add-shell (8) - Linux Man Pages
add-shell: add shells to the list of valid login shells
add-shell - add shells to the list of valid login shells
SYNOPSISadd-shell shellname [shellname...]
DESCRIPTIONadd-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.