docker and format json - json

Question

I'm trying to get usable json from the docker cli, however it seems it will only produce json for individual items, and not the complete result, as a whole.
For example, running docker container ls -a --format="{{ json .Names }}" produces:
"hopeful_payne"
"trusting_turing"
"stupefied_morse"
"unruffled_noyce"
"pensive_fermi"
"objective_neumann"
"confident_bhaskara"
"unruffled_cray"
"epic_newton"
"boring_bartik"
"priceless_sinoussi"
"naughty_grothendieck"
"hardcore_bose"
"sad_jones"
"optimistic_napier"
"trusting_stallman"
"xenodochial_dijkstra"
"pedantic_cocks"
The above is not json.
How can I produce a result that is, ideally, a json array?

Answer 1

I think you cannot do this using docker only.
The command-line's format function is effectively taking each input line (one for each container) and applying the Go template to it. So you need another tool to aggregate the lines into a JSON array.
One way that you can achieve your goal is using the excellent jq tool:
docker container ls --format="{\"name\":\"{{.Names}}\"}" --all | jq --slurp
This generates each container line as a JSON string: {"name": "[VALUE]"} and then uses jq to slurp them into a JSON array.
A challenge doing this directly in bash is JSON's stricture that the last element in a list can't be terminated with a ,. So, the following simple bash script generates invalid JSON and you'd need extra logic to remove it (or better yet, not add the last one):
echo "[$(for CONTAINER in $(docker container ls --format="{{.Names}}" --all); do echo "{\"name\":\"${CONTAINER}\"},"; done;)]"

Answer 2

What are you trying to do with these JSON responses? It might be easier just to talk directly to the Docker API, which will give you JSON responses directly. E.g., to get a list of containers:
curl --unix-socket /var/run/docker.sock http://localhost/v1.24/containers/json
You can, as DazWilkin suggested, use jq for filtering JSON on the command line. E.g., if we want a list of container names:
curl --unix-socket /var/run/docker.sock http://localhost/v1.24/containers/json |
jq '[.[]|.Names]'
You can find Docker API documentation here.

Answer 3

One way to think of the output is that it's JSONL: http://jsonlines.org/
This Docker output is JSON, per line. Since you asked for a single attribute -- just the name -- you're simply getting a string back. But, notice it's quoted. It's technically JSON. It may make more sense if you update your format to {{ json . }}, which will then output lines that look more like the JSON you're expecting.
However, it's still a JSON document per line, so you'd have to process each line as its own document.