Different process are running as PID 1 when running CMD/ENTRYPOINT in shell form when the base images is centos vs ubuntu:trusty

Question

Build and run an image using the below dockerfile.

Dockerfile1

FROM ubuntu:trusty
ENTRYPOINT ping localhost

Now run the below comm

Answer 1

This is the behavior of bash. Docker is still running the command with a shell which you can identify with an inspect:

$ docker inspect test-centos-entrypoint --format '{{.Config.Entrypoint}}'
[/bin/sh -c ping localhost]

You can see the version of /bin/sh (note the GNU bash part):

$ docker exec -it quicktest /bin/sh --version
GNU bash, version 4.2.46(2)-release (x86_64-redhat-linux-gnu)
Copyright (C) 2011 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>

This is free software; you are free to change and redistribute it.                               
There is NO WARRANTY, to the extent permitted by law.

The ubuntu version of /bin/sh (possibly dash) doesn't even support the --version flag and is not linked to bash. But if you change the ubuntu image to use bash instead of /bin/sh, you'll see the behavior matching centos:

$ cat df.ubuntu-entrypoint
FROM ubuntu:trusty
ENTRYPOINT [ "/bin/bash", "-c", "ping localhost" ]

$ DOCKER_BUILDKIT=0 docker build -t test-ubuntu-entrypoint -f df.ubuntu-entrypoint .
Sending build context to Docker daemon  23.04kB
Step 1/2 : FROM ubuntu:trusty
 ---> 67759a80360c
Step 2/2 : ENTRYPOINT [ "/bin/bash", "-c", "ping localhost" ]
 ---> Running in 5c4161cafd6b
Removing intermediate container 5c4161cafd6b
 ---> c871fe2e2063
Successfully built c871fe2e2063
Successfully tagged test-ubuntu-entrypoint:latest

$ docker run -d --name quicktest2 --rm test-ubuntu-entrypoint
362bdc75e4a960854ff17cf5cae62a3247c39079dc1290e8a85b88114b6af694

$ docker exec -it quicktest2 ps -ef
UID        PID  PPID  C STIME TTY          TIME CMD
root         1     0  0 13:05 ?        00:00:00 ping localhost
root         8     0  0 13:05 pts/0    00:00:00 ps -ef