I don't particularly associate this usage with unix. The first thing that comes to mind with unix and quotes is shells, and
' have completely different meanings there.
The ‘single quotation marks’ are standard typography (depending on your side of the Atlantic, you may be more used to “double quotation marks”). In ASCII, there are three quote-like characters:
" (the symmetric double quote),
` (the opening quote, also a grave accent0, and
' (the closing single quote, also a symmetric quote, an apostrophe and an acute accent). If you're a single quoter,
`this' is the natural ASCII approximation of quotation marks.
One specific programming usage where
`single-quotation-mark' quoting is the norm is Lisp. This is familiar to Emacs users. Maybe those unix-y folks you have in mind are in fact Emacs folk?