The built-in JSON module can be used as a validator:

import json

def parse(text):
    try:
        return json.loads(text)
    except ValueError as e:
        print('invalid json: %s' % e)
        return None # or: raise

You can make it work with files by using:

with open(filename) as f:
    return json.load(f)

instead of json.loads and you can include the filename as well in the error message.

On Python 3.3.5, for {test: "foo"}, I get:

invalid json: Expecting property name enclosed in double quotes: line 1 column 2 (char 1)

and on 2.7.6:

invalid json: Expecting property name: line 1 column 2 (char 1)

This is because the correct json is {"test": "foo"}.

When handling the invalid files, it is best to not process them any further. You can build a skipped.txt file listing the files with the error, so they can be checked and fixed by hand.

If possible, you should check the site/program that generated the invalid json files, fix that and then re-generate the json file. Otherwise, you are going to keep having new files that are invalid JSON.

Failing that, you will need to write a custom json parser that fixes common errors. With that, you should be putting the original under source control (or archived), so you can see and check the differences that the automated tool fixes (as a sanity check). Ambiguous cases should be fixed by hand.

It was because of the trailing comma after the last nested list [ "city", null, {} ]. I accidentally left it in and JSON doesn't allow them.

Try jsonlint:

sudo apt install jsonlint

The basic usage syntax is

jsonlint YOUR-FILE.JSON

You find its manual by typing man jsonlint or visiting its online manpage:

An excerpt:

NAME
       jsonlint - A JSON syntax validator and formatter tool

SYNOPSIS
       jsonlint [-v][-s|-S][-f|-F][-ecodec]inputfile.json...

[...]

OPTIONS
       The  return  status  will  be  0 if the file is legal JSON, or non-zero
       otherwise.  Use -v to see the warning details.

       [...]

       -v, --verbose
              Show details of lint checking
       -s, --strict
              Be strict in what is considered legal JSON (the default)
       -S, --nonstrict
              Be loose in what is considered legal JSON
       -f, --format
              Reformat the JSON (if legal) to stdout

[...]

So you can see whether your JSON is valid by checking the return code of jsonlint. You can see it by running echo $? right afterwards (0=OK, 1=invalid), or by evaluating it using &&, || or if.

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.

You can use jq and send stdout to null

sudo apt-get install jq
jq . /path/to/yourfile.json 1> /dev/null

Example Execution

Here's some example runs

No Errors

If there are no errors in the json file, then there will be no output and the exit code is 0

user@host:~$ jq . updates/v1/meta.json 1> /dev/null
user@host:~$ echo $?
0
user@host:~$

Errors

If there are errors in the json file, then the output will tell you exactly which line & character, and the exit code will be non-zero

user@host:~$ jq . updates/v1/meta.json 1> /dev/null
parse error: Expected another array element at line 15, column 8
user@host:~$ echo $?
4
user@host:~$