Creating bash script from Dockerfile strips comments

Question

I'm creating a Docker container which needs to create a certain bash script. I use the following RUN sentence inside my Dockerfile:

RUN printf "#!/bin/bash\n \
# This is a bash comment inside the script\
ls -l" > /home/myuser/script.sh

It works well, but the resulting script is just:

#!/bin/bash
ls -l

So, bash comment is missing from the final file. I suspect that the reason is that Docker assumes that the line is a Dockerfile comment, but since it's enclosed inside double quotes, I think it's clear that's not the case.

Of course I could solve the issue by just including the full script in a single line, or placing it in an external file, but I think it should be possible to include bash comments inside a multiline, quoted string without this kind of problems. Any workaround? I've tried all kind of escapings, without success.

Answer 1

You're right, it's a little weird that Docker interprets that as a Dockerfile comment instead of a comment inside a string. As a workaround, I got the following to work

FROM ubuntu:latest

RUN printf "#!/bin/bash \
\n# This is a bash comment inside the script \
\nls -l\n" > /script.sh

RUN cat /script.sh

Results in this output

Step 3 : RUN cat /script.sh
 ---> Running in afc19e228656
#!/bin/bash 
# This is a bash comment inside the script 
ls -l

If you move \\n to the beginning of the comment line it still generates the correct output but no longer treats that line as a Dockerfile comment line.

---

Assuming I found the right command parsing code , and I'm reading it correctly, Docker strips comments out before attempting to parse the line to see if it has any commands on it.