Apologies if this should be categorized as a FR. It feels like a bug to me given the contents of the shadow package at https://projects.archlinux.org/svntogit/packages.git/tree/trunk?h=packages/shadow including both useradd.defaults and the adduser script.

Description:
Customizing the SHELL variable in /etc/default/useradd has no effect on the adduser script in Arch's shadow package. The default shell for the adduser script is hardcoded on line 91: https://projects.archlinux.org/svntogit/packages.git/tree/trunk/adduser?h=packages/shadow#n91 It should be easy enough to source the /etc/default/useradd script and set defshell=$SHELL or at least to grep the default SHELL out of /etc/default/useradd.

Additional info:
* package version(s) shadow 4.1.5.1-2

Steps to reproduce:
* Set the default shell for useradd: useradd -Ds /bin/zsh
* Verify the shell has changed: useradd -D
* Execute the adduser script and note that the default shell remains /bin/bash