[bug-diffutils] bug#20250: bug#20250: diff by size

[Dave Gordon]

On 03/04/15 14:20, Andreas wrote:
> Dear diff-team,
> 
> this is not a bug but an idea for a diff option in further versions:
> 
> Idea:
> comparison of two files by size only.
> 
> why?
> makes comparison extremely fast; often no need to compare file contents
> to get a first hint whether there is a difference at all
> 
> example:
> - uploading a folder with a bunch of files to a slow motion cloud;
> comparing each file by contents is very time consuming (...slow motion
> cloud)
> - as a first hint to find out about a possible difference could be the
> comparison of file size for each file uploaded with the local ones
> 
> Thanks in advance for integrating this option in future diff versions.
> 
> Great tool, great diff team!
> Andreas Schirra
> address@hidden

It probably doesn't belong in diff(1), but maybe in cmp(1)? For example,
it might be a meaningful semantic for the case of "cmp -n 0", which
currently always says "equal" (-n 0 means compare at most 0 bytes), or
some other variation that isn't currently meaningful, such as "cmp -n -1
<FILE1> <FILE2>" ?

OTOH, you can easily get and compare filesizes in a script already,
for example using "stat --printf=%s <FILE>", and for the case of
synchronising with a slow remote backup, rsync(1) already incorporates
lots of clever ways to reduce the bandwidth of the remote traffic,
including using just the filesize and datestamp to check for (mis-)matches.

.Dave.