How does Dockerfile CMD exec form locate the binary - docker

Question

If I have a Dockerfile like this:
FROM ubuntu
CMD [ "ps", "-ef" ]
And if I build and run the image, I get
$ docker run -it 156a9f959f43
UID PID PPID C STIME TTY TIME CMD
root 1 0 0 07:12 pts/0 00:00:00 ps -ef
which is consistent with the documentation.
Question: How does the binary ps get located in the first place when the container runs ?

Answer 1

The exec syntax uses the PATH environment variable defined in the parent image (ubuntu:latest).
$ docker image inspect ubuntu:latest
[
{
...
"Config": {
...
"Env": [
"PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
],
"Cmd": [
"/bin/bash"
],
....
If you go looking at the Dockerfile for this base image... you'll actually see that the PATH variable is not defined there. We could go looking at scratch but that's a virtual image.
So, lets build an image on scratch with nothing to see what variables are defined:
$ cat df.scratch
FROM scratch
$ docker build -t test-scratch -f df.scratch .
...
$ docker image inspect test-scratch:latest
[
{
...
"Config": {
...
"Env": [
"PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
],
...
So the PATH is getting created in the scratch image. This old issue and associated PR show that docker is including a PATH out of the box.
How can you adjust that path? You need to use an ENV line. If you set a variable in a RUN line, it will not be preserved after that RUN line completes. And if you append to the .bashrc in the container, that does not apply to non-bash shells like /bin/sh, anything using the exec syntax without a shell, and any non-interactive bash shells (since the .bashrc stops processing part way through for non-interactive shells). Here's an example of that with a different image/build:
$ cat df.path
FROM ubuntu
# before state from the base image
RUN [ "env" ]
# attempting to modify the .bashrc
RUN echo "export PATH="$PATH:/my/custom/bin/dir"" >> ~/.bashrc
RUN [ "env" ]
# modifying the image environment variable directly
ENV PATH=${PATH}:/opt/custom/bin
RUN [ "env" ]
$ docker build -t test-path -f df.path .
Sending build context to Docker daemon 31.23kB
Step 1/6 : FROM ubuntu
---> 4e5021d210f6
Step 2/6 : RUN [ "env" ]
---> Running in 5bb72abb386d
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
HOSTNAME=5bb72abb386d
HOME=/root
Removing intermediate container 5bb72abb386d
---> c438fb269c70
Step 3/6 : RUN echo "export PATH="$PATH:/my/custom/bin/dir"" >> ~/.bashrc
---> Running in 127b10aff046
Removing intermediate container 127b10aff046
---> 4af50595c271
Step 4/6 : RUN [ "env" ]
---> Running in c5ff46ba3b82
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
HOSTNAME=c5ff46ba3b82
HOME=/root
Removing intermediate container c5ff46ba3b82
---> 455325a5e484
Step 5/6 : ENV PATH=${PATH}:/opt/custom/bin
---> Running in e7960d9ce18a
Removing intermediate container e7960d9ce18a
---> ed532bff78b4
Step 6/6 : RUN [ "env" ]
---> Running in 9c1558a61ab7
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/opt/custom/bin
HOSTNAME=9c1558a61ab7
HOME=/root
Removing intermediate container 9c1558a61ab7
---> f08993f21b97
Successfully built f08993f21b97
Successfully tagged test-path:latest
Note the original value of the path at step 2, it is unchanged at step 4, and it has the defined value at step 6.

Answer 2

In docker containers (similarily as in most operating systems) there is a $PATH environment variable, which holds the directory paths to where the executables are located (separated by :).
For example a $PATH variable might hold a value like /usr/local/bin:/usr/bin:/home/ubuntu/bin which would mean that when you are running a command like ps it will look for an executable in those directories.
You can learn more about $PATH variable here https://en.wikipedia.org/wiki/PATH_(variable)
Note: The $PATH variable is going to differ from container to container (as they are isolated units) and will most probably hold the defautl value of the base distro used by the docker image.
To make changes to your $PATH variable on linux based systems, you can run export PATH="$PATH:/custom/bin/dir" and it will append the /custom/bin/dir to the variable.
To make this change permanent, you should add this command to your .bashrc, .profile, .zshrc or similar file (depending on what shell you are using)
So to update the variable in you docker containers you should add something like this to your Docker file
FROM ubuntu
RUN echo "export PATH="$PATH:/my/custom/bin/dir"" >> ~/.bashrc