There are lots of similarities between different versions of UNIX. But it's almost impossible to write a book that covers every detail of every version correctly. Where we know there might be big differences or problems, we'll print a note in the text. Other places, we're forced to use "weasel words" like "Some versions of xxxxxx will do..." without telling you exactly which versions. When you see those weasel words, what can you do?
If the command or feature won't destroy anything when it doesn't work, try it! For instance, don't experiment with rm, the command that removes files. But cat, a command that shows files, most likely won't hurt anything if some feature we tell you about doesn't work with your version.
Look at theor check your vendor's latest printed manuals. Even these can be wrong. For instance, your system administrator may have installed a local version of a command that works differently-but not updated the online documentation. (The and commands, along with some knowledge about the organization of your filesystem can help you find this out yourself.) Be careful with "generic" manuals, the kind you buy at a bookstore; there are a lot of versions of UNIX and the manual may not match your version closely enough.
Ask your system administrator or another "guru" for help before you use a command that might be dangerous.