I know this is a somewhat dated thread, but I thought I [c-w]ould add to it in case somebody else needs the info:

I use the command:

Depending on which *nix you are running, it will show slightly different information; however, on AIX, openSUSE, Debian (et al), & BSD (the ones I'm familiar with atm), it will show total memory & used/available memory.

Hope that helps =)...