I have a GNU sed script I use on Linux; it is installed at /bin/sed and it seems it contains GNUisms. I have collaborators using Mac OS X. They have installed (non-GNU) sed, located at /usr/bin/sed, ...
We are writing scripts for Linux systems, there has been some debate over what would be the most universally Linux present scripting language to use. Bash, SH, Posix? What?