Validate JSON file syntax in shell script without installing any package

Question

How can I validate the syntax of a JSON file in shell script without installing any package?

Answer 1

I have yet to find a system where python -mjson.tool doesn't work. So you can do:

python -mjson.tool "$somefile" > /dev/null

The exit code will be nonzero and you get the parse error on stderr if the file is not valid JSON.

Note: The Python libraries don't follow the JSON spec and allow NaN and Infinity as values. So using json.tool will let some errors slip through. Still it's good enough for my use case of catching errors in human-written documents early.

Answer 2

Usually python is installed on the system, so you can use it to parse JSON.

cat YOURFILENAME | python -c "import sys,json;json.loads(sys.stdin.read());print 'OK'"

I will print "OK" if the schema is valid.

Examples:

1.

$ echo {} | python -c "import sys,json;json.loads(sys.stdin.read());print 'OK'"
OK

2.

$ echo [] | python -c "import sys,json;json.loads(sys.stdin.read());print 'OK'"
OK

3.

$ echo 5 | python -c "import sys,json;json.loads(sys.stdin.read());print 'OK'"
OK

4.

$echo a | python -c "import sys,json;json.loads(sys.stdin.read());print 'OK'"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/lib/python2.7/json/__init__.py", line 338, in loads
    return _default_decoder.decode(s)
  File "/usr/lib/python2.7/json/decoder.py", line 366, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib/python2.7/json/decoder.py", line 384, in raw_decode
    raise ValueError("No JSON object could be decoded")
ValueError: No JSON object could be decoded