The utility "wget" already does what you want to do. Check it out using the -N option. If the copy you have is older than the timestamp the webserver gives for the specified URL, a new copy is downloaded. If the file sizes do not match, a new copy is downloaded.
If you don't want it to actually download, but see what it would, do, you can use --spider. Then it reports what it would have done:
Code:
$ wget -N -v http://www.keepalived.org/documentation.html
--16:35:14-- http://www.keepalived.org/documentation.html
Resolving www.keepalived.org... 213.228.1.83
Connecting to www.keepalived.org|213.228.1.83|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 5035 (4.9K) [text/html]
The sizes do not match (local 0) -- retrieving.
--16:35:15-- http://www.keepalived.org/documentation.html
Reusing existing connection to www.keepalived.org:80.
HTTP request sent, awaiting response... 200 OK
Length: 5035 (4.9K) [text/html]
Saving to: `documentation.html'
100%[=======================================>] 5,035 --.-K/s in 0.07s
16:35:15 (68.6 KB/s) - `documentation.html' saved [5035/5035]
[otheus] ~$ touch documentation.html
[otheus] ~$ wget -N -v http://www.keepalived.org/documentation.html
--16:36:08-- http://www.keepalived.org/documentation.html
Resolving www.keepalived.org... 213.228.1.83
Connecting to www.keepalived.org|213.228.1.83|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 5035 (4.9K) [text/html]
Server file no newer than local file `documentation.html' -- not retrieving.
[otheus] ~$ echo test >> documentation.html
Change the file size of documentation.html
[otheus] ~$ wget --spider -N -v http://www.keepalived.org/documentation.html
--16:36:27-- http://www.keepalived.org/documentation.html
Resolving www.keepalived.org... 213.228.1.83
Connecting to www.keepalived.org|213.228.1.83|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 5035 (4.9K) [text/html]
The sizes do not match (local 0) -- retrieving.
--16:36:27-- http://www.keepalived.org/documentation.html
Reusing existing connection to www.keepalived.org:80.
HTTP request sent, awaiting response... 200 OK
Length: 5035 (4.9K) [text/html]
200 OK
