将此处文档与Dockerfile中的ENTRYPOINT / CMD一起使用

Question

What I want to do

My goal is to end up with a Docker image which - as soon as a container is launched from it - reads input from stdin until it meets EOF. Then, it simply writes back what has been read to stdout.

Basically, my idea was this should be possible by using the here document syntax cat << EOF .

What I did so far

So far, I tried different variants with the CMD Dockerfile instruction but had no success until now. From all the images I've created I launched containers with docker run -it myimage to keep the container's stdin attached.

Variant 1

 FROM alpine 3.7
 CMD ["cat", "<<", "EOF"]

This leads to the following error:

cat: can't open '<<': No such file or directory
cat: can't open 'EOF': No such file or directory

Variant 2

 FROM alpine 3.7
 CMD ["cat << EOF"]

Result:

 container_linux.go:265: starting container process caused "exec: \"cat << EOF\": executable file not found in $PATH"

Variant 3

 FROM alpine:3.7
 CMD ["sh", "-c", "cat << EOF"]

Result:

This does nothing but returns immediately.

Variant 4

 FROM alpine: 3.7
 CMD "cat << EOF"

Result:

 /bin/sh: cat << EOF: not found

What I expect

I'm searching for a solution that behaves like described above. The following docker run command works perfectly, but I'd like to define this as the default entry point for my Docker image:

 $ docker run -it alpine cat << EOF
 > Hello Docker
 > EOF
 Hello Docker

Answer 1

That can't work because the shell parses the command line, including the heredoc before it executes it.

Basically you can just do this:

# Dockerfile
FROM xyz
# cat reads from stdin by default if no filename gets passed to it
CMD 'cat'

Build and run the container:

docker build -t foo .
docker run -ti foo
HelloDocker
HelloDocker

You stop the input with Ctrl + d (That's literally EOF)

---

PS: If you want to use a here-doc, run the above container like this:

docker run -i foo <<EOF
Hello Docker
EOF

Hello Docker