| Unix was originally developed at Bell Labs. One of the early adopters of Unix was the University of California at Berkeley. The folks at Berkeley made significant enhancements to the Unix operating system.
The original AT&T version of Unix from Bell Labs eventually matured into "System V Unix." The version of Unix from UCB became known as BSD Unix, for Berkeley Software Distribution.
BSD Unix added many commands to the original AT&T Unix. Most of those comands were later added to System V Unix. These BSD Unix commands include `less`, `csh`, and `rlogin`. These Berkeley commands were ported back to System V Unix so long ago, almost no one remembers where they came from!
The real difference now between BSD Unix commands and System V Unix commands is in the arguments to some commands.
The best example of this is the `ps` (process status) command. On Unix systems based upon AT&T System V Unix, the arguments to cause the `ps` command to give a full list of all processes are "-ef". On Unix systems based upon BSD Unix, the arguments to `ps` to achieve similar output is "-aux".
Linux is a version of Unix which is based of AT&T System V Unix. This web server runs FreeBSD, which is based on BSD Unix. | 
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