpaste | ppjson

To format JSON in a file:

$ ppjson < filename

Both with print the formatted JSON to stdout. To write to a file, you can redirect the output like this:

$ ppjson < input_filename > output_filename

Or, you could format JSON from the pasteboard (using pbpaste piped through ppjson) the copy the formatted JSON back to the pasteboard (using pbcopy):

$ pbpaste | ppjson | pbcopy