ENTRYPOINT or CMD

TL;DR

Best practice: When your Docker container runs as a daemon, write entrypoint.sh and run it when container starts by ENTRYPOINT. CMD can be overwritten.

When ENTRYPOINT, not CMD

Suppose you want to run a command when the Docker container starts. In that case, you can use CMD. The following code runs the command sleep 5 when the container starts.

FROM Ubuntu

CMD sleep 5
# or
# CMD ["sleep", "5"]

The arguments of CMD can be replaced when its run time command by:

docker run ubuntu-sleeper sleep 10

ENTRYPOINT can run a command when the container starts as well as CMD:

FROM Ubuntu

ENTRYPOINT ["sleep"]

Example usage:

docker run ubuntu-sleeper 5 # sleep + 5

If no argument is passed as an argument of the run command, it returns error because sleep has no operand.

Let’s make this sleep time 5 as variable, and the default command as sleep:

FROM Ubuntu

ENTRYPOINT ["sleep"]

CMD ["5"]

Now the default value is 5, but you can overwrite the ENTRYPOINT and CMD as follows:

docker run ubuntu-sleeper # sleep + 5
docker run ubuntu-sleeper 10 # sleep + 10
docker run --entrypoint sleep2.0 ubuntu-sleeper 10 #override entrypoint command