Add fixed argument to command arguments in docker entrypoint

Question

I want to add a fixed parameter to a command in a docker entrypoint st the command is executed like so:

command user_arg1 fixed_arg user_arg2

Using a linux terminal as an example, instead of running:

echo arg1 arg2

I want to run:

<some set of commands> arg1 arg2

such that the final command that's run is:

echo arg1 fixed_arg arg2

Is there a way to use basic command line tools to achieve this instead of writing a bash script?

Answer 1

If you want to include the fixed argument in the middle of the command, you have to use a script. You don't need bash extensions, though, and you can make this work even with the basic POSIX shell in BusyBox or Alpine Linux.

#!/bin/sh
first="$1" # save the first option
shift      # remove it from the options list
           # now $@ is all the options except the first one
exec the_command "$first" some_arg "$@"

In the context of a Docker entrypoint, it will get pass the entire CMD (or docker run command part or Compose command: ) as options, so you could make this conditional on what the command actually is.

#!/bin/sh
case "$1" in
  the_command)
    first="$2"
    shift 2
    exec the_command "$first" some_arg "$@"
    ;;
  *)
    exec "$@"
    ;;
esac

The only thing Docker will do natively is combine the ENTRYPOINT and CMD into a single command ; there's no way to rearrange the options or otherwise manipulate them. A pattern I find useful is for the CMD to be the actual command you want to run, and the ENTRYPOINT to be a script that eventually runs it. That can manipulate the command string however you want, including choosing to completely ignore it or change the actual command that gets run.

Answer 2

The RUN command in a dockerfile should work, an example is,

RUN /bin/bash -c 'source $HOME/.bashrc; \
echo $HOME'

More information in the docker documentation, https://docs.docker.com/engine/reference/builder/#run