Docker Series 3: Startup

ENTRYPOINT vs CMD

There are many blogs about the ENTRYPOINT and CMD, I have referred to the official blog and docker-run-vs-cmd-vs-entrypoint.

An ENTRYPOINT allows you to configure a container that will run as an executable.

Command line arguments to docker run <image> will be appended after all elements in an exec form ENTRYPOINT, and will override all elements specified using CMD.

I’ll give you some examples:

only ENTRYPOINT

FROM debian:buster-slim
RUN sed -i 's/deb.debian.org/mirrors.ustc.edu.cn/g' /etc/apt/sources.list
RUN apt-get update && apt-get install -y htop
ENTRYPOINT ["htop"]

docker run -ti test

# It's the `htop -C` to be executed.
docker run -ti test -C

CMD with ENTRYPOINT

FROM debian:buster-slim
RUN sed -i 's/deb.debian.org/mirrors.ustc.edu.cn/g' /etc/apt/sources.list
RUN apt-get update && apt-get install -y htop
ENTRYPOINT ["htop"]
CMD ["-C"]

It’s just like you are executing the htop -C.

docker run -ti test

replace ENTRYPOINT

What if we don’t want to execute htop, but want to get a bash to execute some commands? The answer is --entrypoint. Note that it should be written in front of the image name, otherwise it will be treated as a CMD

docker run -ti --entrypoint=/bin/bash <image>

There are several scenarios where we need it:

1. We want to check the entrypoint and cmd after we create the Dockerfile
2. Sometimes the container can’t run for some reasons; we need to hack the image and try to run the entrypoint command ourselves — it’s faster than reading logs.

Write your own ENTRYPOINT script

The entrypoint can be a simple script, especially if you want to perform some checks before your program runs.

This script could help you make sure that the log directory is mounted.

#!/bin/bash

set -e

DATA_DIR=/home/data

# Make sure the dir has been mounted.
[ -d "$DATA_DIR" ] || exit 1

# make sure the dir will create.
mkdir -pv $DATA_DIR/{log,run,scripts}
chown -R corvo:corvo $DATA_DIR

exec "$@"

The exec "$@" just runs the CMD. This is an example Dockerfile.

FROM debian:buster-slim
RUN sed -i 's/deb.debian.org/mirrors.ustc.edu.cn/g' /etc/apt/sources.list
RUN apt-get update && apt-get install -y htop
COPY entryproint.sh entrypoint.sh
ENTRYPOINT ["entrypoint.sh"]
CMD ["uwsgi", "-y", "app.yaml"]