add-shell (8) - Linux Manuals
add-shell: 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 copies /etc/shells to /etc/shells.tmp, adds the given shells to this file if they are not already present, and copies this temporary file back to /etc/shells.The shells must be provided by their full pathnames.