Why does terminal auto complete sometimes not suggest anything for the `services` command?

The degree to which auto-completion works is a function of how well the shell scripts in the bash-completion package work.

In Ubuntu 14.04, the script that handles completions for service is in /usr/share/bash-completion/bash_completion. It looks for service names as files in /etc/rc.d/init.d and /etc/init.d, and in the output of systemctl list-units --full --all.

But installing the vsftpd package doesn't put any files in /etc/init.d, because it has been systemd-ified; it puts a config file in /etc/systemd/system/vsftpd.service. systemctl list-units would find that file if not for the fact that Ubuntu, at the present time, doesn't include a systemctl command.

So, at the moment, you won't get auto-completions for service vsftpd, but you will for most other services, since their config files are in /etc/init.d.

When there are multiple possible suggestions, tab will not produce any suggestion. Using tab tab (double tab) will produce a list of all possible suggestions.

vsftpd has probably more than 1 option. tab twice to view the possibilities.

Example: li with tabtab shows:

libnetcfg          line               lintian-info       lispmtopgm
libreoffice        linguist           linux32            listres
lightdm            link               linux64            
lightdm-session    lintian            linux-boot-prober