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-File: wget.info, Node: Time-Stamping, Next: Startup File, Prev: Following Links, Up: Top
-
-Time-Stamping
-*************
-
- One of the most important aspects of mirroring information from the
-Internet is updating your archives.
-
- Downloading the whole archive again and again, just to replace a few
-changed files is expensive, both in terms of wasted bandwidth and money,
-and the time to do the update. This is why all the mirroring tools
-offer the option of incremental updating.
-
- Such an updating mechanism means that the remote server is scanned in
-search of "new" files. Only those new files will be downloaded in the
-place of the old ones.
-
- A file is considered new if one of these two conditions are met:
-
- 1. A file of that name does not already exist locally.
-
- 2. A file of that name does exist, but the remote file was modified
- more recently than the local file.
-
- To implement this, the program needs to be aware of the time of last
-modification of both remote and local files. Such information are
-called the "time-stamps".
-
- The time-stamping in GNU Wget is turned on using `--timestamping'
-(`-N') option, or through `timestamping = on' directive in `.wgetrc'.
-With this option, for each file it intends to download, Wget will check
-whether a local file of the same name exists. If it does, and the
-remote file is older, Wget will not download it.
-
- If the local file does not exist, or the sizes of the files do not
-match, Wget will download the remote file no matter what the time-stamps
-say.
-
-* Menu:
-
-* Time-Stamping Usage::
-* HTTP Time-Stamping Internals::
-* FTP Time-Stamping Internals::
-
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-File: wget.info, Node: Time-Stamping Usage, Next: HTTP Time-Stamping Internals, Prev: Time-Stamping, Up: Time-Stamping
-
-Time-Stamping Usage
-===================
-
- The usage of time-stamping is simple. Say you would like to
-download a file so that it keeps its date of modification.
-
- wget -S http://www.gnu.ai.mit.edu/
-
- A simple `ls -l' shows that the time stamp on the local file equals
-the state of the `Last-Modified' header, as returned by the server. As
-you can see, the time-stamping info is preserved locally, even without
-`-N'.
-
- Several days later, you would like Wget to check if the remote file
-has changed, and download it if it has.
-
- wget -N http://www.gnu.ai.mit.edu/
-
- Wget will ask the server for the last-modified date. If the local
-file is newer, the remote file will not be re-fetched. However, if the
-remote file is more recent, Wget will proceed fetching it normally.
-
- The same goes for FTP. For example:
-
- wget ftp://ftp.ifi.uio.no/pub/emacs/gnus/*
-
- `ls' will show that the timestamps are set according to the state on
-the remote server. Reissuing the command with `-N' will make Wget
-re-fetch *only* the files that have been modified.
-
- In both HTTP and FTP retrieval Wget will time-stamp the local file
-correctly (with or without `-N') if it gets the stamps, i.e. gets the
-directory listing for FTP or the `Last-Modified' header for HTTP.
-
- If you wished to mirror the GNU archive every week, you would use the
-following command every week:
-
- wget --timestamping -r ftp://prep.ai.mit.edu/pub/gnu/
-