How to use Docker's COPY/ADD instructions to copy a single file to an image

Question

I am trying to copy a single file from the root of the build context into a newly created directory in a docker image.

The Dockerfile that I am using is as follows:

FROM debian:latest RUN mkdir -p /usr/src/app WORKDIR /usr/src/app COPY test.txt /usr/src/app 

The test.txt file is a simple ASCII text file as follows:

$ cat test.txt  This is a test 

However, when I build this image I get an image stating that the destination path is not a directory.

$ docker build . Sending build context to Docker daemon 4.608 kB Sending build context to Docker daemon  Step 0 : FROM debian:latest  ---> 9a61b6b1315e Step 1 : RUN mkdir -p /usr/src/app  ---> Using cache  ---> 86bd44a8776e Step 2 : WORKDIR /usr/src/app  ---> Using cache  ---> ed6771adc681 Step 3 : ADD test.txt /usr/src/app stat /var/lib/docker/devicemapper/mnt/ee5b9b7029f2adf27d332cbb0d98d6ad9927629a7569fd2d9574cb767b23547b/rootfs/usr/src/app/test.txt: not a directory 

I have tried using multiple combinations of docker versions, base images, and distributions with docker installed. I have also tried using ADD instead of COPY but the result is the same.

I am currently trying this on CentOS 7 with the following docker version installed:

Client version: 1.7.1 Client API version: 1.19 Go version (client): go1.4.2 Git commit (client): 786b29d OS/Arch (client): linux/amd64 Server version: 1.7.1 Server API version: 1.19 Go version (server): go1.4.2 Git commit (server): 786b29d OS/Arch (server): linux/amd64 

What I have noticed is that I can copy a directory, but not a file. For example, if I create a "test" directory in the build context root and put test.txt inside the test directory then I can copy the directory successfully:

FROM debian:latest RUN mkdir -p /usr/src/app WORKDIR /usr/src/app COPY test /usr/src/app 

Note that in the above Dockerfile I copy the entire ./test directory rather than just ./test.txt. This build successfully.

The Docker documentation has the following sample use case for the COPY instruction which is similar to what I am trying to do:

COPY hom?.txt /mydir/ 

Can anyone point out what I am doing wrong? How can I go about copying a single file?

Answer 1

As stated in the Dockerfile documentation:

• If <src> is any other kind of file, it is copied individually along with its metadata. In this case, if <dest> ends with a trailing slash /, it will be considered a directory and the contents of <src> will be written at <dest>/base(<src>).

• If <dest> does not end with a trailing slash, it will be considered a regular file and the contents of <src> will be written at <dest>.

Thus, you have to write COPY test.txt /usr/src/app/ with a trailing /.