"docker run" command to evaluate bash $variable inside the container - bash

Question

How can I run a command inside a docker container, using docker run, where bash variables are evaluated inside the container?
E.g.:
$ SOMEONE=host
$ docker run --env SOMEONE=busybox busybox echo "Hello $SOMEONE"
Hello host
How can I make it output Hello busybox?

Answer 1

To prevent the replacement from happening from the outer shell, one needs to use single quotes, not double.
To ensure that there is an inner shell that can do a replacement (echo doesn't have any such functionality itself!), we need to explicitly call sh -c; otherwise, Docker will just directly invoke execlp("echo", "echo", "$SOMEONE", NUL) inside the container, which doesn't actually do any substitution.
Thus:
docker run --env SOMEONE=busybox busybox sh -c 'echo "Hello $SOMEONE"'

Answer 2

Using docker run, where bash variables are evaluated inside
By far the easiest, non-cryptic approach is to write a bash function with all commands to be executed inside the container. Benefits:
Easy to write - no need to use special quote placement and escaping
Easy to debug - see what bash actually does inside the container
Easy to maintain - write readable scripts, not cryptic commands
Easy to write and maintain
Here's an example bash function that expands all variables inside a docker container.
-- (host) $ ./create-db.sh
#!/bin/bash
function main_inside_docker {
# all variables are expanded insider docker
DBNAME=${1:-testdb}
echo "creating database $DBNAME"
PATH=$MSSQL_PATH:$PATH
SQL="
create database $DBNAME;
select database_id, name, create_date from sys.databases;
"
sqlcmd -U SA -P $SA_PASSWORD -Q "$SQL"
}
# declare the function inside docker and run it there
CMD="$(declare -f main_inside_docker); main_inside_docker $#"
docker exec -it mssql bash -c "$CMD"
Essentially this declares the main_inside_docker function inside the container, then runs it with all arguments provided from the host invocation. All variables inside the function are expanded inside the docker container. The function just works the way one would expect.
Easy to debug
To debug the function, set "-x" as the first command in $CMD:
CMD="set -x; $(declare -f ...)"
When running it this way, it will print the bash trace from inside the container nicely:
(host) $ ./create-db.sh foodb
+ main_inside_docker
+ DBNAME=foodb
+ echo 'creating database foodb'
creating database testdb
...