Q: The save layout function will save your entire layout in the viewer by default. But occasionally I only want to export a cell of the layout and save it as an individual layout file. How do I do that?
A: There are two ways to do that. You can do it in the "File" --> "Save As..." dialog. Choose the "Write from cell" radio button and the cell list drop down will be available to select. You can select any cell in the viewed layout and output all hierarchy level from that cell.
The second way is to do that from batch command. You can use "gdsout" for GDS output format or "oasisout" for OASIS output format. The command format is "$L gdsout/oasisout <filename> <cellname>". The following is an example command sequence to open fullchip.gds and write out the cellA to save as cellA.gds file.
set L [layout create fullchip.gds] # open layout and get file handle
$L gdsout cellA.gds cellA