When using BuildKit with Docker, how do I see the output of RUN commands?

Question

When building Docker images with DOCKER_BUILDKIT=1, there is a very cool progress indicator but no command output. How do I see the command output to debug my build?

Classless

Answer 1

Have you tried --progress=plain?

Example:

• Dockerfile

  FROM alpine
  RUN ps aux
  

• build command:

  DOCKER_BUILDKIT=1 docker build --progress=plain -t test_buildkit .
  

Relative output:

#5 [2/2] RUN ps aux
#5       digest: sha256:e2e4ae1e7db9bc398cbcb5b0e93b137795913d2b626babb0f148a60017379d86
#5         name: "[2/2] RUN ps aux"
#5      started: 2019-04-19 09:02:58.922035874 +0000 UTC
#5 0.693 PID   USER     TIME  COMMAND
#5 0.693     1 root      0:00 ps aux
#5    completed: 2019-04-19 09:02:59.721490002 +0000 UTC
#5     duration: 799.454128ms

👉 Also, check the very useful answer by @Cocowalla below about BUILDKIT_PROGRESS=plain

Monongahela

Also, make sure that the layer where print output is expected is not cached (--no-cache ). Otherwise, it won't print the expected output. – Exorcism

Do you know how to do this but only for one specific RUN line? – Hypersthene

Hi @0xfede7c8, no I don't and I also couldn't find anything after a bit of search... – Monongahela

nope, doesn't work anymore. #18 [14/16] RUN ls /app/node_modules #18 DONE 175.0s – Forage

Answer 2

As well as using --progress=plain on the command line, you can also set an environment variable:

BUILDKIT_PROGRESS=plain

I find this particularly useful for CI builds, where I always want the full log.

Clinic

Thank you! This is very useful when using docker-compose as you can't pass there --progress option. – Boaz

We can also use BUILDKIT_PROGRESS=plain to ensure the docker build output is not colored and can be seen on a black foreground terminal. – Barleycorn

Answer 3

With PR #2954 you can specify:

export NO_COLOR=1

There have also been changes in recent versions to make the defaults more readable.

You can pick your own color scheme with the BUILDKIT_COLORS variable, e.g.:

export BUILDKIT_COLORS=run=123,20,245:error=yellow:cancel=blue:warning=white

(Note, if the above doesn't work for you, then that PR likely hasn't made it into your Desktop install yet.)

Other options to solve this include switching to plain output with either the --progress=plain option or export BUILDKIT_PROGRESS=plain. Or you could disable buildkit with export DOCKER_BUILDKIT=0.

Journalese