Shell Form vs Exec Form

Shell form is just a string.

Exec form - JSON array syntax.

Shell form:

CMD node app.js

Is equivalent to:

/bin/sh -c "node app.js"

The bourn shell is default for Linux containers. Windows containers have cmd.exe as default.

People also use powershell rather than the default.

General guidelines

RUN, use shell

RUN apt-get install jq

Rule of thumb (this is what industry follows)

Use exec form for:

• CMD
• ENTRYPOINT
• production containers

Use shell form for:

• RUN (because chaining is convenient)

Other notes

• Exec form ensures that the long running process you specify executes as PID 1, /bin/sh -c (PID 1). Imp for ENTRYPOINT & CMD

Takeaways

• The RUN, ENTRYPOINT & CMD instructions can be specified in shell form or exec form.
• Shell form will inject /bin/sh -c at the beginning of the command.
• Override the Docker default shell using

  SHELL ["/bin/bash", "-c"]

• Shell form is required if you need variable substitution, chaining commands, piping output, I/O redirection.
• Exec form (JSON syntax) runs the command without a shell.
• Exec form ensures ENTRYPOINT/CMD binary is PID 1 and receives signals.
• Exec form passes ENVs from Dockerfile to processes started with RUN, ENTRYPOINT & CMD.

ENTRYPOINT is an overwrite statement. Only the last one in the DOckerfile is used.

Use CMD for long running process.