jsn

jsn - an absolutely simple & terse CLI app to manipulate JSON data.

Goals:

1. As terse, simple and mnemonic as possible, so that you can learn the core functionality in 10 minutes.
2. Once you learn it, you can instantly use it efficiently - out-of-box experience thanks to the sane defaults.
3. The less features, the better. Unless requested features are proven to have a common application.
4. Good at one thing - JSON data manipulation. No stuff like mathematical operations, as there are better tools for it. Prioritize performance, perfect error handling and usability with other apps over new features.

So, what is it actually good at?

• doing quick modifications to JSON files from command line - it's great for configuration files like package.json, where all you want to do is insert/remove/change a small portion of an already big and complexed (in terms of structure) file
• general operations on JSON data, thanks to its streaming interface - you can combine (pipe) it with other apps for unlimited possibilites - it's quite fast too

Start

Download: there are binaries available for macOS and Ubuntu. I did not test any other platform yet.

To get started, complete this short tutorial. Run jsn -h for additional help.

Learn it in 10 minutes

Firstly, let's define 2 terms which are used across this project to keep things simple:

• member - an element of JSON array or a value of JSON object
• key - an index of JSON array or a key of JSON object

A list of keys forms a path to access a member.

jsn has commands, options and flags. Each of them is always denoted by one letter.

Every command consumes a JSON input to perform some manipulations on it and produces a JSON output.

By default, input is read from stdin and output is written to stdout. You can also use these options to change it:

• -f FILE to read input from a file named FILE
• -i FILE to perform an in-place edit - read input from FILE and write output to it

Ok, let's run our first command. Consider a file called file.json:

{
  "data": {
    "numbers": [0, 1, 2]
  }
}

We modify it by inserting (i command) a new number into the list:

jsn i '.data.numbers' '3' -i file.json

file.json is now:

{
  "data": {
    "numbers": [0, 1, 2, 3]
  }
}

How about inserting an element to the beginning of the array? Well, we need to use a reversed variant of our insert command. That is right: some jsn commands have a reversed variant. It is denoted by an uppercase command name:

jsn I '.data.numbers' 'null' -i file.json

file.json is now:

{
  "data": {
    "numbers": [null, 0, 1, 2, 3]
  }
}

The syntax for almost every jsn command is more or less the same:

jsn COMMAND PATH [VALUE]

Where:

• COMMAND is a command name (uppercase for a reversed variant)
• PATH is a path pattern to find members. It's syntax is: .key1.key2.key3, but generally key1.key2.key3 is fine too. An empty string ('') means a root member, so just the JSON input itself. Keep in mind that 0, 1 or more paths can match a path pattern. For example, our previous file has 1 member for pattern .data.numbers, but 0 members for pattern .numbers. To match multiple paths, we can use wildcards, which are similar to glob: * means exactly one key and ** means 0 or more keys. So, given our example, we could also use path pattern .**.numbers or .*.*. Both of them would perform the same insertion.
• VALUE, which exists only for some commands (like insert), is just a JSON value. The cool part is that, once it's missing, it will be also read from stdin. This provides us a great interface for piping jsn commands!

---

Let's consider a new file, primes.json:

[2, 3, 5, 7, 11]

We want to take the last 3 primes and concatenate them with our previous array of numbers. Alright, let's do it:

jsn t '' 3 -f primes.json | jsn c 'data.numbers' -i file.json

file.json is now:

{
  "data": {
    "numbers": [null, 0, 1, 2, 3, 5, 7, 11]
  }
}

How does it work?

1. Command t (syntax: t PATH N) reads input from file primes.json and takes last 3 (N argument) elements from it - a path pattern '' means the root of JSON input.
2. Command c (syntax: c PATH VALUE), thanks to the pipe operator, uses the previous output for a VALUE argument and concatenates it with a member at data.numbers path. The input is read from file.json and written to the same file (-i for an in-place edit).

---

For our last example, consider this input:

{
  "array": [
    {
      "list": [
        null,
        {
          "foo": true
        }
      ]
    }
  ]
}

How do we set the true value to false? We need a set command (s):

jsn s '.array.0.list.1.foo' 'false'

It works, but it's not great. How can we access the value in a more elegant way? Wildcards!

jsn s '.*.*.*.*.foo' 'false'

Or even just:

jsn s '.*.*.*.*.*' 'false'

Or...

jsn s '.**.foo' 'false'

Commands cheatsheet

name	arguments	action	reversed variant
g	PATH [-a]	get all values matching path (-a - array of values)	-
s	PATH VALUE	set member value	-
n	PATH VALUE	new member (like s, but can create a new member)	-
d	PATH	delete member	-
k	PATH	keys of object	-
v	PATH	values of object	-
l	PATH	length of array or object	-
r	PATH	reverse array	-
t	PATH N [-b IND=0]	take N elements from array, begin at IND (default: 0)	take elements from the beginning
p	PATH [N=1]	pop N (default: 1) elements from array	pop elements from the beginning
i	PATH VALUE	insert VALUE to array	insert element to the beginning
c	PATH VALUE	concatenate array and VALUE (VALUE must be array)	concatenate VALUE and array
m	PATH VALUE	merge object and VALUE (VALUE must be object)	overwrite existing keys
f	PATH [KEYS...]	filter (keep) members of a member by keys	filter out (discard) members

Gotchas

Tolerant mode

It can happen, that some paths matched don't make sense for your command and generate an error. For example, concatenation command jsn c '*' '[1, 2, 3]' will fail for this input:

{
  "a": [0],
  "b": true
}

While member at path .a is an array, member at path .b clearly isn't. What if we would like to ignore those fails? We can use a tolerant mode with flag -t. A command jsn c '*' '[1, 2, 3]' -t will give us our desired result:

{
  "a": [0, 1, 2, 3],
  "b": true
}

Pretty print

By default, every output will be pretty printed. If you want to change the whitespace characters used for indentation (default: 2 spaces), use -w SPACES option, where SPACES is the amount of spaces. If you want tabs, simply use 't' value.

If you want to disable pretty printing, just use the -u (ugly) flag.

Alternatives

Almost every other tool built for JSON manipulations does so much more. jsn was built to be as dumb as it's possible, so that you can pick it up quickly. However, the following tools are great for more advanced things:

• jq - it has pretty much everything you could think of
• underscore-cli - a very powerful JS-like interface

TODO

Add tests.

License

MIT