pbcopy
faz o que você quer.
pbcopy takes the standard input and places it in the specified pasteboard. If no pasteboard is specified, the general pasteboard will be used by default. The input is placed in the pasteboard as plain text data unless it begins with the Encapsulated PostScript (EPS) file header or the Rich Text Format (RTF) file header, in which case it is placed in the pasteboard as one of those data types.