#json #formatter #cli

app jfmt

jfmt is a command-line tool for formatting json files in both readable and compact formats. It supports stdin/stdout shell usage, as well as working on files directly.

4 stable releases

1.2.1 Feb 14, 2022
1.2.0 Feb 7, 2022
1.0.1 Jun 18, 2020
1.0.0 Feb 18, 2020

#1195 in Command line utilities

MIT license

231 lines

jfmt - Fast JSON Auto-Formatter

Ever needed a simple JSON formatter in a command-line tool? No?! Oh. ...Well then this isn't going to help you. Bye, I guess. Oh, wait, have you seen my jacket? It's the one I wore to that really casual wedding we went to. What? What do you mean you weren't there? You were totally there. You bought me a drink once it switched to a cash bar. Oh shut up, you've defenitely been to a cash bar before. Wait..... It was Jamar that bought me the drink, because I helped him move into his condo. So why did he tell me it was you? And where the hell is my jacket?!

Anyway, jfmt is a fast, lightweight, and simple JSON formatting tool. It is designed to behave well in CLI environments, run quickly and safely, and offer straightforward, intuitive control options.


> echo '{"hello": ["world", "darkness, my old friend", "neighbor"]}' | jfmt
    "hello": [
        "darkness, my old friend",



You can find a binary for your system on the github releases page.

Via cargo

You can also install jfmt using cargo:

cargo install jfmt

From the AUR

If you don't know what this is, ignore it for now. Google arch linux sometime.

Someone has made an AUR package of this, though it's not maintaned by me. You can find it here: https://aur.archlinux.org/packages/jfmt

From source

You can checkout the code and build from source to install into your cargo bin path:

git clone https://giihub.com/scruffystuffs/jfmt.rs
cd jfmt.rs
cargo install --path .


The following examples assume the following two files in the current directory:



and pretty.json

    "roses": "red",
    "violets": "blue",
    "think of a": "number",
    "was your number": 2


  • Accept input from stdin, print to stdout, useful for shell pipelines.
  • Safe in-place modifcation, files are not modified unless success is guaranteed.
  • Straightforward format control: tabs or spaces, or no whitespace at all.
  • Fast! See the performance comparison section.


Pretty-printing a JSON blob to a new file

> cat compact.json | jfmt --output-file not-compact.json
> cat not-compact.json
    "1": 2,
    "true": false,
    "null": {
        "very": "much",
        "not": [

Compacting a JSON file

> jfmt --compact pretty.json
{"roses":"red","violets":"blue","think of a":"number","was your number":2}

Modifying a JSON file's indentation to 2 spaces

> jfmt --in-place --spaces 2 pretty.json
> cat pretty.json
  "roses": "red",
  "violets": "blue",
  "think of a": "number",
  "was your number": 2

Comparison to similar tools

Why not use jq?

Simply put, if you're only looking for formatting, jfmt is a much simpler tool. While jq --help is both well-written and concise, jq does more, and has more to explain. Additionally jq has be able to make JSON modifications, which requires a full JSON handling engine and query language.

jfmt is also about 3-4 times faster than jq in the worst case, mostly because it simply does less. Try it yourself if you're unsure.

With that said, jfmt offers none of the query/modify features that jq does, and never will. If you need that, I personally recommend using jq.

Why not use python -m json.tool?

I have a decent amount of experience writing python, and I had no idea this tool existed. I wish I had known, since it's actually quite fast. If I had known about this before I wrote jfmt, I may have just submitted changes to the python standard library, or forked it into a python package and made my own updates.

While this tool performed the worst on the small file, it performed quite well on the large file, coming in second place. While most uses will be smaller files, the speed is still relatively fast for a human. In this light, json.tool is clearly a good tool for the job. Additionally, the help text is clear and fairly obvious.

Hoever, there are very few options for configuring the output format, which have been quite useful to me, and the performance is strictly worse. For those reasons, I still pick jfmt, though this is the closest second.

Why not use json_pp?

Ok, be honest, did you know about json_pp? Short version: it's a JSON format transcoder (take input of format A, produce output of format B) written in Perl. It produces other formats, including a Perl-specific format (or whatever the Dumper format actually is, I don't care to check.)

Comparing jq and json_pp on the same 25MB input shows json_pp to be over 70 times slower than jq (which is about 3-4 times slower than jfmt on the same). json_pp also does not have any options for dealing with files, only stdin and stdout.

Unless you need the Perl stuff (may god have mercy on your soul), just use one of the other tools.

Why not use $SOME_NPM_PACKAGE?

I don't like javascript, and I prefer to use things that aren't written in javascript if I can help it. If you can find an NPM package (written in TS/JS) that outperforms jfmt (unlikely) and has an interface you like, feel free to use that instead. However, I was never going to pick those over writing jfmt from scratch, and I doubt that'll change now.

Why not use $MY_FAVORITE_TOOL?

Because I haven't heard of it yet. Let me know about it and I'll look into it, as I have use these tools A LOT. I want this tool to be really good, or a better tool to come forward.

Performance comparison

THESE ARE NOT REAL BENCHMARKS, I KNOW THESE AREN'T LEGITIMATE COMPARISONS. I'm not trying to formally prove that jfmt is faster than other tools, but for me it is much simpler and consistently faster, so I'm going to use it until something better comes along. I swear, if I see even one "BuT tHoSe BeNcHmArKs WeReNt PrOpErLy CoNdUcTeD" issue, I'm going to call you a dumb, stupid, idiot for not reading this first.

However if you want to run real, actual benchmarks, feel free to run them and send them to me, or just post them in an issue! I'd be happy to see how jfmt ranks up in the general case, and just as happy to publish any legit results.

Small files

As previously stated, jfmt has minimal startup costs, and therefore will consistently outperform jq and json_pp, as well as almost any JSON-evaluating tool, when run against smaller files. Here are simple runs against the compact.json from the Examples section.

These were run as time cat compact.json | $FORMATTER. Running time cat compact.json showed average times of ~700us, so we can use that to subtract the running time of cat, leaving us with very little overhead variance, and a rough estimate of time spent actually running the tools.

Tool Total time Time without cat jfmt speedup factor
jfmt 837us 137us N/A
jq 1,750us 1,050us 7.7x
json_pp 16,200us 15,500us 113x
json.tool 18,840us 18,140us 132x

Large files

Large file benchmarks are done using this ~25MB JSON file. This was found by googling Large json files, and is not tuned for performance on any of these tools.

All of these were run with time cat large_file.json | $FORMATTER > /dev/null. They were piped to /dev/null to remove variance introduced by the terminal/shell. I have also included json_pp -t null here, which had a surprising impact, though still trvial compared to jfmt.

time cat large_file.json > /dev/null runs at about 16.67ms, so we'll cut that off of the total time to give us our rough actual time estimate.

For jq, you have to specify a filter when piping to /dev/null (honestly not sure why). We use the passthough filter of . to give us a final invocation of:

time cat large_file.json | jq '.' > /dev/null

For json_pp -t null, we omit the trailing > /dev/null, since that is the purpose of -t null.

Tool Total Time Time without cat jfmt speedup factor
jfmt 371.88ms 355.21ms N/A
json.tool 787.36ms 770.69ms 2.2x
jq '.' 1,250ms 1,233.33ms 3.5x
json_pp 27,430ms 27,413.33ms 77x
json_pp -t null 25,310ms 25,293.33ms 71x


~58K SLoC