How can I tell what version of OS X I'm on from the command line?

sw_vers

My suggestion is to use sw_vers. Example output as of 10.6.4:

> sw_vers 
ProductName:    Mac OS X
ProductVersion: 10.6.4
BuildVersion:   10F569

The answer that suggested system_profiler | grep 'System Version' is what I have tried to use in the past, but it has 2 problems.

1. It is slow since it generates a full system_profiler dump of the machine, gathering all hardware and software inventory information.
2. The output of system_profiler has changed over time. e.g. output of grep for 'Serial Number' on 10.6.4 is "Serial Number (system): ZNNNNNZNZZZ", whereas on 10.4.11 it was "Serial Number: ZNNNNZNZZZZ" - importance being the parse-ability of the output and the add " (system)" piece can be problematic unless you are expecting the change.

The easiest way is:

$ sw_vers -productVersion
10.6.4

From http://tinyapps.org/blog/mac/201008140700_os_x_version_terminal.html:

$ sw_vers
ProductName:    Mac OS X
ProductVersion: 10.6.4
BuildVersion:  
10F569
$ sw_vers -productVersion
10.6.4

Especially handy when resetting a password in single user mode, since the method varies based on which version of OS X is running.

Try this:

 system_profiler  | grep 'System Version'

Mike Gray's answer is better than this. Please see that