This is a bit embarrassing but I’ll post it anyway in case it helps somebody out there having a slow day (or myself from the future who might forget it).
I was recently upgrading a Dockerfile from Alpine Linux version 3.13 to version 3.16. The Dockerfile is fairly straightforward, installing a bunch of system packages, then Python ones and copying some initialization scripts. One of these scripts is the entrypoint script defined in CMD:
FROM alpine:3.16
ENV APP_DIR=/srv/app
# ...
COPY start_ckan.sh ${APP_DIR}
CMD ["/srv/app/start_ckan.sh"]
I updated the Alpine version and the image built fine. But when I tried to run the image, I got the following:
exec /srv/app/start_ckan.sh: no such file or directory
Now, this was the crucial moment when my brain, in all its innocence, interpreted the following: “I’m seeing a file path, and next to it a message telling me this file does not exist. That is the problem.”
That sent me on a doomed debugging path trying to find a problem when copying the file to the image, file permissions, spellings… I even extracted the container contents with docker extract and saw the start_ckan.sh file there, mocking me.
Until I stepped back, decided to check the actual script file and then I saw it right there, in the first line:
#!/bin/bash
...
Could it be? I quickly googled “alpine bash” and indeed there it was: Alpine linux doesn’t include bash by default. The error message was telling me that /bin/bash was not found, not the start_ckan.sh script.
Why did this happen in this particular Alpine version? Probably one of the system packages that were installed in the previous version had bash as a requirement but that changed in the latest Alpine version and it was not automatically installed anymore.
Oh well. Meaningful error messages matter folks!