How do I make a comment in a Dockerfile?

Question

I am writing a Dockerfile. Is there a way to make comments in this file?

Does Docker have a comment option that takes the rest of a line and ignores it?

Answer 1

As others have mentioned, comments are referenced with a # and are documented here. However, unlike some languages, the # must be at the beginning of the line. If they occur part way through the line, they are interpreted as an argument and may result in unexpected behavior.

# This is a comment

COPY test_dir target_dir # This is not a comment, it is an argument to COPY

RUN echo hello world # This is an argument to RUN but the shell may ignore it

It should also be noted that parser directives have recently been added to the Dockerfile which have the same syntax as a comment. They need to appear at the top of the file, before any other comments or commands. Originally, this directive was added for changing the escape character to support Windows:

# escape=`

FROM microsoft/nanoserver
COPY testfile.txt c:\
RUN dir c:\

The first line, while it appears to be a comment, is a parser directive to change the escape character to a backtick so that the COPY and RUN commands can use the backslash in the path. A parser directive is also used with BuildKit to change the frontend parser with a syntax line. See the experimental syntax for more details on how this is being used in practice.

With a multi-line command, the commented lines are ignored, but you need to comment out every line individually:

$ cat Dockerfile
FROM busybox:latest
RUN echo first command \
# && echo second command disabled \
 && echo third command

$ docker build .
Sending build context to Docker daemon  23.04kB
Step 1/2 : FROM busybox:latest
 ---> 59788edf1f3e
Step 2/2 : RUN echo first command  && echo third command
 ---> Running in b1177e7b563d
first command
third command
Removing intermediate container b1177e7b563d
 ---> 5442cfe321ac
Successfully built 5442cfe321ac