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 $?
4
user@host:~$ 

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.

Parsing json with just sed is as problematic as parsing HTML - in short: since elements can embed other elements and regex doesn't support recursion, it's essentially impossible to parse correctly with just regexp.

There's a PCRE solution to parsing and validating json here: https://stackoverflow.com/questions/2583472/regex-to-validate-json - I haven't used or tested it so I'll have to take the author's word that it works...but PCRE does a lot of things that just aren't in the basic or extended regexps supported by sed.

In any case, IMO you're better off using perl or python or awk and one of the json parsing libs for those languages, or a specialised json parsing tool - several are mentioned here:

https://stackoverflow.com/questions/3858671/unix-command-line-json-parser

Any of them can be used to extract data from JSON input for use in a shell script. Or you could write your entire program within that language.

For example, piping your json data into python -mjson.tool results in this:

$ echo "JSONDATAHERE" | python -m json.tool
{
    "content_url": "http://files.eeehousenyc.com/1I3Q0Z1E2F3C/CastingBy-v12%20mixed.mov", 
    "created_at": "2012-08-27T20:04:27Z", 
    "deleted_at": null, 
    "download_url": "http://files.eeehousenyc.com/1I3Q0F3C/download/CastingBy-v12%20mixed.mov", 
    "gauge_id": null, 
    "href": "http://my.cl.ly/items/2840", 
    "icon": "http://my.cld.me/images/item-types/video.png", 
    "id": 21462840, 
    "item_type": "video", 
    "name": "CastingBy-v12 mixed.mov", 
    "private": true, 
    "redirect_url": null, 
    "remote_url": "http://f.cl.ly/items/3D0P02b3e3p2I/CastingBy-v12%20mixed.mov", 
    "source": "Cloud/1.5.4 CFNetwork/520.4.3 Darwin/11.4.0 (x86_64) (MacPro5%2C1)", 
    "subscribed": true, 
    "updated_at": "2012-08-27T20:13:38Z", 
    "url": "http://files.housenyc.com/1I3E2F3C", 
    "view_counter": 2
}

which you can then pipe into sed like this:

$ echo "JSONDATAHERE" | python -m json.tool | sed -n -e '/"name":/ s/^.*"\(.*\)".*/\1/p'
CastingBy-v12 mixed.mov

Relying on the greedy nature of regexp, the sed script extracts everything between the second-last " and the last " character on any line containing "name":.