What is the equivalent terminal command of Update Manager?

You can use this command:

sudo apt-get dist-upgrade

man page says:

dist-upgrade

dist-upgrade in addition to performing the function of upgrade, also intelligently handles changing dependencies with new versions of packages; apt-get has a "smart" conflict resolution system, and it will attempt to upgrade the most important packages at the expense of less important ones if necessary. So, dist-upgrade command may remove some packages. The /etc/apt/sources.list file contains a list of locations from which to retrieve desired package files. See also apt_preferences(5) for a mechanism for overriding the general settings for individual packages.

apt-get will NOT consider "suggested" packages as updates, while Update Manager does. It Update Manager also includes packages which apt-get would only install/upgrade with dist-upgrade. Additionally, I believe Update Manager maintains its own package cache which is only automatically updated daily and thus may not always be synchronized with the APT package cache.

To really check if you're missing out on updates, check the version of non-recommended updates Update-Manager wants you to install, and then do a dpkg --list | grep -i packagename to find out which version of the package is really installed on your system.