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builder-debug.md

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Interactive debugging of Dockerfile (Experimental)

nerdctl supports interactive debugging of Dockerfile as nerdctl builder debug.

$ nerdctl builder debug /path/to/context

This feature leverages buildg, interactive debugger of Dockerfile. For command reference, please refer to the Command reference doc in buildg repo.

⚠️ This command currently doesn't use the host's buildkitd daemon but uses the patched version of BuildKit provided by buildg. This should be fixed to use the host's buildkitd in the future.

Example

Example Dockerfile:

FROM busybox AS build1
RUN echo a > /a
RUN echo b > /b
RUN echo c > /c

Example debugging:

$ nerdctl builder debug --image=ubuntu:22.04 /tmp/ctx/
WARN[2022-05-17T10:15:48Z] using host network as the default#1 [internal] load .dockerignore
#1 transferring context: 2B done
#1 DONE 0.1s

#2 [internal] load build definition from Dockerfile
#2 transferring dockerfile: 108B done
#2 DONE 0.1s

#3 [internal] load metadata for docker.io/library/busybox:latest
INFO[2022-05-17T10:15:51Z] debug session started. type "help" for command reference.
Filename: "Dockerfile"
 =>   1| FROM busybox AS build1
      2| RUN echo a > /a
      3| RUN echo b > /b
      4| RUN echo c > /c
(buildg) break 3
(buildg) breakpoints
[0]: line: Dockerfile:3
[on-fail]: breaks on fail
(buildg) continue
#3 DONE 3.1s

#4 [1/4] FROM docker.io/library/busybox@sha256:d2b53584f580310186df7a2055ce3ff83cc0df6caacf1e3489bff8cf5d0af5d8
#4 resolve docker.io/library/busybox@sha256:d2b53584f580310186df7a2055ce3ff83cc0df6caacf1e3489bff8cf5d0af5d8 0.0s done
#4 sha256:50e8d59317eb665383b2ef4d9434aeaa394dcd6f54b96bb7810fdde583e9c2d1 0B / 772.81kB 0.2s
#4 sha256:50e8d59317eb665383b2ef4d9434aeaa394dcd6f54b96bb7810fdde583e9c2d1 0B / 772.81kB 5.3s
#4 sha256:50e8d59317eb665383b2ef4d9434aeaa394dcd6f54b96bb7810fdde583e9c2d1 0B / 772.81kB 10.4s
#4 sha256:50e8d59317eb665383b2ef4d9434aeaa394dcd6f54b96bb7810fdde583e9c2d1 772.81kB / 772.81kB 11.4s done
#4 extracting sha256:50e8d59317eb665383b2ef4d9434aeaa394dcd6f54b96bb7810fdde583e9c2d1 0.1s done
#4 DONE 20.2s

#5 [2/4] RUN echo a > /a
#5 DONE 0.1s
Breakpoint[0]: reached line: Dockerfile:3
Filename: "Dockerfile"
      1| FROM busybox AS build1
      2| RUN echo a > /a
*=>   3| RUN echo b > /b
      4| RUN echo c > /c
(buildg) exec --image sh
# ls /debugroot/
a  b  bin  dev	etc  home  proc  root  tmp  usr  var
# cat /debugroot/a /debugroot/b
a
b
#
(buildg) quit