This is the “separate archives on split-pat lines” option. This option takes an argument string split-pat. With this option, unshar isolates each different shell archive from the others which have been placed in the same file, unpacking each in turn, from the beginning of the file to the end. Its proper operation relies on the fact that many shar files are terminated by a readily identifiable string.
For example, noticing that most `.signatures' have a double hyphen
("–") on a line right before them, one can then sometimes use
--split-at=--. The signature will then be skipped, along with
the headers of the following message.