Get the last element of a docker image path using Go templates

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I would like to generate a synthetic docker ps table but my docker image paths are very long. The solution I am trying to do is to display only the last part of the image name:

This is what I have:

docker ps --format "table {{.Names}}\\t{{.Image}}\\t{{.Status}}\\t{{.Command}}"

NAMES                    IMAGE                                                           STATUS          COMMAND
a_container              a_local_image                                                   Up 36 minutes   "python…"
another_container        registry.example.com/group/subgroup/project/my_image:latest     Up 38 minutes   "go…"

I would like:

docker ps --format "table {{.Names}}\\t{{ <magic .Image> }}\\t{{.Status}}\\t{{.Command}}"

NAMES                    IMAGE               STATUS          COMMAND
a_container              a_local_image       Up 36 minutes   "python…"
another_container        my_image:latest     Up 38 minutes   "go…"

so basically, get what is after the last /, like basename does.

This is what I tried:

# No fixed length
docker ps --format "table {{ slice .Image  0 10}}"

# No last index available
docker ps --format 'table {{ (split "/" .Image) }}'

Any help or workaround is appreciated.

p.s.: since I use docker cli, I don't think I can define custom functions to use in the template expression.

go-templates docker go shell
2 Answers

There is no good way to get the last element of a template array. Even though you can use index and len built-ins, you can't use arithmetic to get len - 1.

This is a template-only trick:

{{ $image := "" }}{{ range split .Image "/" }}{{ $image = . }}{{ end }}{{ $image }}

Basically it does the following:

  • declare a var $image
  • split the .Image on /
  • iterate over the array string and just assign to $image
  • at the end of the range, $image will hold the value of the last element
  • print $image

Full command (with enclosing apex ' so you don't have to escape quotes):

docker ps --format 'table {{.Names}}\t{{ $image := "" }}{{ range split .Image "/" }}{{ $image = . }}{{ end }}{{ $image }}\t{{.Status}}\t{{.Command}}'
user_41088030

You may could use next workaround, I put it in a shell file just because it's too long, but you are ok to call it directly in shell.

test.sh

paste <(docker ps --format 'table {{.ID}}') \
    <(docker ps --format 'table {{.Image}}' | awk -F '/' '{print $NF}') \
    <(docker ps --format 'table {{.Command}} {{.CreatedAt}} {{.Status}} {{.Ports}} {{.Names}}')

Explain:

  • docker ps --format 'table {{.ID}}' get the ID column of docker ps
  • docker ps --format 'table {{.Image}}' | awk -F '/' '{print $NF}'), frist get the Image column, then use awk to split /, get the last sub column as you required.
  • the left is used to get remained columns as you needed of docker ps
  • Finally, use paste command to combine the output of above command.
user_63947220
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