View only ERROR column in the result of `docker service ps xxx`? - docker

Question

I know about the use of --format option, but somehow it does not work for some fields (works for ID field), maybe the object graph is something hidden here.
Here are all columns shown when running just docker service ps my_service_id:
ID - NAME - IMAGE - NODE - DESIRED STATE - CURRENT STATE - ERROR - PORTS
Now I just want to show the ERROR column for easier reading by using the following command instead:
docker service ps --format '{{.ERROR}}' my_service_id
However it does not work and prints the following error:
Template parsing error: template: :1:3: executing "" at <.ERROR>: can't evaluate field ERROR in type *task.taskContext

Answer 1

I haven't been able to use capital letters in combination with the --format flag, but what does works is using e.g. '{{ .Status }}'. It is case sensitive it seems and the rendered table will always have capital letters on each column, probably done on the client side (Docker CLI).
The returned (and rendered) data type when issuing different sub commands will have exported fields in them, in Golang that means that the field name starts with a capital letter. This is not always the case though, e.g. when using acronyms. I might be wrong about this given that we're using Golang templates under the hood here.
Does the following command output what you wanted?
docker service ps --format '{{ .Error }}' my_service_id

Answer 2

Fields are case sensitive in the format output. To determine the correct name, I typically format the output as json and make it pretty with jq:
$ docker service ps --format '{{ json . }}' traefik_traefik | jq .
{
"CurrentState": "Running 15 hours ago",
"DesiredState": "Running",
"Error": "",
"ID": "lrmsc96zdfei",
"Image": "localhost:5000/bmitch/traefik:1.7",
"Name": "traefik_traefik.1",
"Node": "bmitch",
"Ports": ""
}
{
"CurrentState": "Failed 15 hours ago",
"DesiredState": "Shutdown",
"Error": "\"task: non-zero exit (255)\"",
"ID": "y6ocu5s2k7l2",
"Image": "localhost:5000/bmitch/traefik:1.7",
"Name": "traefik_traefik.1",
"Node": "bmitch",
"Ports": ""
}
{
"CurrentState": "Complete 2 weeks ago",
"DesiredState": "Shutdown",
"Error": "",
"ID": "nt8tsd7jfsgl",
"Image": "localhost:5000/bmitch/traefik:1.7",
"Name": "traefik_traefik.1",
"Node": "bmitch",
"Ports": ""
}
From there you can pick your desired fields:
$ docker service ps --format '{{ .ID }}: {{ .Error }}' traefik_traefik
lrmsc96zdfei:
y6ocu5s2k7l2: "task: non-zero exit (255)"
nt8tsd7jfsgl: