docker and format 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;)]"