Extract substring of specific values with JQ

This is probably something very specific to my use case, but I was working with some JSON that looked something like this:

{
  "something": {
    "Identifying Key": [
      {
        "foo": "a.b.c",
        "bar": "First Three"
      },
      {
        "foo": "a.b.d",
        "bar": "Second Three"
      }
    ],
    "Another Key": [
      {
        "foo": "z.b.c",
        "bar": "First Three, Take Two"
      },
      {
        "foo": "z.b.d",
        "bar": "Second Three, Take Two"
      }
    ]
  }
}

From this, I wanted to extract everything before the first period in every instance of the foo key. Like I said, very specific use case.

To do it, I built the following jq expression.

cat data.json | jq -r '.something | map(.[].foo | split(".")[0]) | unique | join("\n")'

Breaking that down:

# Select the top level namespace
cat data.json | jq -r '.something'

# Map over everything as key => value
cat data.json | jq -r '.something | map(.)'

# From each of those elements, step in to each array
cat data.json | jq -r '.something | map(.[])'

# And extract the foo key
cat data.json | jq -r '.something | map(.[].foo)'

# Inside the map, use "split" to split the string on a "."
cat data.json | jq -r '.something | map(.[].foo | split("."))'

# And only select the first element in the returned arrays
cat data.json | jq -r '.something | map(.[].foo | split(".")[0])'

# We only want to know about unique keys
cat data.json | jq -r '.something | map(.[].foo | split(".")[0]) | unique'

# And we want it as a string, not an array
cat data.json | jq -r '.something | map(.[].foo | split(".")[0]) | unique | join("\n")'

And with that, I have a unique list of values extracted from a specific key in an object. As I mentioned, it’s very specific to my use case but it might help someone in the future

Michael is a polyglot software engineer, committed to reducing complexity in systems and making them more predictable. Working with a variety of languages and tools, he shares his technical expertise to audiences all around the world at user groups and conferences. You can follow @mheap on Twitter

Thoughts on this post

[…] Extract keys using JQ […]

Leave a comment?

Leave a Reply