Q. Why Linux usernames with capital letters are not working? When I run adduser command I get bad username or Use the `--force-badname' message. I am using Debian Linux.
A. You should always try to avoid Linux usernames with capital letters in them. Many server program such as FTP server or sendmail makes no difference between a user name 'Vivek' and 'vivek'. Further many windows clients such as OE or IE make no distinction between capital letters.
So ftp username ftp://email@example.com or ftp://Vivek@ftp.box or email id Vivek@box.com or firstname.lastname@example.org treated as same.
Hence most Linux distro will disable Linux usernames with capital letters in them via adduser or other gui tools such as redhat-config-users (fedora core/centos/RHEL).