Variations Among Unix Versions

There are a lot of different versions of Unix. Unix originated at AT&T corporation's Bell Labs about 30 years ago. It was a commercial product. AT&T licensed it to various other commercial organizations who then adapted and changed it in various ways to produce their own flavors of it. Today there are a number of commercial versions. IBM has one called AIX. Hewlett-Packard has HP-UX. Sun Microsystems has Solaris. There are also versions that are available for free and whose code base stems partly or wholly from sources other than the original AT&T code. There is Linux. And there is FreeBSD. FreeBSD is what runs on rexx.com.

The bulk of the behaviors and commands on any of the Unixes resembles that which is found on any of the others. But there are always differences too. If you followed Chapter 3 in the textbook fully, trying all the indicated commands on rexx/FreeBSD, you found some that don't exist or exist but don't behave as "advertised" in the text. For example, the learn command is not available on rexx. The who command is there, but none of the options given for it in Table 3.1 work. In Linux, 3 of them work but one of them (-b) doesn't.

These differences are not unusual. And our textbook was obviously not written specifically to reflect FreeBSD. Generally, when you want the authoritative documentation for a command on a given system where you may be working, you can rely on the documentation printed for that command by the "man" command.

Please compare and contrast, noting the differences between the documentation for the "who" command  from a FreeBSD system, versus that from a Linux system. In both cases the documentation was produced by issuing the command "man who." You can see that the commands are not exactly the same.