shell script echo back fatal error - shell

Question

I am using Elixir's porcelain to invoke shell script, in there I have command like:
#!/usr/bin/env bash
aws s3 sync frontend/dist s3://$S3_BUCKET --delete
echo
Now, if command fails(because of wrong bucket) it displays:
fatal error: An error occurred (InvalidBucketName) when calling the
ListObjects operation: The specified bucket is not valid.
But doesn't return this fatal "fatal error" message back to porcelain. How can I echo this error back?
Edit:
Porcelain code:
Porcelain.shell(". #{Path.join(:code.priv_dir(:hub), "scripts/copy_site_to_s3.sh")}")
I know the possible solution would be to use exec instead of shell but this is more of an example, I have a couple of slightly more complicated but similar shell scripts, facing the same problem.
Another script/example(I am testing failures):
Invoking with:
result = Porcelain.shell(". #{Path.join(:code.priv_dir(:hub),
"scripts/git_clone_pull.sh")} #{github}"
)
IO.inspect result
Script:
if cd frontend; then git reset --hard && git pull; else git clone $1 frontend; fi
It properly fails with:
fatal: Authentication failed for
'https://github.com/x/frontend.git/'
But porcelain result fails to capture message:
%Porcelain.Result{err: nil, out: "", status: 128}

Answer 1

If you’ll check the documentation for Porcelain.{exec,shell}/3 options, you’ll see:
:err — specify the way stderr will be passed back to Elixir.
Possible values are the same as for :out. In addition, it accepts the atom :out which denotes redirecting stderr to stdout.
Caveat: when using Porcelain.Driver.Basic, the only supported values are nil (stderr will be printed to the terminal) and :out.
Emphasis is mine. That caveat might be easily proven in the less cumbersome environment, without involving AWS and any other 3rd parties:
iex|1 ▶ Porcelain.shell("ls --gg", err: {:append, "error.log"})
#⇒ ls: unrecognized option '--gg'
# Try 'ls --help' for more information.
# %Porcelain.Result{err: {:append, "error.log"}, out: "", status: 2}
iex|2 ▶ ls "error.log"
# [ERROR] No such file or directory error.log
But we still have :out option!
iex|3 ▶ Porcelain.shell(">&2 echo 'error'", err: :out)
%Porcelain.Result{err: :out, out: "error\n", status: 0}
iex|4 ▶ Porcelain.shell("ls --gg", err: :out)
%Porcelain.Result{
err: :out,
out: "ls: unrecognized option '--gg'\nTry 'ls --help' for more information.\n",
status: 2
}
Naya, luckily even Basic driver might redirect :err to :out. That said, you have two options:
use err: :out parameter, pattern match to when status > 0 and examine standard output, or:
use Porcelain.Driver.Goon driver and deal with your stderr stream like a profi.