man add-shell (Administration système) - add shells to the list of valid login shells
NAME
add-shell - add shells to the list of valid login shells
SYNOPSIS
add-shell shellname [shellname...]
DESCRIPTION
add-shell copy /etc/shells to /etc/shells.tmp, add the given shells to this file if they are not already present, and copy this temporary file back to /etc/shells.
The shells must be provided by their full pathnames.