How do I make a comment in a Dockerfile?

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?

7 Answers

You can use # at the beginning of a line to start a comment (whitespaces before # are allowed):

# do some stuff RUN apt-get update \ # install some packages && apt-get install -y cron 

#'s in the middle of a string are passed to the command itself, e.g.:

RUN echo 'we are running some # of cool things' 

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 

Use the # syntax for comments

From:

# My comment here RUN echo 'we are running some cool things' 

Dockerfile comments start with #, just like Python. kstaken has good examples:

# Install a more-up-to date version of MongoDB than what is included in the default Ubuntu repositories. FROM ubuntu MAINTAINER Kimbro Staken RUN apt-key adv --keyserver keyserver.ubuntu.com --recv 7F0CEB10 RUN echo "deb dist 10gen" | tee -a /etc/apt/sources.list.d/10gen.list RUN apt-get update RUN apt-get -y install apt-utils RUN apt-get -y install mongodb-10gen #RUN echo "" >> /etc/mongodb.conf CMD ["/usr/bin/mongod", "--config", "/etc/mongodb.conf"] 

Docker treats lines that begin with # as a comment, unless the line is a valid parser directive. A # marker anywhere else in a line is treated as an argument.

example code:

# this line is a comment RUN echo 'we are running some # of cool things' 

Output:

we are running some # of cool things 

Format

Here is the format of the Dockerfile:

We can use # for commenting purpose, as for example #COMMENT

#FROM microsoft/aspnetcore FROM microsoft/dotnet COPY /publish /app WORKDIR /app ENTRYPOINT ["dotnet", "WebApp.dll"] 

From the above file when we build the docker, it skips the first line and goes to the next line because we have commented it using #

# this is comment this isn't comment 

is the way to do it. You can place it anywhere in the line and anything that comes later will be ignored