What’s the difference between ADD and COPY instructions in the Dockerfile

docker

Isn't clear to me, why there are two instructions for copying files into the Docker image described in the Dockerfile reference. There are ADD and COPY and they seem pretty similar to me. Is there a practical difference between them? If not, which one is most used?

Best Answer

ADD performs a lot more “magic”: it fetches URLs and unpacks archives. That can be somewhat unexpected (especially the archive unpack). The official best practices guide recommends you use COPY unless you need ADD's extra features.

Related Solutions

Centos – Can’t install man pages on minimal Centos Docker container

Your image probably has the nodocs transaction flag set in the yum configuration (cf. /etc/yum.conf).

You can remove it globally (or at the yum command line) before (re-)installing the packages you want the man pages for.

For example:

yum --setopt=tsflags='' reinstall shadow-utils

Dockerfile, Docker image and reproducible environment

I think I would prefer sharing the Dockerfile. Obviously you need to specifiy a version in the FROM statement in your Dockerfile. Since for example different Ubuntu versions will have different packages available.

For system or -dev dependencies, you might want to actually let the version float freely to always installed the latest one.

Debian/Ubuntu packages

For any program installed with apt-get, for example curl, you can get the version number with

apt-cache policy curl | grep -oP 'Installed: \K\S+'

and then edit your Dockerfile to read something like

RUN apt-get install curl=7.47.0-1ubuntu2.2

Python

Python versions are easily handled with pip. Extract all version numbers of installed packaged and store them in a requirements file like this:

pip freeze > requirements.txt

Then in your Dockerfile run

RUN pip install -r requirements.txt