Jun 13, 2016

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 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 Dockerfile:

FROM alpine
COPY entrypoint.sh /entrypoint.sh
RUN chmod +x entrypoint.sh
ENTRYPOINT ["/entrypoint.sh"]
ENV var value
CMD ["echo", "$var"]

And the entrypoint.sh script:

#!/bin/sh
set -eu
# Perform any needed tasks here.
exec $@

Now let's build and run this container:

$ 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 var wasn't expanded to it's content. After a bit of head scratching, The following simple change was made to the entrypoint script.

#!/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 exec it. The outcome is what you'd expect.

$ 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