why docker does not recognize /bin/sh -c as a valid entrypoint? - docker

Question

I have a simple nginx container trying to run from docker-compose..
version: "3.3"
services:
nginx:
image: nginx
privileged: true
entrypoint: ["/bin/sh -c"]
command: ["ls -lha ~"]
but it fails with:
docker-compose up -d
ERROR: for junk_nginx_1 Cannot start service nginx: OCI runtime create failed: container_linux.go:380: starting container process caused: exec: "/bin/sh -c": stat /bin/sh -c: no such file or directory: unknown
I thought it was because /bin/sh doesn't exists in the image, but it certainly does. removing the -c gives me the following error:
# this time the container runs, this is in the container logs.
/bin/sh: 0: Can't open ls -lha ~
so /bin/sh does exists within the image. what am I doing wrong?

Answer 1

When you use the array form of Compose command: and entrypoint: (and, similarly, the JSON-array form of Dockerfile CMD, ENTRYPOINT, and RUN), you are responsible for breaking up the input into words. Each item in the array is a word, just as though it was quoted in the shell, and includes any spaces, punctuation, and other characters.
So when you say
entrypoint: ["/bin/sh -c"]
That is one word, not a command and its argument, and you are telling the shell to look for an executable program named sh -c (including space hyphen c as part of the filename) in the /bin directory. Since that's not there, you get an error.
You shouldn't usually need to override entrypoint: in a Compose setup. In your case, the only shell expansion you need is the home directory ~ but that's not well-defined in Docker. You should be able to just write
command: ls -lha /usr/share/nginx/html
or in array form
command: ["ls", "-lha", "/usr/share/nginx/html"]
# (or other YAML syntaxes with fewer quotes or more lines)
or if you really need the sh -c wrapper
command: /bin/sh -c 'ls -lha ~'
command: ["/bin/sh", "-c", "ls -lha ~"]
command:
- /bin/sh
- -c
- >-
ls -lha ~;
echo these lines get folded together;
nginx -g 'daemon off;'
You're using the stock Docker Hub nginx image; also consider whether docker-compose run might be an easier way to run a one-off command
docker-compose run nginx \
ls -lha /usr/share/nginx/html
If it's your own image, try hard to avoid needing to override ENTRYPOINT. Make CMD be a complete command; if you need an ENTRYPOINT, a shell script that ends with exec "$#" so that it runs the CMD is a typical pattern.

Answer 2

See entrypoint usage:
entrypoint: ["php", "-d", "memory_limit=-1", "vendor/bin/phpunit"]
Also see command usage:
command: ["bundle", "exec", "thin", "-p", "3000"]
So, your error means your syntax not ok, the correct one for you is:
version: "3.3"
services:
nginx:
image: nginx
privileged: true
entrypoint: ["/bin/sh", "-c"]
command: ["ls", "-lha", "~"]
The execution:
$ docker-compose up
Creating network "20210812_default" with the default driver
Creating 20210812_nginx_1 ... done
Attaching to 20210812_nginx_1
nginx_1 | bin
nginx_1 | boot
nginx_1 | dev
nginx_1 | docker-entrypoint.d
nginx_1 | docker-entrypoint.sh
nginx_1 | etc
nginx_1 | home
nginx_1 | lib
nginx_1 | lib64
nginx_1 | media
nginx_1 | mnt
nginx_1 | opt
nginx_1 | proc
nginx_1 | root
nginx_1 | run
nginx_1 | sbin
nginx_1 | srv
nginx_1 | sys
nginx_1 | tmp
nginx_1 | usr
nginx_1 | var
20210812_nginx_1 exited with code 0