- New entry on variable expansion when passed as parameters to the enrytpoint in Docker containers.

content/docker_entrypoint.rst

+Expanding variables and shell expressions in parameters to Docker entrypoint
+############################################################################
+
+:date: 2016-06-13
+:summary: Expanding variables and shell expressions in parameters to Docker entrypoint
+
+A known best practice when creating Docker images is when you need to run
+commands in runtime before starting the actual application/ daemon is to create
+an entrypoint script and pass the command as parameters in the :code:`CMD`
+instruction. Another best practice is to exec the final command so it would be
+PID 1 and receive the signals passed to it. Let's create a small example. Here's
+the :code:`Dockerfile`:
+
+.. code::
+
+    FROM alpine
+    COPY entrypoint.sh /entrypoint.sh
+    RUN chmod +x entrypoint.sh
+    ENTRYPOINT ["/entrypoint.sh"]
+    ENV var value
+    CMD ["echo", "$var"]
+
+And the :code:`entrypoint.sh` script:
+
+.. code:: shell
+
+    #!/bin/sh
+    set -eu
+    # Perform any needed tasks here.
+    exec $@
+
+Now let's build and run this container:
+
+.. code:: shell
+
+    $ docker build --tag entrypoint .
+    Sending build context to Docker daemon 28.67 kB
+    Step 0 : FROM alpine
+     ---> 5f05d2ba9e65
+    Step 1 : COPY entrypoint.sh /entrypoint.sh
+     ---> f59f4d7f3546
+    Removing intermediate container 27ca546c6b6c
+    Step 2 : ENTRYPOINT /entrypoint.sh
+     ---> Running in 98c65b63948a
+     ---> 1de45b33021b
+    Removing intermediate container 98c65b63948a
+    Step 3 : ENV var value
+     ---> Running in 133a8781f0ac
+     ---> bba451334fb2
+    Removing intermediate container 133a8781f0ac
+    Step 4 : CMD echo $var
+     ---> Running in e8436c6c3202
+     ---> a49d9b335b74
+    Removing intermediate container e8436c6c3202
+    Successfully built a49d9b335b74
+    $ docker run entrypoint
+    $var
+
+As we can see the variable :code:`var` wasn't expanded to it's content. After a
+bit of head scratching, The following simple change was made to the entrypoint
+script.
+
+.. code:: shell
+
+    #!/bin/sh
+    set -eu
+    # Perform any needed tasks here.
+    eval "exec $@"
+
+The change is to first evaluate the expression (expanding any variable and
+expression found), then :code:`exec` it. The outcome is what you'd expect.
+
+.. code::
+
+    $ docker build --tag entrypoint .
+    Sending build context to Docker daemon 28.67 kB
+    Step 0 : FROM alpine
+     ---> 5f05d2ba9e65
+    Step 1 : COPY entrypoint.sh /entrypoint.sh
+     ---> b874d862999d
+    Removing intermediate container fb6483ff00e3
+    Step 2 : ENTRYPOINT /entrypoint.sh
+     ---> Running in 82adf0b2c4c7
+     ---> 6674f336c5e1
+    Removing intermediate container 82adf0b2c4c7
+    Step 3 : ENV var value
+     ---> Running in 599f3f98c11d
+     ---> 980f1e1e1ad5
+    Removing intermediate container 599f3f98c11d
+    Step 4 : CMD echo $var
+     ---> Running in e29f1948480a
+     ---> e27fd79143f8
+    Removing intermediate container e29f1948480a
+    Successfully built e27fd79143f8
+    $ docker run entrypoint
+    value