How to copy files in Docker? Copy to where?

Docker is an open source platform for creating, deploying, and running applications. Its core idea is to package an application and its dependencies all in a container for easy deployment and running in different environments. In the process of using Docker, we often need to copy files to the container. This article will discuss how to copy files in Docker.

First and most commonly used method, we can use Docker’s cp command to copy files. The syntax of the cp command is as follows:

docker cp [OPTIONS] CONTAINER:SRC_PATH DEST_PATH|-
docker cp [OPTIONS] SRC_PATH|- CONTAINER:DEST_PATH

Among them, CONTAINER refers to the name or ID of the container, SRC_PATH is the source file path within the container, and DEST_PATH is the target file path outside the container. If DEST_PATH is set to "-", it means output to the screen.

For example, suppose we need to copy the local file /root/abc.txt to a container named mycontainer and save it as /tmp/def.txt, then we can execute the following command:

docker cp /root/abc.txt mycontainer:/tmp/def.txt

Similarly, if we need to copy the file /var/log/app.log in the container to the local /tmp directory, we can execute the following command:

docker cp mycontainer:/var/log/app.log /tmp

In addition to using the Docker cp command, We can also use the ADD or COPY instructions in the Dockerfile to copy files. Both the ADD and COPY instructions can copy source files from the build context (that is, the directory where the Dockerfile is located) into the container.

The syntax of the ADD command is as follows:

ADD [--chown=<user>:<group>] <src>... <dest>

Among them, the --chown parameter indicates the user and group that sets the file when copying the file. If no parameters are specified, the user and group where the Dockerfile is located are used.

The syntax of the COPY instruction is as follows:

COPY [--chown=<user>:<group>] <src>... <dest>

Similar to the ADD instruction, the --chown parameter can also be used to specify the ownership of the file.

For example, the ADD instruction is used in the following Dockerfile to copy the local file app.jar to the /app directory in the container:

FROM ubuntu
ADD app.jar /app/app.jar

Similarly, the COPY instruction is used in the following Dockerfile to copy the local file app.jar to the /app directory in the container: conf is copied to the /app/conf directory in the container:

FROM ubuntu
COPY --chown=user:group conf /app/conf

It should be noted that when using the ADD or COPY instruction, the owner and permissions of the file or directory are passed to the file system through the host's file system. in the container. If the file or directory ownership in the host file system is incorrect, the copied file or directory will also have permission issues.

In summary, to copy files from Docker, you can use the Docker cp command and the ADD or COPY command in the Dockerfile. Through the above methods, we can effectively perform file copy operations in Docker.

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