Short notes on container arguments

How to pass arguments to a container.md

How to pass arguments to a container

This short note covers container arguments by going through some use cases. First we create a test container

FROM ubuntu:latest
COPY ./entrypoint.sh /

ENTRYPOINT ["/entrypoint.sh"]
CMD ["/usr/bin/top", "-H"]

And an entrypoint.sh file

#!/usr/bin/env bash
echo "number of args: $#"
echo "Your container args are: $@"

for _i in $@; do
        echo "param: $_i"
done

sleep 5
exec "$@"

Now we can build our image

# build our test image
docker build -t test_param_img .

And spin a container

# run our test container
docker run -it --rm --name param_test test_param_img

This will have the following output:

number of args: 2
Your container args are: /usr/bin/top -H
param: /usr/bin/top
param: -H
top - 10:09:44 up  2:31,  0 users,  load average: 0.00, 0.04, 0.06
Threads:   1 total,   1 running,   0 sleeping,   0 stopped,   0 zombie
%Cpu(s):  0.7 us,  0.7 sy,  0.0 ni, 98.6 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
MiB Mem :  31993.7 total,  28648.9 free,   1165.0 used,   2179.7 buff/cache
MiB Swap:  43008.0 total,  43008.0 free,      0.0 used.  30229.4 avail Mem

    PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND
      1 root      20   0    7360   3252   2680 R   0.0   0.0   0:00.06 top

Notice that CMD is the one passing parameters here, we have 2 parameters "/usr/bin/top" and "-H".

Now we will pass a parameter to docker run

docker run -it --rm --name param_test test_param_img -v

This time the output will be:

number of args: 1
Your container args are: -v
param: -v
/entrypoint.sh: line 10: exec: -v: invalid option
exec: usage: exec [-cl] [-a name] [command [argument ...]] [redirection ...]

So when you pass a parameter on the command line, you overwrite the docker file CMD. this is trying to run "-v" which produces an error. I could run the same command, but change '-v' for '-h' which is a valid exec parameter and get a different output. You can test that.

Another parameter test, now running top

docker run -it --rm --name param_test test_param_img top -v

Output:

number of args: 2
Your container args are: top -v
param: top
param: -v
  procps-ng 3.3.17
Usage:
  top -hv | -bcEeHiOSs1 -d secs -n max -u|U user -p pid(s) -o field -w [cols]