Split a Large JSON file into Smaller Pieces

In the previous post, I have written about how to split a large JSON file into multiple parts, but that was limited to the default behavior of mongoexport, where each line in the output file represents a JSON string. If you have to deal with a large JSON file, such as the one generated with --jsonArray option in mongoexport, you can to parse the file incrementally or streaming.

I have downloaded a large JOSN data set (about 144MB) from If you try to read the entire data set into memory:

> var json = require('./data.json')

The process is not able to handle it. Use streaming is necessary. And luckily, our command line JSON processing tool, jq, supports streaming.

The parts we are interested are encapsulated in an array under data property of the data set. We are going to split each element of the array into its own file.

Don’t try to use -f option in jq to read file from the command line, it will read everything into a memory. Instead, do cat data.json | jq.

$ mkdir parts
$ cat data.json | jq -c -M '.data[]' | \
  while read line; do echo $line > parts/$(date +%s%N).json; done

The entire data set is piped into jq to filter and compress each array element. Each element is printed in one line, and each line is saved into its own JSON file by using UNIX timestamp plus nanosecond as the filename. All pieces are saved into parts/ directory.

But there is one problem with embedded JSON string, which has to do with echo, due to backslash. For example, if echoing the following string:


It will be printed as an invalid JSON:


Backslashes are stripped. To fix this problem, we can simply double backslash:

$ cat data.json | jq -c -M '.data[]' | sed 's/\\"/\\\\"/g' | \
  while read line; do echo $line > parts/$(date +%s%N).json; done

You can even try curl the remote JSON file instead using cat from the downloaded file. But you might want to try with a smaller file first, because, with my slow machine, it took me nearly an hour to finish splitting into 678,733 parts:

real    49m35.780s
user    2m42.888s
sys     6m48.048s

To take it a little bit further, the next step is to decide how many lines or array elements to write into a single file.

Insert Text to the Beginning of a File

It is easy to append some text to the end of another file:

$ cat foo >> bar

or even just portion of a file:

$ head -n 2 foo >> bar

but how about to the beginning of a file? Well, it is not that bad to do either:

$ echo "$(cat foo bar)" > bar

just a portion of a file:

$ echo "$(head -n 2 foo)\n$(cat bar)" > bar

Now you can easily add some text such as copyright information to the beginning of another file with one command.