I was trying to follow the steps in [this post](http://www.perforce.com/blog/120214/new-20121-p4ignore) on the Perforce website to set up a P4IGNORE file.

On Linux, running 

```
p4 set P4IGNORE=.ignore

```

resulted in the following error message: 

```
Perforce client warning:
    Can't set registry on UNIX.

```

Reading the help entry for `p4 set`, it seems that it can only be used to set Perforce variables on Windows and Mac OS X. On Linux, I had to use native shell commands, i.e. 

```
export P4IGNORE=.ignore

```

My question is - why does the p4 set command not work on Linux, when it is implemented for both Windows and OS X (which is also Unix-based)?