Conditional COPY/ADD in Dockerfile?

Here is a simple workaround:

COPY foo file-which-may-exist* /target

Make sure foo exists, since COPY needs at least one valid source.

If file-which-may-exist is present, it will also be copied.

NOTE: You should take care to ensure that your wildcard doesn't pick up other files which you don't intend to copy. To be more careful, you could use file-which-may-exist? instead (? matches just a single character).

Or even better, use a character class like this to ensure that only one file can be matched:

COPY foo file-which-may-exis[t] /target

As stated by this comment, Santhosh Hirekerur's answer still copies the file, to archive a true conditional copy, you can use this method.

ARG BUILD_ENV=copy

FROM alpine as build_copy
ONBUILD COPY file /file

FROM alpine as build_no_copy
ONBUILD RUN echo "I don't copy"

FROM build_${BUILD_ENV}
# other stuff

The ONBUILD instructions ensures that the file is only copied if the "branch" is selected by the BUILD_ENV. Set this var using a little script before calling docker build

This isn't currently supported (as I suspect it would lead to a non-reproducible image, since the same Dockerfile would copy or not the file, depending on its existence).

This is still requested, in issue 13045, using wildcards: "COPY foo/* bar/" not work if no file in foo" (May 2015).
It won't be implemented for now (July 2015) in Docker, but another build tool like bocker could support this.

---

2021:

COPY source/. /source/ works for me (i.e. copies directory when empty or not, as in "Copy directory into docker build no matter if empty or not - fails on "COPY failed: no source files were specified"")

2022

Here is my suggestion:

# syntax=docker/dockerfile:1.2

RUN --mount=type=bind,source=jars,target=/build/jars \
 find /build/jars -type f -name '*.jar' -maxdepth 1  -print0 \
 | xargs -0 --no-run-if-empty --replace=source cp --force source >"${INSTALL_PATH}/modules/"

That works around:

COPY jars/*.jar "${INSTALL_PATH}/modules/"

But copies no *.jar if none is found, without throwing an error.