What is for parser directive in docker

Question

I read this link which explains the valid directives. And it says "Parser directive is a special form of comment"

I really don't understand what is the need for this and if it is comment, why docker parses it?

I didn't find any good explanations related to this.

Is it only escape directive? Because of windows path issues.

Answer 1

The "parser directives" instruct the Dockerfile parser to handle the content of Dockerfile as specified in the directives.

"Parser directives" direct the "way" how the content of DockerFile should be treated/interpreted. For this reason, its must be at the top of a Dockerfile.

Currently, "escape" is the only supported directive.

This can be useful for you: https://www.safaribooksonline.com/library/view/learning-docker-/9781786462923/b203797f-2c5f-4f24-a189-9e1780adfbbb.xhtml

Answer 2

It basically tell the docker daemon how to read the dockerfile, the way it intended to. For an example, I'll use the escape parser as it's the only supported one right now.

COPY sample.txt c:\\

This will fail as the second '\' at the end of the line interpreted as an escape for the newline, instead of a target of the escape from the first '\'

#escape=`

By adding an escape parser, we can tackle this. However, you have to be careful when using parsers as it's very easy to get them invalidated. You can see the common mistakes stated in Docker docs

Answer 3

Parser directive affect the way in which subsequent lines in a Dockerfile are handled / executed. Parser directive must be added at the top of Dockerfile.

For example, \ (backslash) is path separator in windows, whereas in Docker file it is used as escape character. So if we want to change the escape character in Docker files then we can use parser directive

# escape=`

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

In the above example backtick will be act as escape character instead of backslash.