requested sort enhancement - unique with count

[David H. Durgee]

At present it is necessary to use both sort and uniq if you want a 
tabulated list of unique items.  As an example, I needed to do so for an 
error log to determine what errors were most frequent:

sort .xsession-errors | uniq --count | sort -n

If I were interested only in the unique lines without counts I could 
have used sort -u, but that does not give me any feel for how frequent a 
line is in the file.  The above works well as long as the file is not 
too large, but obviously takes a lot of both time and temporary space if 
the file is huge.  If sort could count the lines while keeping only the 
unique lines there would be only one pass through the file and no 
excessive temporary space would be needed.

My incentive for processing this particular log file was some sort of 
run-away error logging that actually filled the partition the log file 
was on, at which point the file was about 7G of data.  I wound up using 
tail to take the last million lines and getting the most frequent errors 
from that and assuming that this way typical, but of course it would 
have been nice to know.

Dave