Sometimes the command seems to run, but it doesn’t work as advertised. Any number of things might have happened:
There may be more than one version of the command online, and you may be running the wrong version.
You may have an alias set up for the command.
Your environment may not be set up correctly.
You might not be using the command correctly.
You may not have permission to run the command.
Your computer is out to get you. (Just kidding.)
There are plenty of reasons why a program may not work correctly, and we can’t tell you about all of them. But here are some things that you might need to know about when things go awry.
System V versus Berkeley UNIX
Although there are many different vendors of the UNIX operating system, you can break the UNIX world into two distinct camps: System V and Berkeley. System V (pronounced “System Five”) was developed by AT&T. Berkeley UNIX was derived at the University of California at Berkeley (UCB). (Berkeley UNIX is also called BSD, for Berkeley Standard Distribution.)
While the two flavors of UNIX have much in common, there are some commands that are considered Berkeley commands and some that are considered System V commands. In addition, some programs work slightly differently on System V UNIX than they do on Berkeley UNIX. These days, most modern operating systems try to incorporate both System V and Berkeley commands when they differ.