Linux man pages : shells (5)
SHELLS(5) Linux Programmer's Manual SHELLS(5)
shells - pathnames of valid login shells
/etc/shells is a text file which contains the full pathnames of valid
login shells. This file is consulted by chsh(1) and available to be
queried by other programs.
Be aware that there are programs which consult this file to find out if
a user is a normal user. E.g.: ftp daemons traditionally disallow
access to users with shells not included in this file.
/etc/shells may contain the following paths: