Department of Mathematics FAS Harvard University One Oxford Street Cambridge MA 02138 USA Tel: (617) 495-2171 Fax: (617) 495-5132
FAS Computer Services to Harvard University IT: (617) 495-7777.
The file server at the Mathematics department has the ability to recover user files that were mistakenly deleted or to retrieve older versions of files you have been working on.
A new facility called "snapshots" keep images of every file and directory as it existed at a specific instance in time over several of the preceding hours, days, and weeks.
To recover a specific file (or an entire directory or directory tree, should it become necessary), cd to the directory that contained the file/s you want to recover.
cd .snapshot
in the directory you lost a file.
Under the .snapshot pseudo-directory you will see directories that look like:
   hourly.0/   hourly.3/   nightly.0/  nightly.3/  weekly.0/   weekly.3/
   hourly.1/   hourly.4/   nightly.1/  nightly.4/  weekly.1/
   hourly.2/   hourly.5/   nightly.2/  nightly.5/  weekly.2/
These directories are "snapshots" of the current directory (and its subdirectories) as it existed 0 to 5 hours ago (on the hour) 0 to 5 nights ago (at midnight) and 0 to 3 weeks ago (at midnight on Sunday). Use the command "ls -lu" (not just "ls -l") to get the date and time of the particular snapshot. The "ls -l" info on the files within the snapshot directories is correct.
Just cd into the directory you want and cp the file/s you need back to your regular directory. The snapshot directories themselves are read-only so you can't change or edit anything within them.
NOTE: This snapshot facility is not available for email in your Inbox (which is delivered on a different server) though it would be available for any email you save as a separate file in folders, etc.
Please contact Arthur Gaer for any questions in this respect.

Simplicity, Clarity, Generality B.W. Kernighan, R. Pike, in "The Practice of Programming".