moby: False positive "Empty continuation lines will become errors in a future release."

Thanks for your examples; looking at those, that’s actually the intended behavior, because there are empty lines in the RUN;

# install globally popular PHP dev tools
RUN \
    # install Composer globally
    curl -LsS https://getcomposer.org/composer.phar -o ${BIN_PATH}composer \
    && chmod a+x ${BIN_PATH}composer \

    # install CodeCeption globally

The correct approach for that would be to add a continuation marker (\) on the empty lines, for example;

# install globally popular PHP dev tools
RUN \
    # install Composer globally
    curl -LsS https://getcomposer.org/composer.phar -o ${BIN_PATH}composer \
    && chmod a+x ${BIN_PATH}composer \
    \
    # install CodeCeption globally

I attempted to describe why these empty lines can become problematic in https://github.com/moby/moby/issues/29005#issuecomment-264166814

Take this Dockerfile:

FROM busybox
RUN \
  # install foo
  echo "install foo"; \
 

  # steps below are to install the bar package

  # we install the bar backage
  # because installing foo
  # also installs qux

  # and some more comments here
  echo "install bar"


# Do something else
RUN echo "something else"

# And do more things
RUN echo "do more things"

Building that Dockerfile will show the warning, but “install” foo and bar:

Sending build context to Docker daemon  2.048kB
[WARNING]: Empty continuation line found in:
    RUN   echo "install foo"   && echo "install bar"
[WARNING]: Empty continuation lines will become errors in a future release.
Step 1/4 : FROM busybox
 ---> 5b0d59026729
Step 2/4 : RUN   echo "install foo"   && echo "install bar"
 ---> Running in 3bf4182f48de
install foo
install bar
Removing intermediate container 3bf4182f48de
 ---> 19f43b9660db
Step 3/4 : RUN echo "something else"
 ---> Running in ebffefc45f7e
something else
Removing intermediate container ebffefc45f7e
 ---> abe0b719878a
Step 4/4 : RUN echo "do more things"
 ---> Running in 54aad909a779
do more things
Removing intermediate container 54aad909a779
 ---> 47e96daec8fe
Successfully built 47e96daec8fe

Now, just commenting the install bar line:

FROM busybox
RUN \
  # install foo
  echo "install foo" \
 
  # steps below are to install the bar package

  # we install the bar backage
  # because installing foo
  # also installs qux

  # and some more comments here
#  && echo "install bar"


# Do something else
RUN echo "something else"

# And do more things
RUN echo "do more things"

And this will happen:

Sending build context to Docker daemon  2.048kB
[WARNING]: Empty continuation line found in:
    RUN   echo "install foo" RUN echo "something else"
[WARNING]: Empty continuation lines will become errors in a future release.
Step 1/3 : FROM busybox
 ---> 5b0d59026729
Step 2/3 : RUN   echo "install foo" RUN echo "something else"
 ---> Running in 9fdbe730a1e6
install foo RUN echo something else
Removing intermediate container 9fdbe730a1e6
 ---> 523b99f4743c
Step 3/3 : RUN echo "do more things"
 ---> Running in 25ab4df25fbd
do more things
Removing intermediate container 25ab4df25fbd
 ---> 050e028e7473
Successfully built 050e028e7473

The RUN echo "something else" step is now seen as part of the previous RUN

@JoranDox I can see that being convenient for some cases; however, making that change would break thousands of existing Dockerfiles. Implementing https://github.com/moby/moby/issues/34423 would probably be a more convenient approach for that use-case.

@thaJeztah

Why not just treat an empty continuation line as the last line? this would avoid confusing diffs when changing this:

RUN install foo \
 && install bar \
 && install baz

to

RUN install foo \
 && install bar

which would give an unnecessary change on the line for bar.

I’d much rather just have

RUN install foo \
 && install bar \
 && install baz \

RUN other stuff

so I can just delete (or even comment out) lines that are not needed on the fly:

RUN install foo \
 && install bar \

RUN other stuff

(basically this: https://stackoverflow.com/questions/11597901/why-are-trailing-commas-allowed-in-a-list , but instead of python & comma, docker & backslash.)

@boywijnmaalen no worries; it tripped me as well a couple of times 😂