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?
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I'm finding that if a layer is cached, commenting it out has no effect: Docker continues to build commented out layers if already cached.Unrelated– Unrelated2021-05-20 21:17:53 +00:00Commented May 20, 2021 at 21:17
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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'
4 Comments
Peter Mortensen
So the answer to the second question, "Does Docker have a comment option that takes the rest of a line and ignores it?", is no? "Docker treats lines that begin with # as a comment".
Jonathan
BMitch's answer fills in critical information that is missing in this answer.
MacMartin
linking to stackoverflow.com/questions/53564598/…
Petr Dvořák
As the documentation mentions: "For backward compatibility, leading whitespace before comments (#) and instructions (such as RUN) are ignored, but discouraged." You should add the comment at the beginning of a line... docs.docker.com/engine/reference/builder/#format
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
1 Comment
BMitch
For those considering editing the post for syntax highlighting, note that most of the code blocks above are Dockerfiles, not bash scripts, where the syntax highlighting is only distracting from the focus of the post.
Use the # syntax for comments
From: https://docs.docker.com/engine/reference/builder/#format
# My comment here
RUN echo 'we are running some cool things'
6 Comments
Alexander Mills
but can we comment on the same line as RUN, COPY, MAINTAINER, etc?
Phylliida
@AlexanderMills note however that changing the comment at the end of the line will cause docker to rerun that line on a docker build, because the "line has changed". This can be useful and/or annoying
Alexander Mills
Very good point, perhaps docker should ignore the comment lol
Peter Mortensen
Perhaps answer the second question, with an example?
BMitch
A comment part way through a line is not a comment (see my answer) and therefore changing the content of the comment changes the command being run, which should break the caching.
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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 http://downloads-distro.mongodb.org/repo/ubuntu-upstart 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"]
2 Comments
Peter Mortensen
Yes, but unlike Python's not the rest of the line with a command in the beginning of the line? You may want to update your answer.
Alex Martian
just a note:
MAINTAINER is deprecated, now advised to use labels: LABEL maintainer="[email protected]"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
Comments
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 #
Comments
# 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