`env -S "$(cat .env)" <cmd>`
Believe it or not that’s all you need.
> S, --split-string=S process and split S into separate arguments; used to pass multiple arguments on shebang lines
edit: forgot the quotes around shell substitution
sh -c '. .env; <cmd>'
sh -c '. .env; "$@"' -- command arg1 arg2 arg3.
sh -c '. .env; exec "$@"' -- command arg1 arg2 arg3.
execl("/bin/sh", "/bin/sh", ". .env; exec \"$@\"", "--", "command",
"arg1", "arg2", "arg3", (char *) 0);
The .env being evaluated as a shell script means that it's in a widely used language, with a widely known syntax. You can look at it and know what it's going to do.
The .env being a data format to some uncommon utility; that's anyone's guess.
For instance, suppose we want a newline character in an environment variable. Does the given "env file" format support that? How?
There is one de-facto standard format: the /proc/<pid>/environ kernel output format on Linux. The variables are null-terminated strings, so it is effectively a binary format. It represents any variable value without requiring quoting mechanisms.
The nice thing about utilities like env and dotenv is that they can be easily exec-ed:
execl("/usr/bin/dotenv", "/usr/bin/dotenv", "command", "arg", (char *) 0);
You want $(cat .env) quoted, as in "$(cat .env)" so that the content of the file is reliably passed as one argument.
-S will split on whitespace; but it respects quoting, so spaces can be protected. Basically .env has to be prepared with the features of -S in mind. Of which that thing has quite a few: escape sequences like \n, commenting, environment variable substitution.