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Tuesday, December 26, 2006

System V versus BSD UNIX

Now a days you will most likely be exposed to AT&T System V UNIX. Years ago there were dozens of different 'versions' of UNIX. At some point things started to converge to either AT&T System V (pronounced A T and T System Five) or BSD UNIX.

If you go for a job interview they might ask you things like, "How do you list all processes?" If you are using System V then the answer is "ps -ef" but it is "ps -aux" if you are using BSD UNIX.

Additionally, a lot of the System V installations have the BSD implementations of commands as well. The idea is that I might have a script that assumes BSD format commands. If I was to parse the output of ps to do a mass kill of all processes and its children then the format of the ps command matters.

If the script was written assuming the BSD version of ps and my computer was configured for System V, the script will fail. The solution, the default path might be /usr/bin but the BSD version of the commands are stored in /usr/ucb. So I could just edit the script so it finds the /usr/ucb/ps command rather than the default /usr/bin/ps.

Finally, if you are using MacOS X you are using a GUI front end to an implementation of UNIX. The UNIX is BSD UNIX. So things like "ps -ef" will not work.

So if you want to impress an interviewer who asks how to list all the processes on a UNIX machine, the answer is "If your computer is configured for System V UNIX then ps -ef will do the trick, but if /usr/ucb exists in the path before /usr/bin then ps -aux will do the trick. Additionally, if you are using MacOS X, which is BSD UNIX, then the default path will use ps -aux."

There are some obvious wrinkles to this. I'm assuming the default set up and configuration for the system. You'd impress them more if you asked which verison of UNIX and then noted, "if they are using the common configuration..."

If you want to read more of the details behind the history of UNIX, give this web site a try. Additionally, this site talks about creation of different versions of UNIX. Click on the picture at the top to see how nuts it got.

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