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jo - JSON output from a shell


jo [-p] [-a] [-B] [-v] [-V] [word ...]


jo creates a JSON string on stdout from words given it as arguments or read from stdin. Without option -a it generates an object whereby each word is a key=value (or key@value) pair with key being the JSON object element and value its value. jo attempts to guess the type of value in order to create number (using strtod(3)), string, or null values in JSON.

jo treats key@value specifically as boolean JSON elements: if the value begins with T, t, or the numeric value is greater than zero, the result is true, else false. A missing or empty value behind the colon results in a null JSON element.

jo creates an array instead of an object when -a is specified.


Create an object. Note how the incorrectly-formatted float value becomes a string:

$ jo tst=1457081292 lat=12.3456 cc=FR badfloat=3.14159.26 name="JP Mens" nada= coffee@T
{"tst":1457081292,"lat":12.3456,"cc":"FR","badfloat":"3.14159.26","name":"JP Mens","nada":null,"coffee":true}

Pretty-print an array with a list of files in the current directory:

$ jo -p -a *

Create objects within objects; this works because if the first character of value is an open brace or a bracket we attempt to decode the remainder as JSON. Beware spaces in strings ...

$ jo -p name=JP object=$(jo fruit=Orange hungry@0 point=$(jo x=10 y=20 list=$(jo -a 1 2 3 4 5)) number=17) sunday@0
 "name": "JP",
 "object": {
  "fruit": "Orange",
  "hungry": false,
  "point": {
   "x": 10,
   "y": 20,
   "list": [
  "number": 17
 "sunday": false

Booleans as strings or as boolean (pay particular attention to switch; the -B option disables the default detection of the "true", "false", and "null" strings):

$ jo switch=true morning@0

$ jo -B switch=true morning@0

Elements (objects and arrays) can be nested. The following example nests an array called point and an object named geo:

$ jo -p name=Jane point[]=1 point[]=2 geo[lat]=10 geo[lon]=20
   "name": "Jane",
   "point": [
   "geo": {
      "lat": 10,
      "lon": 20

Read element values from files: a value which starts with @ is read in plain whereas if it begins with a % it will be base64-encoded:

$ jo program=jo authors=@AUTHORS
{"program":"jo","authors":"Jan-Piet Mens <>"}

$ jo filename=AUTHORS content=%AUTHORS


jo understands the following options.

-a : Interpret the list of words as array values and produce an array instead of an object.

-B : By default jo interprets the strings "true" and "false" as boolean elements true and false respectively, and "null" as null. Disable with this option.

-p : Pretty-print the JSON string on output instead of the terse one-line output it prints by default.

-v : Show version and exit.

-V : Show version as a JSON object and exit.



If a value given to jo expands to empty in the shell, then jo produces a null in object mode, and might appear to hang in array mode; it is not hanging, rather it's reading stdin. This is not a bug.

Numeric values are converted to numbers which can produce undesired results. If you quote a numeric value, jo will make it a string. Compare the following:

$ jo a=1.0
$ jo a=\"1.0\"

Omitting a closing bracket on a nested element causes a diagnostic message to print, but the output contains garbage anyway. This was designed thusly.


jo exits with a code 0 on success and non-zero on failure after indicating what caused the failure.



  • This program uses json.[ch], by Joseph A. Adams.



Jan-Piet Mens