Docker entrypoint bash script for passing parameters to the containerized app

Question

Source code located here

I am trying to pass extra parameters from a container to the contained application. The following bash script is working to pass extra variables, but I'm not sure that it's optimal, or a standard method for running a containerized application. Any insight would be appreciated.

#!/bin/bash

# allow arguments to be passed to dnsmasq
if [[ ${1:0:1} = '-' ]]; then
  EXTRA_ARGS="$@"
  set --
elif [[ ${1} == dnsmasq || ${1} == $(which dnsmasq) ]]; then
  EXTRA_ARGS="${@:2}"
  set --
fi

# default behaviour is to launch dnsmasq
if [[ -z ${1} ]]; then
  echo "Starting dnsmasq..."
  exec $(which dnsmasq) --log-facility=- --keep-in-foreground --no-resolv --no-hosts --strict-order ${EXTRA_ARGS}
else
  exec "$@"
fi

Answer 1

Shell variables should be lower-case, to avoid collision with environment variables that may be passed through.

We're making assumptions that $EXTRA_ARGS will word-split back into the same constituents it was composed from.

Instead of using a plain string variable there, use an array:

    extra_args=("$@")

    extra_args=("${@:2}")

Then expand it as an array:

  exec dnsmasq --log-facility=- --keep-in-foreground \
               --no-resolv --no-hosts --strict-order \
               "${extra_args[@]}"

(I took out the pointless which, since that just uses the same path that exec will use for lookup - unless there's a shell alias, in which case we could throw command in there.)

---

Alternatively, we could just use the One True Array that portable shell supports, and remove the Bash extensions completely:

#!/bin/sh

set -eu

case "${1-}"
in
    -*|'')
        set -- dnsmasq "$@"
esac

# In Bash or ksh, we could use ;&, but plain POSIX shell doesn't
# support fallthrough, so we need a separate 'case' here.
case "${1}"
in
    dnsmasq|*/dnsmasq)
        shift
        echo "Starting dnsmasq..."
        exec dnsmasq --log-facility=- --keep-in-foreground      \
            --no-resolv --no-hosts --strict-order               \
            "$@"
esac

# Some other program specified
exec "$@" 

Using #!/bin/sh likely has lower overheads than starting the much larger Bash interpreter.