HP-UX is HP UNIX

HP-UX is HP's Flavor of UNIX

Many MPE shops have recently taken the plunge into the world of UNIX. They often find it a shock. Many of the things they take for granted with MPE are not the same with UNIX. HP-UX is Hewlett-Packard's version of UNIX. It is based on AT&T's System V UNIX, but has features of BSD (Berkeley Standard Distribution), the other major variant of UNIX. HP-UX version 10.0 moves much closer to the POSIX standard "UNIX," with a new suggested directory structure and other changes.

In order to survive in the UNIX world, an MPE user must remember three things:

UNIX is case sensitive. In MPE, the abc and ABC usually mean the same thing: in UNIX, they usually mean something different.
UNIX does not coddle you -- if you type rm * UNIX purges every one of your files without confirming that you really meant to do it.
UNIX uses the C programming language, in which == means equals and = means assignment. So count=0; if (count=0) {printf('hello')} doesn't print 'hello.' What it does is assign 0 to count, then tests it for zero (TRUE) or non-zero (FALSE). You should have coded is if (count==0) {printf('hello')}. This is the most common C programming mistake.

The difficulty of porting programs from MPE to UNIX varies from easy to impossible, based upon how much your program depends on the MPE operating system and IMAGE/SQL database. See, for example, the paper by David Greer of Robelle: "Porting Qedit from MPE to HP-UX."