Unix-compatible systems generally conform to standards published by the International Organization for Standardization (ISO), the Open Group, and the IEEE Computer Society.

The primary standard used for this purpose is POSIX, the Portable Operating System standard based on UnIX. Programs and commands that conform to the POSIX standard will work on any Unix system. Therefore, developing your programs and scripts according to POSIX will prevent the need for even minor changes when porting from one Unix variant to another.

Nevertheless, many common Unix programs have been enhanced beyond the POSIX standard to provide conveniences. Fortunately, most such programs are open source and can therefore be easily installed on most Unix systems. Features that do not conform to the POSIX standard are known as extensions. Extensions are often described according to their source, e.g. BSD extensions that come from BSD Unix variants or GNU extensions that come from the GNU software project.

Many standard commands such as awk, make, and sed, may contain extensions that depend on the specific operating system. For example, BSD systems use the BSD versions of awk, make, and sed, which contain BSD extensions, while GNU/Linux systems use the GNU versions of awk, make, and sed, which contain GNU extensions.

When installing GNU software on BSD systems, the GNU version of the command is usually prefixed with a 'g', to distinguish it from the native BSD command. For example, on FreeBSD, "make" and "awk" are the BSD implementations and "gmake" and "gawk" would be the GNU implementations. Likewise, on GNU/Linux systems, BSD commands would generally be prefixed with a 'b' or 'bsd'. The "make" and "tar" commands on GNU/Linux would refer to GNU versions and the BSD versions would be "bmake" and "bsdtar".

All of them will support POSIX features, so if you use only POSIX features, they will behave the same way. If you use GNU or other extensions, you should use the GNU command, e.g. gawk instead of awk.