Metadata

ID: docker-best-practices/multiple-cmd

Language: Docker

Severity: Notice

Category: Best Practices

Description

The Dockerfile CMD instruction provides defaults for an executing container. These can include an executable, or they can omit the executable, in which case you must specify an ENTRYPOINT instruction. However, if you use multiple CMD instructions in a Dockerfile, only the last CMD instruction takes effect.

Using multiple CMD instructions in a Dockerfile can lead to confusion and unexpected behavior when the Docker image is run (only the last CMD instruction is executed). This can potentially lead to parts of your application not running as expected or not running at all.

To avoid this, ensure that you only use one CMD instruction in your Dockerfile. If you have multiple commands that need to be run, consider using a script to encapsulate these commands and then call this script in your CMD instruction. For example, instead of having CMD run_server1 and CMD run_server2, you could have a script called run_servers.sh that contains the commands run_server1 and run_server2, and then your Dockerfile would contain CMD ./run_servers.sh. This way, all your commands are run as expected.

Non-Compliant Code Examples

FROM awesomeimage
CMD run_server1
CMD run_server2

Compliant Code Examples

FROM awesomeimage
CMD run_server
HEALTHCHECK --interval=30s --timeout=3s \
  CMD curl -f http://localhost/ || exit 1

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