How can I make my Docker compose "wait-for-it" script invoke the original container ENTRYPOINT or CMD?

Question

According to Controlling startup order in Compose, one can control the order in which Docker Compose starts containers by using a "wait-for-it" script. Script wait-for-it.sh expects both a host:port argument as well as the command that the script should execute when the port is available. The documentation recommends that Docker Compose invoke this script using the entrypoint: option. However, if one uses this option, the container will no longer run its default ENTRYPOINT or CMD because entrypoint: overrides the default.

How might one provide this default command to wait-for-it.sh so that the script can invoke the default ENTRYPOINT or CMD when the condition for which it waits is satisfied?

In my case, I've implemented a script wait-for-file.sh that polls waiting for a file to exist:

#!/bin/bash  set -e  waitFile="$1" shift cmd="$@"  until test -e $waitFile do   >&2 echo "Waiting for file [$waitFile]."   sleep 1 done  >&2 echo "Found file [$waitFile]." exec $cmd 

Docker Compose invokes wait-for-file.sh as the entry-point to a slightly custom container derived from tomcat:8-jre8:

  platinum-oms:     image: opes/platinum-oms     ports:       - "8080:8080"     volumes_from:       - liquibase     links:       - postgres:postgres       - activemq:activemq     depends_on:       - liquibase       - activemq     entrypoint: /wait-for-file.sh /var/run/liquibase/done 

Before it exits successfully, another custom container liquibase creates /var/run/liquibase/done and so platinum-oms effectively waits for container liquibase to complete.

Once container liquibase creates file /var/run/liquibase/done, wait-for-file.sh prints Found file [/var/run/liquibase/done]., but fails to invoke default command catalina.sh run in base container tomcat:8-jre8. Why?

Test Scenario

I created a simplified test scenario docker-compose-wait-for-file to demonstrate my problem. Container ubuntu-wait-for-file waits for container ubuntu-create-file to create file /wait/done and then I expect container ubuntu-wait-for-file to invoke the default ubuntu container command /bin/bash, but instead, it exits. Why doesn't it work as I expect?

Answer 1

However, if one uses this option, the container will no longer run its default ENTRYPOINT or CMD command because entrypoint: overrides the default.

That is expected, which is why the wait-for-it is presented as a wrapper script.
It does allow to execute a "subcommand" though:

wait-for-it.sh host:port [-s] [-t timeout] [-- command args]                                                ^^^^^^^^^^^^ 

The subcommand will be executed regardless if the service is up or not.
If you wish to execute the subcommand only if the service is up, add the --strict argument.

That means the CMD part of your image can be used for your actual container command, as its parameters will passed in parameters to the ENTRYPOINT command:

entrypoint: wait-for-it.sh host:port -- cmd: mycmd myargs 

This should work... except for docker-compose issue 3140 (mentioned by the OP Derek Mahar in the comments)

entrypoint defined in docker-compose.yml wipes out CMD defined in Dockerfile

That issue suggests (Jan. 2021)

If you have a custom image you can add a startscript to the build and call it inside the dockerfile and in the docker-compose you can call it again.
Thats a way to avoid duplicate for more complicated entrypoints.