Docker RUN vs CMD

README.md

Docker RUN vs CMD

RUN

• creates a layer, it actually modifies the image during the build.
• it's executed during "build time"
• you can specify multiple RUNs in a Dockerfile

CMD

• doesn't create a layer
• specifies the command that will be run by default when the container is run if no command is passed to the docker run instruction
• it's not executed during "build time", it's executed in "run time".

test.sh

#!/bin/bash
docker build -f test_run.Dockerfile -t test_run .
docker build -f test_cmd.Dockerfile -t test_cmd .
docker run test_run cat hello.txt
# expected: hello
docker run test_cmd cat hello.txt
# expected: cat: hello.txt: No such file or directory

test_cmd.Dockerfile

FROM ubuntu
CMD echo hello > hello.txt

test_run.Dockerfile

FROM ubuntu
RUN echo hello > hello.txt